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Protein and Exercise

Author: Stan Reents, PharmD
Original Posting: 11/14/2011 11:35 AM
Last Revision: 09/14/2013 05:15 PM

Most athletes know that carbohydrates are a critical fuel source during exercise. However, the role of protein is often misunderstood....not only by athletes, but by health care professionals as well (Wolfe RR, et al. 2008).

In the following review, I will provide a perspective on protein consumption before, during, and after exercise.


Before I summarize the recommendations for athletes and people who exercise strenuously, we need to review the current dietary protein guidelines in general, as there are some important principles to consider.

Historical Perspective

Beginning in 1997, the Food and Nutrition Board of the Institute of Medicine (IOM) began publishing Dietary Reference Intakes (DRIs), a list of precise amounts for specific nutrients. These were developed to expand upon the "Recommended Dietary Allowance (RDA)" values which had been in existence for decades. Today, for many nutrients, in addition to the RDA, specific amounts exist for "Estimated Average Requirements," "Average Intakes," and "Tolerable Upper Intake Levels." I seriously doubt that your physician can explain these to you, and I'm certain that your personal trainer cannot.

Optimum Protein Intake for Adults

Protein intake is sometimes listed in terms of "daily servings," which is helpful, but most scientific resources use "grams per kilogram of body weight per day" (g/kg/day). So that we can make useful comparisons, I will use "g/kg/day" and focus mainly on the RDA in this review.

Currently, the RDA of protein for adults is 0.8 g protein/kg/day, and this has been the guideline going all the way back to at least 1974. It is important to keep in mind that RDA's represent the minimum amount required to meet nutritional needs, not the optimal amount, as the name seems to imply (Wolfe RR, et al. 2008). Because of this confusion, it has been suggested that the term "Recommended Dietary Allowance" should be renamed "Minimal Daily Requirement" as that is a better description of what it really is (Wolfe RR, et al. 2008).

In fact, some experts feel that the RDA for protein is too low to even meet minimal needs for the average adult (Wolfe RR, et al. 2008), and definitely too low for elite athletes (Wolfe RR. 2000). In one study, elderly adults who were maintained on 0.8 g protein/kg/day lost muscle mass after just 14 weeks (Campbell WW, et al. 2001). In 1989, researchers at Tufts University showed that distance runners required a minimum protein intake of 0.94 g/kg/day to maintain nitrogen balance (Meredith CN, et al. 1989), adding further evidence that the RDA value of 0.8 g/kg/day is not enough.

In 1974, it was acceptable for protein to constitute just 8% of daily calories. As recently as 2000, the American Heart Association stated that a diet where protein makes up 15% of daily calories "should be adequate" (Krauss RM, et al. 2000). In 2005, the official Dietary Guidelines for Americans (from the USDA and the Department of HHS) modified protein recommendations from 12-20% of total daily calories (prior to 2005) to 10-35% (after 2005).

Protein intake recommendations published in the most recent Dietary Guidelines For Americans, 2010 allow for a liberal "10-35% per day" (percent of daily calories from protein), but specify a relatively modest total daily amount of "46-54 grams per day" for adult females and males, respectively. These amounts are based on 0.8 g/kg/day. However, according to Robert Wolfe, PhD, 0.8 g/kg/day doesn't even meet the low end of the 10-35% range. (In an 80-kg person consuming 2800 calories per day, this works out to only 9% of total daily calories.)

In the September 2006 issue of the American Journal of Clinical Nutrition, Wolfe claims that the DRI's for protein intake from the Institute of Medicine are based on the wrong parameters: nitrogen balance studies, and, the development of disease. He points out that even in states of starvation, nitrogen balance can be maintained. And, regarding the development of disease, Wolfe argues that it just isn't logical to base guidelines for the general population on diseases that mostly occur in old age. Instead, muscle mass, muscle strength, and muscle function should be the focus for determining daily protein intake recommendations. He states that skeletal muscle is a vital reservoir of protein for the entire body (skeletal muscle contains 50-75% of the protein in the human body) and, if dietary protein intake is too low, then, muscle protein declines.

And when this happens, health problems result.

Wolfe's argument (in AJCN) is elegant and research supports his claims: Men and women ages 70-79 yrs. in Pittsburgh, PA and Memphis, TN were followed for 3 years. Subjects who consumed the lowest amount of dietary protein lost the most skeletal muscle (Houston DK, et al. 2008).

A sedentary lifestyle also has profound effects on skeletal muscle: Even in young healthy males, as little as 7 days of bed rest can substantially reduce muscle protein synthesis (Ringholm S, et al. 2011). Thus, when you combine aging with a sedentary lifestyle, it's not surprising that health issues arise.

It is well-recognized that resistance exercise (eg., weight-lifting, etc.) increases muscle size and strength, even in the elderly. ACSM guidelines recently added strength training as a recommendation for the elderly. This brings up another principle regarding dietary protein and skeletal muscle:

"The effects of dietary protein and resistance exercise on muscle protein synthesis are considered to be independent and additive." (Campbell WW, et al. 2007)

What this statement means is something like this: If strength training increases muscle mass by 5%, and an increased consumption of dietary protein increases muscle mass by 3%, then, doing both might produce a net increase of 8%. But, for many people, increasing dietary protein intake even without doing any strength training can increase muscle mass.

Thus, this is another argument for increasing your daily protein intake.

Maximizing protein synthesis is an important concept, especially in an aging society where frailty and loss of muscle mass is nearly universal in the final decades of life. It seems clear that a daily consumption of 0.8 g/kg/day is not enough. Wolfe claims a daily intake of as much as 1.8 g protein/kg/day would be more appropriate for most adults (Wolfe RR. 2006). However, some research suggests that muscle protein synthesis reaches a maximum when dietary protein rises to somewhere around 1.1 g/kg/day:

Two separate studies evaluated elderly subjects who regularly consumed 1.1 g protein/kg/day: In one study, neither muscle mass nor muscle strength improved further when additional protein was consumed immediately prior to and immediately after weight-training (Verdijk LB, et al. 2009). In the other, the opposite result was seen (Holm L, et al. 2008). (This may be explained by the specific type of protein used in these 2 studies.....see below). In a 3rd study, young males receiving the same amount (1.1 g/kg/day), muscle protein synthesis was enhanced during exercise, but not afterwards, if a carb+protein beverage was consumed (Beelen M, et al. 2008).

So, if muscle protein synthesis "maxes out" at about 1.1 g/kg/day, why, then, does Wolfe (and others) suggest even higher daily protein intake? Answer: There is a dynamic relationship between the effects of exercise and dietary protein on skeletal muscle. "How" protein is consumed during the day, and, "when" it is consumed in relation to an exercise session are critical details. Note that a single exercise session can stimulate muscle synthesis for up to 48 hrs (Rasmussen BB, et al. 2003), however, when amino acids are supplied, muscle protein synthesis is stimulated for only 1-2 hours (Bohe J, et al. 2001) (Rasmussen BB, et al. 2000). Wolfe derives his recommendations from research showing that muscle protein synthesis is maximal after ingesting 15 grams of essential amino acids, extrapolating that to an equivalent amount of protein, and, then, assuming that this amount can be consumed multiple times per day (Wolfe R. 2006).

Other research shows that muscle protein synthesis is maximum after a single 20 gram "dose" of protein (Moore DR, et al. 2009). This suggests that consuming massive single doses of protein in hopes of accelerating muscle growth is futile. Excessive intake of protein might instead be utilized as an energy source (see below).

In other words, a higher daily intake of protein IS beneficial if consumed in small amounts multiple times per day.

If Wolfe's recommendation of 1.8 g/kg/day seems high, note that in an 80-kg person consuming 2800 calories per day, this amount of protein would represent 20% of total daily calories, well within the 10-35% range currently recognized as acceptable. In healthy people, daily intakes of 2 g protein/kg/day pose no problems (Wolfe R. 2006).

(See "APPENDIX" at the end of this article for lists of specific sources of protein.)


Whey protein is often considered the best protein for building muscle. This is based on several factors:

• Whey is a good source of branched-chain amino acids (BCAAs): Branched-chain amino acids are thought to be better at promoting muscle growth than non-branched chain amino acids. However, consuming individual branched-chain amino acids doesn't always work (Leenders M, et al. 2011).

• Whey is considered a "fast" protein: After consumption, the amino acids in whey protein are released rapidly and make their way to hungry skeletal muscle cells quickly.

In contrast, casein is considered a "slow" protein. This is because the amino acids in casein are released into the bloodstream (after digestion) much more slowly and over a longer period of time (ie., 6-8 hours).

Two separate studies evaluated elderly subjects who regularly consumed 1.1 g protein/kg/day: When casein was consumed immediately prior to and immediately after weight training, neither muscle mass nor muscle strength improved further (Verdijk LB, et al. 2009). In the other study, when whey protein was consumed immediately after weight training, muscle strength was improved (Holm L, et al. 2008). While several other variables might explain the different results seen in these 2 studies, it does suggest that whey protein is the preferred type of protein to consume immediately following a weight-training session. Nevertheless, despite casein's "slow-release" property, the consumption of casein immediately after exercise can still enhance muscle building (Levenhagen DK, et al. 2002).

Unfortunately, commercial sports drinks don't always clarify the specific protein they contain. Note, however, that milk is a good source of casein and whey: the protein in milk is 80% casein and 20% whey.


The potential benefits of consuming protein before, during, and after exercise are:

• Improved performance: Consuming protein during exercise can improve performance of that specific exercise session. And, consuming protein during and immediately following exercise can improve performance of a subsequent exercise session due to enhanced recovery.

• Less muscle injury/soreness: Consuming protein during and after exercise has been shown to reduce muscle injury.

• Reestablish energy stores: Consuming protein during and after exercise enhances the uptake of amino acids by skeletal muscle, but, also enhances the replenishment of muscle glycogen as well.

It is important to realize that each of these 3 actions are interrelated to the others. Also, it's very difficult to assess the benefits of a high-protein diet on exercise performance without first considering the daily carbohydrate intake of the athlete.


Now that we have a perspective on protein requirements for the average adult, we can discuss what athletes and those who exercise strenuously need.

The current "official" source of dietary recommendations for athletes and those who exercise strenuously comes from the American College of Sports Medicine (ACSM). In March 2009, ACSM, along with the American Dietetic Association and Dieticians of Canada, published the position statement Nutrition and Athletic Performance. In this paper, they make the following recommendations for daily protein intake:

  • Endurance athletes: 1.2 - 1.4 g protein/kg/day.
  • Strength athletes: 1.2 - 1.7 g protein/kg/day.

These guidelines are widely endorsed by sports nutritionists. Note, however, that these amounts are actually lower than what Wolfe recommends for the average adult (Wolfe RR. 2006). Additional recommendations for several other specific groups are offered by Nancy Clark, MS, RD, in her best-selling book Sports Nutrition Guidebook, 4th ed. These are summarized in the table below.

Even higher amounts of daily protein have been evaluated in athletes:

• 2.4 g/kg/day: Researchers at McMaster University in Canada studied strength athletes while consuming 3 different amounts of dietary protein for 13 days:

  • 0.9 g/kg/day
  • 1.4 g/kg/day
  • 2.4 g/kg/day

The diet providing the largest amount of protein (2.4 g/kg/day) did NOT increase protein synthesis more than the diet providing 1.4 g/kg/day. The excess dietary protein was, instead, used for energy (Tarnopolsky MA, et al. 1992).

• 3 g/kg/day: Time-trial performance was better in elite cyclists when they were maintained on a daily diet of 3 g protein/kg/day compared to a diet providing half that amount. (NOTE: Their carbohydrate intake was 6 g/kg/day, and the researchers admitted that this might have been a bit low.) (Witard OC, et al. 2011).

• 3.3 g/kg/day: However, in a separate study, performance was WORSE when cyclists were maintained on a daily diet of 3.3 g protein/kg/day compared to when they were consuming 1.3 g/kg/day (Macdermid PW, et al. 2006). This raises the question: was performance worse because protein intake was too high, or, because carbohydrate intake was too low? While the athletes were on the high protein diet, their carbohydrate consumption averaged 4.9 g/kg/day, which is considered low for elite aerobic athletes (Clark N. 2008).

Thus, reputable sources provide a wide range of recommendations for daily protein intake:

• sedentary, but otherwise
healthy, adults
0.8 g protein / kg / day (RDA) Institute of Medicine
DRI values
• weekend warrior, adult 1.0 - 1.5 g protein / kg / day Clark N.
Sports Nutrition
Guidebook (2008)
• endurance athletes 1.2 - 1.4 g protein / kg / day ACSM (2009)
• strength athletes 1.2 - 1.7 g protein / kg / day ACSM (2009)
• sedentary, but otherwise
healthy, adults
1.8 g protein / kg / day Wolfe R.
AJCN, Sept. 2006
• growing teenage athlete 1.5 - 2.0 g protein / kg / day Clark N.
Sports Nutrition
Guidebook (2008)
• research conducted
in athletes
2.4 - 3.0 g protein / kg / day (various)

Recommendations: Daily Protein Intake

If you are confused at this point regarding how much protein to consume on a daily basis, I don't blame you; this research is difficult to sort out. An important concept to keep in mind is that the effects of modifying dietary protein intake cannot be evaluated without also considering the intake of carbohydrates as well. Nevertheless, to try and provide a perspective, here are my thoughts:

• To maximize protein synthesis: Regardless of your current exercise program, you may want to increase your daily protein intake. Wolfe's recommendation is 1.8 g/kg/day. Multiple, small amounts throughout the day provide the best response. (Be careful to not increase your fat intake!)

• To maximize athletic performance: Here the recommendation is much more fuzzy. First, "performance" can be measured in a variety of ways. For example, an NFL lineman wants to improve muscle strength, whereas a marathon runner is interested in muscle endurance. Second, performance is also dramatically affected by what you consume not only on a daily basis, but, what you consume during, and after exercise. And third, carbohydrate intake still needs to be considered, even if your protein intake has been optimized. Halson et al. showed that, when protein intake was 1.3 - 1.4 g/kg/day, a high carbohydrate diet was better than a low carbohydrate diet in cyclists during an 8-day high-intensity training phase (Halson SL, et al. 2004). To maintain muscle glycogen, daily carbohydrate must be adequate. Nancy Clark, MS, RD, suggests a range of 6-10 g carbs/kg/day. An elite athlete undergoing high-intensity training needs 8-10 g carbs/kg/day but the average weekend warrior doesn't need that much. Nevertheless, the principle applies to everyone: If daily carbohydrate intake is too low, then some of the protein that is ingested is converted to glycogen and burned for energy (Witard OC, et al. 2011).


The potential benefits of consuming protein DURING exercise are:

  • less muscle protein loss
  • additional source of calories
  • better recovery due to decreased muscle injury and/or faster repair

Exercise Physiology

What happens when protein is ingested during exercise is very complicated (Koopman R, et al. 2004). How muscles utilize dietary protein is largely determined by how much carbohydrate is being consumed.

During exercise, carbohydrates and fats are the 2 major sources of energy. Glycogen, a carbohydrate stored in muscle, is the primary source of fuel during short-term, intense exercise. Strenuous exercise can substantially deplete muscle glycogen after 1-2 hours. As the duration of a single exercise session increases, there is a gradual shift from carbohydrates (glycogen) to fats to meet energy needs (ie., the human body contains 60,000 - 100,000 kcal stored as fat, but only about 1800 kcal stored as carbohydrate). Thus, you must replenish carbs during a prolonged exercise session.

While the amounts of carbs and fats used for energy vary depending on the type and duration of exercise, protein supplies a relatively small amount. According to Nancy Clark, MS, RD, only about 5% of energy comes from protein during exercise, however, John Ivy, PhD, and Robert Portman, PhD, in their book The Performance Zone (2004) state that as much as 15% of energy needs comes from protein.

Regardless of the exact amount, the principle to keep in mind is that protein shouldn't be thought of as a primary energy source. Yes, amino acids can be converted into glycogen, but the best way to manufacture glycogen is to consume carbohydrates.

• Consuming Protein During Exercise - Effects on Muscle Protein: Ingesting protein during exercise does enhance the resynthesis of muscle protein, but, the story is a bit complicated: It appears that protein synthesis doesn't increase in the hardest working muscle(s) until after exercise is over. Protein synthesis in other muscles, DOES increase while exercise is occuring if protein is ingested (Hulston CJ, et al. 2011).

• Consuming Protein During Exercise - Effects on Muscle Glycogen: The relationship between dietary carbohydrates and muscle glycogen is pretty straight-forward: supplying carbs during (and after) exercise restores muscle glycogen. But insulin is required for this process and it turns out that ingesting protein along with carbs can generate higher insulin levels than after ingesting carbs alone.......up to a point (Miller SL, et al. 2003).

Effects on Athletic Performance

Athletes and weekend warriors don't much care about the physiology of what is going on inside muscles. What matters to them is the effect on performance. So, does the consumption of protein during exercise improve performance?

The answer is, "it depends", it depends on the amount of carbohydrates being ingested.

Endurance Exercise: Two studies of elite cyclists reported that consuming a carb+protein sports drink during endurance exercise increased performance as compared to a carbohydrate-only drink. (Ivy JL, et al. 2003) (Saunders MJ, et al. 2004.)

However, these 2 studies were criticized by Van Essen and Gibala (Van Essen M, et al. 2006):

a) The rate of carbohydrate intake in these studies was too low: In the Ivy study, carbs were ingested at a rate of 47 g/hr (they used Accelerade®); the rate in the Saunders study was 37 g/hr. It is widely accepted that the optimum rate of carbohydrate intake for exercising muscles is 60 g/hr. Van Essen and Gibala found that when elite cyclists consumed 60 grams of carbs/hr during exercise, the addition of protein offered no improvement in performance over the carb-only sports drink.

b) The exercise test did not mimic real-life competition: In the other studies, the athletes cycled at a fixed workload until exhaustion. Van Essen and Gibala felt that it was more realistic to have athletes cycle a fixed distance as fast as they could. To further simulate a real race, they had each athlete ride their own personal bike, not a laboratory bike, and offered monetary rewards to the winner.

Thus, when carbohydrate intake was optimized (eg., 60 g/hr), and, the testing procedure more closely mimicked a real race, the addition of protein to a sports beverage made no least, in terms of performance.

However, there are other reasons to favor a carb+protein sports drink over a carb-only sports drink during exercise:

The first issue is a practical one: The limit for sugar content for sports drinks is considered to be about 6-7%. Above that, GI emptying slows, increasing the risk for abdominal cramps, vomiting, etc. Cyclists might be able to tolerate higher concentrations, but runners may not due to more jostling of their internal organs while running.

Conversely, if the sports drink is substantially less than a 6% carbohydrate solution, then the athlete must ingest a pretty large volume to meet the carb goal of 60 g/hr:

TO DELIVER 60 g carbs/hr
6% carbohydrate sol. 33 fl. oz. per hour
4% carbohydrate sol. 50 fl. oz. per hour
2% carbohydrate sol. 100 fl. oz. per hour

Let's use a specific example to illustrate this:

Ready-to-drink original Gatorade® ("Thirst Quencher") contains 5.8% carbs (14 g carbs per 8 fl.oz.). Using this product, an athlete would have to consume 34 fl. oz. per hour to meet the goal of 60 g of carbs per hour. This works out to roughly 8 fl. oz. every 15 minutes, or, 11 fl. oz. every 20 minutes. I talked to 2 elite athletes; one is a triathlete and the other is a cyclist. They both told me that they don't drink that much fluid during training or racing and felt that it would be tough to meet that goal.

One solution to this (no pun intended), is to use a more concentrated beverage, but, as the sugar concentration goes up, so does the osmolality of the sports drink and, also, the risk of GI intolerance. Thus, opting for a carb+protein sports drink might be a better choice.

The second -- and, perhaps the more important -- reason to consume protein during exercise is that it may promote better muscle recovery. This appears to be the real benefit of consuming protein during exercise. Muscle damage is often assessed by measuring creatine kinase (CPK or CK) levels. Several studies in cyclists have shown that CK levels postexercise were lower if the athletes consumed a combination of protein+carbs during exercise, as opposed to only carbohydrates: In one study, CK levels were 83% lower (Saunders MJ, et al. 2004) and in another study, CK levels were 53% lower (Romano-Ely, BC, et al. 2006).

So, to summarize, here are the potential benefits of consuming protein during exercise:

a) Protein can improve performance: If your carbohydrate intake is not optimum, the additional consumption of protein offers another source of calories, and, in this scenario, protein can help improve performance.

b) A combination of protein+carbs produces higher insulin concentrations: If carbohydrate intake is not optimum, then, consuming protein along with carbs can help to boost insulin levels higher. This may facilitate the uptake of vital nutrients into skeletal muscle.

c) Protein reduces muscle injury and/or enhances recovery: Even if the addition of protein doesn't improve your performance, it is still possible that you might have less muscle soreness and may do well the following day due to faster recovery. This, in my opinion, is the best reason to consume protein during exercise.

Recommendations: Protein Intake During Exercise

Ivy and Portman state clearly in their book The Performance Zone that protein (and carbs!) should be consumed during exercise. They recommend 8-15 grams of protein per hour of exercise. Whey protein (and high glycemic-index carbs) are preferred because they are rapidly digested. This amount can be divided into 2-3 "doses" and consumed every 20-30 minutes during exercise. (Their specific formula is summarized in the table below.)


The potential benefits of consuming protein AFTER exercise are:

  • replenishment of muscle protein
  • enhanced replenishment of muscle glycogen
  • improved exercise performance in a subsequent session

Determining the optimum nutritional recovery formula requires a lot of careful analysis. There are 3 critical factors:

  • which nutrients to consume
  • what amount and how often to consume them
  • when to start (post-exercise)

A) Maximizing Muscle GLYCOGEN Synthesis Post-Exercise:

As mentioned above, glycogen stores are limited and can be severely depleted by prolonged exercise. The best way to replenish glycogen stores is by consuming carbohydrates. Consuming protein in addition to carbs can enhance this process, but it depends on how much carbohydrate is ingested:

John Ivy, PhD, et al. showed that, when protein was added to a carbohydrate "dose" of 1.1 g/kg/hr, glycogen synthesis was higher than compared to a carbohydrate intake of 1.5 g/kg/hr without protein (Ivy JL, et al. 2002). Additional research is summarized in the table below:

• 0.8 g carbs/kg/hr enhanced van Loon, et al. 2000
• 1.1 g carbs/kg/hr enhanced Ivy JL, et al. 2002
• 1.2 g carbs/kg/hr no additional
• Howarth KR, et al. 2009
• Jentjens, et al. 2001

This research suggests that there is a critical level of carbohydrate intake: if carbohydrate intake during recovery is at least 1.2 g/kg/hr, then muscle glycogen synthesis is maximum and protein consumption offers no additional benefit.

However, it's not just the also matters how often carbs are consumed (during recovery). Ivy et al. propose that when carbs are consumed every 15 minutes, glycogen synthesis is better than if carb "doses" are consumed every 2 hours (post-exercise) (Ivy JL, et al. 2002).....

So, to summarize: Scientific research suggests that the optimum carbohydrate replenishment formula is at least 1.2 g/kg/hr, and this amount should be divided into 4 doses consumed every 15 minutes. And if this formula is followed, then the additional consumption of protein does not enhance glycogen resynthesis further.

But, are these amounts realistic? Maybe not. In one study utilizing the ideal carbohydrate intake of 1.2 g/kg/hr, the athletes were required to consume 750 ml/hr (divided into 4 doses every 15 minutes) of a recovery beverage that contained 14.4% carbohydrates. Half of the subjects in this study complained of bloating and GI distress during the 4-hr recovery period (Howarth KR, et al. 2009).

B) Maximizing Muscle PROTEIN Synthesis Post-Exercise:

Exercise increases the rates of both protein breakdown and re-synthesis in muscle for up to 48 hours after a workout (Rasmussen BB, et al. 2003). Unless protein is consumed during recovery, breakdown will exceed synthesis, resulting in the loss of muscle mass. Protein consumed after exercise assists in the repair and synthesis of muscle proteins (Howarth KR, et al. 2009) and thus, is vital to the recovery process.

To maximize muscle recovery after exercise, 3 factors must be considered: (a) how much protein to consume, (b) what type of protein to consume, and (c) when it is consumed.

• AMOUNT of Protein: How much protein do you need to consume after exercise to satisfy muscle protein synthesis? Answer: Not much. One study shows that as little as 10 grams of whey protein was effective (Tang JE, et al. 2007). Another study showed that the effect maxed-out at about 20 grams of protein (egg protein was used in this study) (Moore DR, et al. 2009). However, the amount is probably less important than "when" it is consumed (see below).

• TYPE of Protein: The effects of different dietary proteins on skeletal muscle can vary. Whey is generally regarded as the preferred protein for building muscle. It contains branched-chain amino acids (BCAAs), which skeletal muscle prefers, and, also, it is considered to be a "fast", it is absorbed quickly, and, amino acids are delivered to skeletal muscle quickly. The uptake of nutrients by skeletal muscle is most active immediately following the end of exercise. Thus, to properly "feed" skeletal muscle, a "rapid acting" protein is needed. This role is filled by whey.

Casein, on the other hand, is considered a "slow" protein. It is absorbed more slowly, releasing amino acids into the bloodstream for 6-8 hours after ingestion.

The metabolic activity of skeletal muscle is elevated for 24-48 hrs after exercise ends (Rasmussen BB, et al. 2003). During this period, a steady supply of amino acids is needed. This role is filled by casein. Some experts have even recommended that casein should be consumed before going to sleep because of its sustained-release properties.

Combining the rapid actions of whey with the sustained effects of casein appears to be an ideal formula for skeletal muscle recovery after exercise:

One study comparing the response to milk protein (ie., whey + casein) against soy protein revealed that muscle-building was 34% greater after the consumption of milk protein than after soy protein. One way to explain this is that whey protein optimized the synthesis of new muscle protein while casein suppressed the breakdown of existing muscle protein. Soy protein does not offer this dual action (Wilkinson SB, et al. 2007), and, may even be detrimental: some research suggests that soy protein decreases testosterone levels (Weber KS, et al. 2001).

• TIMING: "When" protein is consumed after exercise is more important than "how much" and "what type" of protein to consume. Research shows that skeletal muscle cells uptake nutrients most efficiently within the first 30-60 minutes after exercise ends. Several hours later, this uptake process is greatly diminished. In one study, when recovery nutrition began 3 hours after exercise ended, the response was much less effective compared to beginning immediately after the end of exercise (Levenhagen DK, et al. 2001).

Much of this research has been summarized in an excellent book by Ivy and Portman: Nutrient Timing. On p. 10 they state:

"Consuming a poor-quality protein, such as corn, within 45-min after exercise ends will result in greater protein synthesis than consumption of a higher-quality protein, such as whey, if consumption is delayed by 2 hours."

Putting all this into more simple terms: It is critical to begin consuming carbs and protein within 30 minutes after exercise ends. This is when skeletal muscle cells uptake nutrients most effectively. Because timing is so important, rapidly-absorbed nutrients are preferred: whey protein is ideal, and, high glycemic-index carbs are preferred. Also, small amounts at frequent intervals are more effective than a single huge dose.

Effects on Performance

A lot of science has been presented. But what really matters is whether performance actually improves if recovery nutrition is optimized. There is quite a bit of evidence that optimizing your post-exercise nutritional formula does improve recovery, and, in doing so, may improve performance during the next session:

• Cyclists: Eight trained cyclists rode at a moderate pace for 2 hours to deplete muscle glycogen stores. During one trial, they were given a standard carb+electrolyte beverage afterwards. During the other trial, they were given a carb+protein beverage. They then did a high-intensity performance ride to exhaustion. After consuming the carb+protein beverage, the cyclists rode 55% longer (Williams MB, et al. 2003).

• Distance runners: Distance runners were given roughly 600 kcal as carbohydrates only or 600 kcal as a mixture of carbs + protein after a strenuous glycogen-depleting run. After consuming the carb+protein recovery beverage, their performance during a following maximum effort run was 21% better compared to the carb-only beverage (Niles ES, et al. 2001).

Recommendations: Protein Intake After Exercise

Within 30 minutes after a hard workout (Ivy and Portman suggest intake should begin within 15 minutes after exercise ends), athletes should consume 50-60 grams of high glycemic index carbs, and 12-15 grams of whey protein.

This represents a carb:protein ratio of 4:1. This ratio is not absolutely critical (again, timing is more important), but, if you want a single commercial sports drink that offers that ratio, Accelerade® and Endurox R-4® offer a carb:protein ratio of 4:1. Several other products are listed in the table below:

50-60 g CARBS
12-15 g PROTEIN
GU Recovery Brew
whey 6.5-to-1 3 scoops 1.5-2.0 scoops
Accelerade® powder whey 4.2-to-1 2.5-3.0
Endurox R-4® powder whey 4-to-1 2 scoops 2 scoops
Gatorade G-Series PRO
03 Recover drink,
16.9 oz.
whey 2.7-to-1 1.2-1.4
1 bottle
milk, 1% fat casein
1.4-to-1 31-37 oz. 11-13 oz.

To determine the carb:protein ratio of other commercial sports drinks, refer to the comprehensive table in my review: "Sports Drinks and Recovery Drinks". Note that a 12-oz. glass of milk combined with an additional carbohydrate source (eg., a bagel) might be just as effective as one of these commercial recovery drinks.


I've reviewed a lot of scientific research. Let's boil all this down to some simple guidelines:

Protein is definitely beneficial to athletes and those who exercise strenuously, but the specific benefits that consuming protein will provide depend on if energy needs are being adequately met by carbohydrates. So, the first thing to do is to analyze your carbohydrate intake. The critical values are as follows:

Daily intake • for elite aerobic athletes: 8-10 g/kg/day
• for others: 6-8 g/kg/day
During exercise 60 g carbs/hour
Immediately after
1.2 g carbs/kg/hr

If your intake of carbs is less than any of these amounts, then, some of the protein you consume will be burned for energy and not devoted to rebuilding muscle protein.

In their book The Performance Zone, Ivy and Portman use the label "30W15" to describe how much and when to consume these nutrients: beginning 30 minutes before exercise and ending 15 minutes after exercise ends. They recommend consumption of both carbs and protein during this entire time frame.

Listed below are the specific carbohydrate and protein recommendations by Ivy and Portman (note that I have excluded the vitamin and mineral amounts from their formula):

• 30-min BEFORE EXERCISE high glycemic-index carbs: 20-26 g
whey protein: 5-6 g
• DURING EXERCISE (every 20-30 min) high glycemic-index carbs: 20-26 g
whey protein: 5-6 g
• WITHIN 15-min AFTER EXERCISE high glycemic-index carbs: 50-60 g
whey protein: 12-15 g
glutamine: 1-2 g

Whey protein and high glycemic-index carbs are preferred because they are rapidly absorbed. Glutamine is also recommended because it is the most abundant amino acid in skeletal muscle.


Reviewing the research for this article has caused me to change my opinion regarding the appropriate carb-to-protein ratio of my daily diet. For many years, I have consumed, and been an advocate for, a diet consisting of roughly 70% carbs and 10-15% protein. I based this choice on the following:

  • carbohydrates -- in the form of glycogen -- are the major fuel for exercising muscles
  • glycogen stores can be severely depleted after 1-2 hours of strenuous exercise (and, thus, must be replenished during prolonged exercise)
  • for decades, official nutrition guidelines have recommended a relatively small amount of protein in the daily diet.

However, I now believe that a higher daily intake of protein is justified. Here's why:

1) To Optimize Health

Previously, I mistakenly interpreted the RDA to represent the optimal amount. In reality, it represents the minimal amount necessary to avoid health problems. Robert Wolfe, PhD, makes the case that RDA recommendations for protein intake are based on the wrong criteria (see discussion above). Instead, he proposes that muscle function should be the focus. He states that health and disease do correlate with the functional status of skeletal muscle and suggests that the average adult's daily requirement of protein is closer to 1.8 g/kg/day (Wolfe RR. 2006) as opposed to the much lower RDA amount of 0.8 g/kg/day.

2) To Optimize Athletic Performance and Recovery

The appropriate consumption of protein provides several benefits for athletes and those who exercise strenuously. Protein can supply the body with additional calories if carbohydrate intake is too low. However, the real benefit of consuming protein before, during, and immediately after exercise is due to the effects on restoring muscle protein, reducing muscle injury, and enhancing recovery.

Thus, a theme begins to emerge: all adults (from sedentary people to serious athletes) might do better if protein intake was higher. The most recent Dietary Reference Intakes (2010) state that protein can make up 10-35% of total daily calories. Since there is no evidence of any problems with protein intakes as high as 2 g protein/kg/day (in people without kidney diseases), I have made the decision to reduce my daily consumption of refined carbs and try to consume good quality sources of protein at each meal. My goal will be 1.8 g/kg/day. And, after a hard tennis workout, I always try to consume a recovery beverage that contains a combination of carbs+protein.

Something to think about.....


• Resistance Exercise vs. Consuming Protein: Resistance exercise (weight-lifting, etc.) and consuming protein can each stimulate muscle protein synthesis independently. However, this process is stimulated for up to 48 hours after resistance exercise whereas a single dose of protein stimulates it for only 1-2 hours.

• Daily Protein Intake: If your daily protein intake is close to the RDA of 0.8 g/kg/day, then it seems wise to consume more than that. But, in doing so, be careful to not also increase your intake of fat, salt, and other unwanted nutrients. Also, don't increase your consumption of red meat to achieve this goal. (The October 2011 issue of CSPI's consumer nutrition newsletter Nutrition Action summarizes the detrimental aspects of consuming too much red meat.) Instead, opt for healthy vegetable sources of protein, and consider high-protein bars and shakes that have low amounts of fat. (See "APPENDIX" at the end of this article for lists of specific sources of protein.)

• To Optimize Athletic Performance: The first task is to evaluate how much carbohydrate you are consuming. But, regardless of that amount, consuming protein before, during, and immediately after exercise appears to have benefits. Opt for foods that contain whey protein, and/or commercial products that contain whey protein, essential amino acids, or branched-chain amino acids.

• To Optimize Recovery: In addition to the recommendations above, make sure you begin your nutritional recovery regimen within 30 minutes after exercise ends. The combination of whey with casein is an ideal regimen for skeletal muscle recovery, however, when you consume protein (post-exercise) appears to be an even more critical factor.


Q: Is milk an acceptable sports drink?

ANSWER: Yes. Sports nutrition research has led to a huge interest in "recovery" drinks for athletes. Commercial recovery drinks such as Endurox®, and, new products from Gatorade® are now widely marketed. (These are discussed in detail in: Sports Drinks and Recovery Drinks.) Milk is also an excellent choice for a "recovery" beverage. Milk contains roughly 1 gram of protein per fl. oz. and the carb:protein ratio is about 1.5-to-1. Milk is also much cheaper than commercial sports drinks: milk costs about $2 for a half-gallon.

There are other reasons why milk is a good recovery drink. Milk contains 2 proteins: casein and whey. Recall that whey protein is a good source of BCAA's. But, the other protein in milk -- casein -- is also beneficial. It appears that whey helps support muscle-building while casein helps to inhibit muscle breakdown. Consuming milk protein after weight-training increased muscle protein synthesis 34% more than soy protein (Wilkinson SB, et al. 2007). So, the next time you need to refuel after a hard workout, consider a big glass of milk. Throw in an additional carbohydrate source (eg., a bagel) and you have a good recovery formula. However, milk also contains fat and, for this reason, milk is not a good choice for either before, or, during exercise.

For more information, here is an interesting web site:


The "official" recommendations for protein intake during exercise can be found here:

• For Health Care Professionals: American College of Sports Medicine, American Dietetic Association, and Dietitians of Canada. Joint Position Statement: Nutrition and athletic performance. Med Sci Sports Exerc 2009;41:709-731.

• For Consumers: The official Dietary Guidelines for Americans, 2010 can be found at the USDA's "Choose My Plate" web site.


Three sports nutrition books that I referred to extensively during the research for this article are:

Many other sports nutrition books available. We have reviewed some of them in our BOOK REVIEWS section:

  • Power Eating by Susan Kleiner, PhD, RD offers reasonably good nutrition information for strength athletes and body-builders.
  • Advanced Sports Nutrition by Dan Benardot, PhD, RD is targeted to serious athletes.
  • Sports Nutrition for Endurance Athletes by Monique Ryan, MS, RD is also another excellent book.


If you'd like some help designing your personal fitness program, or have questions regarding exercise, or simply want some regular motivation, sign-up for our Online Fitness and Wellness Coaching Service. It might be the best investment in your health that you'll ever make.


The following table lists the number of servings of specific foods that provide the RDA amount of 0.8 g protein/kg:

60 kg
80 kg
100 kg
@ 0.8 g protein/kg:
= 48 g = 64 g = 80 g
• chicken breast, no skin
3-oz. serving
15 g
per serving
3.2 servings 4.3 servings 5.3 servings
• eggs 7 g
per egg
7 eggs 9 eggs 11 eggs
• hamburger,
3-oz. serving
22 g
per serving
2.2 servings 2.9 servings 3.6 servings
• milk, 1% fat 1 g
per oz.
48 oz. 64 oz. 80 oz.
• peanut butter
("Laura Scudder's Nutty")
3.5 g
per tbsp
14 tbsp 18 tbsp 23 tbsp
• yogurt ("Greek" by Lucerne)
vanilla, 6-oz.
11 g
per cup
4.5 cups 6 cups 7.3 cups

Wolfe's target of 1.8 g protein/kg/day is ambitious, but anyone should be able to meet this goal without the aid of protein/amino-acid supplements, as the typical North American diet is rich in protein-containing foods.

For example, let's say in a given day you consume:

  • 2 eggs for breakfast: 14 g protein
  • a small burger for lunch: 22 g protein
  • a large chicken breast for dinner: 30 g protein
  • three 12-oz. glasses of milk: 36 g protein
  • 2 cups of high-protein Greek yogurt: 22 g protein

This adds up to a total of 124 grams of protein. Depending on your body weight, this would put you pretty close to Wolfe's recommendation of 1.8 g protein/kg/day.

Keep in mind that some sources of protein (eg., bacon and eggs, cheeseburgers) can also be high in fat (and salt!). And, I do not recommend you increase your intake of red meat; there are healthier ways to increase your protein intake. For example, some vegetables are great sources of protein:

• lentils 18 g per cup
• edamame 17 g per cup
• kidney beans 13 g per cup
• tofu 9 g per 3-oz.

Another way to increase your protein consumption without taking in too much fat is to use nutrition bars or protein drinks. Listed below are several that contain relatively higher amounts of protein. Make sure that you read the Nutritional Facts panel on the packaging closely. For example, note that PowerBar markets 3 different product lines all under the similar name "Protein Plus." Also, don't be misled by the product's name: South Beach's "Protein Fit" bars only contain a relatively low 9 g of protein per bar (these are not included in the table).

(per bar)
(per bar)
PowerBar Protein Plus 30g Bar
chocolate brownie
30 g 11 g
EAS Myoplex MASS Bar
(various flavors), 90 g
30 g 20 g
(various flavors), 75 g
25 g 8 g
(various flavors), 70 g
25 g 8 g
PowerBar Protein Plus Bar
cookies 'n cream
23 g 6 g
PowerBar Protein Plus Reduced Sugar Bar
chocolate peanut butter, 70 g
22 g 9 g
Clif Builder's Bar
(various flavors), 68 g
20 g 8 g
PureFit Nutrition Bar
(various flavors), 57 g
18 g 7 g
(various flavors), 60 g
17 g 8 g
Zone Perfect Bar
chocolate peanut butter, 50 g
14 g 7 g
Kashi GoLean Roll
caramel peanut, 55 g
12 g 5 g
Odwalla Protein Bar
chocolate peanut butter, 56 g
12 g 6 g

(source: product labels and mfr. web sites, Aug-Sept. 2011)

I like the taste of Clif's "Builder's" bars, but they are very sweet. I recently tried PureFit bars ( during a hike at the Grand Canyon. They're not super-sweet, they don't contain high-fructose corn syrup or trans fats, and, they don't melt. But, my personal favorites are Odwalla Protein Bars, specifically the "chocolate chip peanut" flavor. They're not pasty, and, not super-sweet, though they only contain 7 g protein per bar.

For those looking for protein drinks, several ready-to-drink products are listed below:

(per bottle)
(per bottle)
shake (various flavors), 17-oz.
42 g 7 g
CytoMax Protein
orange tangerine, 20-oz.
40 g 0 g
chocolate, vanilla, 13.7-oz.
32 g 6 g
shake (various flavors), 14-oz.
25 g 2.5 g
Muscle Milk
vanilla creme, 14-oz.
25 g 9 g
EAS Myoplex LITE
shake (various flavors), 11-oz.
20 g 2 g
milk, 1% fat, 12-oz. 13.5 g 4 g
Gatorade G-Series FIT 03 Recover
Protein Smoothie, mixed berry, 11.2-oz.
12 g 2 g

(source: product labels, Aug-Sept. 2011)

So, you can see that there are various ways to increase your daily protein intake without also increasing your fat intake.


American College of Sports Medicine, American Dietetic Association, and Dietitians of Canada. Joint Position Statement: Nutrition and athletic performance. Med Sci Sports Exerc 2000;32:2130-2145. Abstract

American College of Sports Medicine, American Dietetic Association, and Dietitians of Canada. Joint Position Statement: Nutrition and athletic performance. Med Sci Sports Exerc 2009;41:709-731. Abstract

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Stan Reents, PharmD, is a former healthcare professional. He is a member of the American College of Sports Medicine (ACSM) and holds current certifications from ACSM (Health & Fitness Specialist), ACE (Personal Trainer, Health Coach) and has been certified as a tennis coach by USTA. He is the author of Sport and Exercise Pharmacology (published by Human Kinetics) and has written for Runner's World magazine, Training and Conditioning, Club Solutions, and other fitness publications.

DISCLOSURE: Neither the author, nor, AthleteInMe, LLC, has any financial relationships with any of the manufacturers of products mentioned in this review that would be considered a conflict-of-interest.

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