The key finding in the present study is that consuming a hypercaloric high protein diet has no effect on body composition in resistance-trained individuals. This is the first investigation in resistance-trained individuals to demonstrate that consuming a high protein hypercaloric diet does not result in a gain in fat mass. On average, they consumed 4.4 g/kg/d of protein which is more than five times the recommended daily allowance [16].

It should be noted that in previous studies, subjects that consumed a hypocaloric diet that is higher in protein and lower in carbohydrate, experienced more favorable alterations in body composition [17–20]. However, the effects of consuming extra calories above normal baseline intake coupled with changes in macronutrient content have not been fully elucidated. The current investigation found no changes in body weight, fat mass, or fat free mass in the high protein diet group. This occurred in spite of the fact that they consumed over 800 calories more per day for eight weeks. The high protein group consumed an extra 145 grams of protein daily (mean intake of 307 grams per day or 4.4 g/kg/d). This is the highest recorded intake of dietary protein in the scientific literature that we are aware of [21–30].

The results of the current investigation do not support the notion that consuming protein in excess of purported needs results in a gain in fat mass. Certainly, this dispels the notion that ‘a calorie is just a calorie.’ That is, protein calories in ‘excess’ of requirements are not metabolized by the body in a manner similar to carbohydrate. Recently, Bray et al. demonstrated that a relatively higher amount of protein does not contribute to an additional gain in fat mass [11]. In this investigation, subjects consumed a diet that exceeded their normal caloric intake by 954 kcal/d. Subjects were randomized into one of three groups: low protein (5% of total energy from protein), normal protein (15%) and high protein (25%). After a treatment period of eight weeks, fat mass increased in all three groups equally (~3.5 kg); however, lean body mass decreased by 0.7 kg in the low protein group in contrast to a gain in the normal (2.9 kg) and high protein (3.2 kg) group. According to the investigators, calories alone contributed to the increase in fat mass; however, protein contributed to gains in lean body mass but not fat mass [11]. Thus, eating extra calories will result in a gain in body fat; however, overfeeding on protein will also result in a gain in lean body mass perhaps due to an increase in muscle protein synthesis.

There are profound differences between the investigation by Bray et al. and the current one. For instance, the current investigation used highly trained subjects whereas the participants in the Bray et al. study did not exercise. What is intriguing is that subjects in the high protein group (Bray et al.) consumed 135 grams of protein daily (~1.8 g/kg/d) compared to their baseline intake of 93 grams (~1.2 g/kg/d). This is less than the amount of protein consumed at baseline for subjects in the current study (~1.9-2.3 g/kg/d). The gain in lean body mass experienced by the subjects in the Bray et al. study suggest that their initial protein intake was inadequate to begin with. Therefore, non-exercising subjects should consume protein at levels twice the recommended daily allowance while keeping carbohydrate and fat intake the same. This dietary strategy alone may promote gains in lean body mass.

On the other hand, the subjects in the current study were resistance-trained subjects who were instructed to not alter their training regimen. Thus, the lack of body composition changes in our group may be attributable to the fact that it is very difficult for trained subjects to gain lean body mass and body weight in general without significant changes in their training program.

An overfeeding study by Tchoukalova et al. demonstrated a gain in fat mass with no change in fat free mass [31]. In this investigation, all subjects consumed a diet that consisted of 50% carbohydrate, 15% protein, and 35% fat. Subjects were instructed to eat until they were ‘more full than usual.’ The extra calories were provided via the choice of an ice cream shake (402 kcal, 40% fat), a king-sized Snickers bar (510 kcal) (Mars Inc.), or Boost Plus (360 kcal/8 oz) (Nestle Nutrition). It is therefore not surprising that eight weeks of overfeeding on food that is largely comprised of carbohydrate would result in a fat mass gain. This is in agreement with other studies [11, 12]. Carbohydrate overfeeding has been shown to elevate de novo lipogenesis; moreover, excess carbohydrate may be converted to fat via both hepatic and extrahepatic lipogenesis [13, 32].

Norgan et al. had six young men overfeed for 42 days by 6.2 MJ/d (~1490 kcal) [33]. The composition of the overfed meals was 49% carbohydrate, 34% fat, and 17% protein. The mean increase in body weight, body fat and total body water was 6.03, 3.7, and 1.8 kg, respectively. They did not measure body composition per se; however, it would seem reasonable that part of that weight gain would lean body mass. The 17% protein intake in the Norgan et al. study is comparable to the ‘normal’ protein group in the Bray et al. investigation which demonstrated a gain in both fat and lean mass. However, it is in contrast with the current investigation which did not show any significant changes in either parameter.

One might suggest that the high thermic effect of protein may make it difficult to gain body weight during times of overfeeding. It has been shown that the greater the protein content of a meal, the higher the thermic effect [34]. Both young and old individuals experience an increase in resting energy expenditure after a 60 gram protein meal (17-21% increase) [35]. Also, the thermogenic response to a mixed meal (440 kcal of carbohydrate [glucose], fat, and protein) differs between lean and obese subjects [36]. In a study by Swaminathan et al., the thermic effect of fat was lower in obese (−0.9%) versus lean individuals (14.4%). In contrast, there was no difference in the thermic effect of glucose or protein. When subjects consumed a mixed meal, the thermogenic response was significantly less in the obese (12.9%) versus the lean individuals (25.0%) [36]. Another investigation found that the thermic effect of a 750 kcal mixed meal (14% protein, 31.5% fat, and 54.5% carbohydrate) was significantly higher in lean than obese individuals under conditions of rest, exercise and post-exercise conditions. According to the authors, “the results of this study indicate that for men of similar total body weight and BMI, body composition is a significant determinant of postprandial thermogenesis; the responses of obese are significantly blunted compared with those of lean men” [37].

The subjects in our study were lean, resistance-trained young men and women. Their baseline protein intake as ~2.0 g/kg/d. It has been previously demonstrated that a higher protein intake is associated with a more favorable body composition even in the absence of caloric restriction [38]. One might speculate that the thermic effect of consuming large amounts of dietary protein in trained subjects exceeds that of untrained but normal weight individuals.

It is unusual that despite no change in their training volume, the ~800 kcal increase in caloric intake had no effect on body composition. This is the first overfeeding study done on well-trained individuals; thus, one might speculate that their response differs from sedentary individuals. Although there was no significant change in the mean value for body weight, body fat, lean body mass or percent fat, the individual responses were quite varied. This may be due to the fact that other dietary factors were not controlled (e.g. carbohydrate intake). There was a mean increase in carbohydrate intake (~14%) in the high protein group. This was not significant due to the wide variation in intakes. Of the 20 subjects in the high protein group, 9 consumed more carbohydrate whereas 11 decreased or maintained the same intake. It is unclear if consuming protein only during the overfeeding period in the absence of fat or carbohydrate intake alterations would differentially impact body composition; however, we would speculate that protein overfeeding alone would likely have no effect on fat mass while promoting gains in lean body mass concurrent with a heavy resistance training regimen geared towards skeletal muscle hypertrophy.

Another factor that may have played a role in the current investigation is the type of protein consumed in the high protein group. Because of the difficulty in consuming 4.4 grams of protein per kg body weight daily, every subject in the high protein group acquired their additional protein calories primarily from whey protein powder. It has been shown that the thermic effect is greater with whey versus casein or soy protein [39]. Recently scientists demonstrated that consuming similar calories and protein during resistance training in initially untrained individuals resulted in greater gains in lean body mass in the whey supplemented group versus soy or carbohydrate [40]. Another investigation found that muscle protein synthesis after whey consumption was approximately 93% greater than casein and approximately 18% greater than soy. Furthermore, the same pattern held when measured post-exercise (whey > soy > casein) [41]. On the other hand, 48 grams of both whey and rice protein isolate consumed post resistance exercise improved indices of body composition and exercise performance similarly [42]. Thus, one might speculate that if the protein dose or intake is sufficiently high, it may not matter what that particular protein source may be.