This was a good and needed study that assessed changes in A) weight; B) body composition; and C)metabolic rate in healthy, weight-stable males and females between the ages of 18-35. Subjects were provided with enough calories to sustain weight (pre-intervention period) for about 2 to 4 weeks, and were then randomly divided into three groups: Low, Normal, and High protein, each with 40% more energy (calories) for 8 weeks than needed to sustain weight. This was ~1,000 calories MORE than they were consuming per day in the weight stabilization pre-intervention period. The basic finding was that calories alone, regardless of protein content, accounted for the increase in fat mass when overfed. However, the low protein group experienced less total weight gain than the higher protein groups despite consuming the same amount of excess calories. The higher weight gain of the normal protein group and the high protein group was due to a greater increase in lean body mass, which also resulted in a higher metabolic rate. Therefore, the impact of the protein was to affect the total energy expended (higher) and the amount of lean body mass (higher), but not body fat acquisition (the same as the low protein intake group. Note: So as to keep excess caloric intake the same while varying the protein content of the diet, fat intake was used to make this adjustment. Lower protein intakes, therefore, had a higher proportion of fat intake, and higher protein intakes had a lower proportion of fat intake.
Conclusion. Excess calories, regardless of the energy substrate distribution (i.e., proportion of carbohydrate, protein, or fat) equally increases body fatness. Low protein intakes result in the lowest total weight gain (but not less fat gain), perhaps because of difficulty in sustaining the lean mass.