Eating too much food guarantees that you get fatter, regardless of how much protein you consume, according to new research published in the Jan. 4 issue of the Journal of the American Medical Association.
All patients gained weight in the study, but those in the low-protein group gained less than those who ate normal or high levels of protein when they overate by about 1,000 calories a day, according to the study.
"Fat storage was exactly the same with all three levels of protein," according to George Bray, MD, of Pennington Biomedical Research Center in Baton Rouge, La. "Protein, on the other hand, had no effect on storage offat, but it did affect weight gain."
Some work has suggested that eating a diet high or low in protein could maintain body weight through its potential effects on metabolism; eating too little could spare lean body mass while eating too much could add lean body mass.
The researchers assessed 25 patients -- 16 males, 9 females -- ages 18 to 35 with a body mass index (BMI) between 19 and 30 who lived in-clinic for the duration of the study.
They first had a weight-stabilizing diet, followed by randomization to diets consisting of 5%, 15%, or 25% energy from protein. During this phase, patients overate for eight weeks, increasing their overall energy intake by about 40%, or an additional 954 kcal per day.
The researchers found that all patients gained weight, but those in the low-protein group gained less than those who ate normal or high levels ofprotein (3.16 kg versus 6.05 kg and 6.51 kg, respectively, P=0.002).
Yet those in the low-protein group gained less lean body mass than those in the normal or high protein groups (0.7 kg versus 2.87 kg and 3.18 kg, P0.001), largely accounting for the differencs in weight gain.
And the overall increase in fat mass was similar between groups, rising about 3.51 kg from baseline, they reported.
They also found that resting and total energy expenditure rose among those who had normal- or high-protein diets, but it didn't change in the low-protein group.
"A low-protein diet may mean you weigh less, but might cause higher levels of body fat," Sandon said. "Those that ate the low-protein diet gained less weight overall, but the weight they gained was mostly body fat. Excess bodyfat is related to metabolic syndrome and other health issues even if you are of normal or slightly overweight."
Sandon also noted that the increased gain in muscle mass among those on the normal- and high-protein diets likely accounts for the increase in resting energy expenditure.
"We also know from other research that protein has a higher thermic effect on food," he said in the email. "Higher protein intake requires more calories to digest, absorb, and metabolize compared to carbohydrate or fat. This may partly explain the increased calorie expenditure in the normal-to-high-protein groups."
Bray cautioned that the study was limited because the majority of patients were male and black, and the findings may lack generalizability, but still concluded that "protein does influence what happens to your lean body muscle mass during the course of any dietary intervention, so there's an important value to eating protein, but it doesn't influence your storage of calories."
What this means, according to health and fitness expert Jeff Behar, CEO ofwww.MusclemagFitness.com is that while eating enough protein to maintain muscle mass is important, eating too much protein can increase your overall calories, which can cause weight gain and increased body fat.
The nutrition study was supported by the U.S. Department of Agriculture.
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