Nutrition

May 20, 2014

New Study on Protein Overfeeding: A Critical Analysis

A new research study on protein overfeeding is causing a bit of a stir in the fitness community. The study in question, authored by Dr. Jose Antonio et al, evaluated body composition changes in a group of men and women that consumed an additional 800 calories of protein each day (to the tune of more than 5 times the daily RDA for protein!) versus a group consuming a maintenance diet. Here is a brief rundown of the methodology and findings.

A total of 30 resistance-trained subjects (and these subjects would be considered highly trained, with an average lifting experience of almost 9 years) participated in the study: 10 in the control group who were at caloric maintenance, and 20 in the experimental group who ate a caloric surplus, with virtually all of the additional calories consumed in the form of a whey/casein protein powder. Subjects were instructed to maintain their normal resistance training programs, which were not supervised by the researchers. Total calories and macronutrient intake were calculated by self-reported daily food diaries. Body composition was assessed by a BodPod, which uses air displacement plethysmography to estimate fat mass (FM) and fat-free mass (FFM).

After 8 weeks, no statistically significant differences were seen from baseline levels in either group. That said, the high protein group did gain an average of 1.7 kg (3.7 lbs), all in the form of FFM. The food diaries indicated that subjects adhered to the diets as specified by the protocol, and the self-reported volume of training over the course of the study did not change from pre-study levels.

So what’s the controversy here? Well, some have questioned the study’s validity, claiming results violate the First Law of Thermodynamics (i.e. energy is neither created nor destroyed, but rather changed from one form to another). When extrapolated to nutrition, the First Law of Thermodynamics essentially states that the difference between calories ingested versus calories expended will dictate whether weight is gained or lost. Since the subjects in the high-protein group consumed 800 calories over maintenance, it would stand to reason that the subjects who overate should have gained a fairly extensive amount of weight. Using the generally accepted formula that 3500 equates to one pound of fat (which is a fundamentally flawed concept, but that’s a post for another day), total weight gain should have been somewhere in the range of 12 pounds over the course of the 8-week study period.

A closer look at the evidence, however, shows that the results were generally consistent with thermodynamic principles. Here’s why.

First, the thermic effect of food (TEF) for protein is very high. Simply stated, the TEF refers to the amount of calories expended in the digestion and absorption process. Protein has a much higher TEF than the other macronutrients, equating to approximately 30% of total calories. Thus, if you overeat 800 calories of protein, about 240 of these calories will be lost to thermogenesis.

Moreover, overeating results in an increase in a phenomenon called non-exercise activity thermogenesis (NEAT). As the name implies, NEAT refers to the energy expended during everything other than regimented exercise (i.e. fidgeting, maintenance of posture, activities of daily living, etc). A classic study by Levine et al found that subjects who were overfed 1000 calories a day compensated by increasing NEAT by a daily average of ~350 calories. Assuming a somewhat similar response in the Antonio et al study, this would mean that approximately 600 of the 800 extra calories consumed would have been expended via TEF and NEAT.

So we’re left to account for about 200 extra calories a day. Well, it just so happens that the reported weight gain of just over 3 pounds explains this away very nicely. The most interesting thing here is that all of the added weight was attributed to gains in FFM as opposed to body fat. This suggests that overfeeding protein well above levels normally thought to maintain a positive nitrogen balance may in fact have a small effect on enhancing the hypertrophic response to resistance training. Although the study did not provide any insight into potential mechanisms, one possibility is that very high protein intakes may help to suppress protein breakdown. Given that increases in hypertrophy are the result of the difference between protein synthesis and degradation, this hypothesis warrants further study.

I’ve heard a number of people criticize the fact that caloric intake was assessed by self-report. To this end, research does in fact show that self-reported food intake can be quite inaccurate. While certainly this is a valid concern, it should be noted that subjects in the high-protein group reported their dietary intake prior to the study as well as during the intervention. It seems logical to think that if these subjects misreported caloric intake during the study, they also would have done so to a similar extent when reporting their baseline intake. Thus, the net effect would seemingly be a fairly accurate representation of the extra calories consumed over the study period. So while there could be issues related to over-reporting of food intake, the results would seem to suggest that the factors I mentioned above are a more likely explanation.

Bottom line: The study, while intriguing, really serves as pilot data for future exploration into the topic. A big issue here is that the resistance training component was not supervised by the researchers. Thus, there is no way to verify what was actually done by the subjects and, importantly, how hard they actually trained. From what I understand, a follow-up study is already in the works that will address this issue. In the meantime, the take-home message here seems to be that if you intend to overeat (and care about your body composition), make sure the extra calories come from protein-rich foods.