by Jose Antonio PhD, www.theissn.org – We’ve all done this. And it almost always happens from Thanksgiving to the New Year’s holiday. We overfeed. Pumpkin pie, turkey with gravy, chocolate, wine, more pie, more wine, and god knows what else we stuff down our esophagus. But what really happens when we overfeed? And what happens if we overfeed with varying amounts of carbs, protein and fat? Great question! And I love these overfeeding studies (rather than the diet/weight loss ones) because they tell a tale that is more intriguing, in my opinion. A study from the Journal of the American Medical Association examined 25 healthy, weight-stable 
male and female volunteers, aged 18 to 35 years. After consuming a weight-stabilizing diet for about 2-3 weeks, participants were randomized to diets containing 5% of energy from protein (low protein), 15% (normal protein), or 25% (high protein), which they were overfed during the last 8 weeks of their 10- to 12-week stay in the inpatient metabolic unit. This overfeeding period corresponded to about 954 extra calories per day! That’s equivalent to 3 Dunkin Donuts (glazed of course). During the entire overfeeding period, the study subjects consumed in excess of 50,729 calories. So what happened? Overeating produced significantly less weight gain in the low protein diet group (6.95 pounds) compared with the normal protein diet group (13.31 pounds) or the high protein diet group (14.32 pounds). Interestingly, body fat increased similarly in all 3 protein diet groups and represented 50% to more than 90% of the excess stored calories. Metabolic rate or resting energy expenditure, total energy expenditure, and body protein did not increase during overfeeding with the low protein diet. On the other hand, resting energy expenditure (normal protein diet: 160 kcal/d]; high protein diet: 227 kcal/d) increased in the normal and high protein groups. And the amount of lean body mass went up in the normal protein (6.31 pounds) and high protein (6.38 pounds) groups. So what’s the take home message?
First my editorial comments. A 25% protein diet is not a high protein diet. If anything, it is very average. The better comparison would be a 25% versus a 50% protein diet. But inasmuch as this study compared 5%, 15% and 25%, let’s see what we can glean from this. First off, total calories especially if it is mostly carbs will be stored as fat. If you bump up your protein intake, you’ll still gain fat, but you’ll also gain lean body mass. So overfeeding on a mixed diet of carbs, protein and fat will promote a weight gain that is both fat and muscle. In addition, boosting one’s protein intake will elevate resting metabolic rate. On the flip side, these individuals were not exercising. A diet as low as 5% protein (or even 15%) just doesn’t give you enough protein for the basic needs of recovery. The authors of this study concluded that calories alone account for the increase in fat; protein affected energy expenditure and storage of lean body mass, but not body fat storage. [1] However, we you keep your caloric intake the same, but do an isocaloric replacement of carbohydrate with protein, the end result for most individuals is a loss of body fat and a gain in lean body mass. Basically eating more protein can only be good for you vis a vis resting energy expenditure and lean body mass gain.
Though I wonder what happens when you get well-trained highly fit individuals and have them overfeed on protein? Hmmm…
Reference
1. Bray GA, Smith SR, de Jonge L, Xie H, Rood J, Martin CK, Most M, Brock C, Mancuso S, Redman LM: Effect of dietary protein content on weight gain, energy expenditure, and body composition during overeating: a randomized controlled trial. JAMA 2012, 307:47-55.
