When it comes to weight gain, it’s all about the calories. That might seem obvious, but popular diets continue to suggest that lowering or increasing certain dietary components — carbs or protein, say — is the key to weight loss. A clever new American study  shows, however, that it’s not what you eat but how much that matters when it comes to body weight. However, it's not quite as simple as that...

For the study, researchers at Pennington Biomedical Research Center in Baton Rouge, led by Dr George Bray, tested three high-calorie diets in a group of 25 healthy, normal-weight people. Each randomly assigned diet delivered the same amount of excess calories — 954 extra calories per day, or about 40% more than the participants needed, the better to fatten them up — but the difference lay in where most of those surplus calories came from, protein or fat.

The researchers wanted to know, when it comes to weight gain, does it make any difference if people stuff themselves with too much protein, too little or the typical amount?

Overall, not surprisingly, everyone gained weight after eight weeks of overeating. But the participants eating low-protein and, thus, higher-fat diets (carbohydrates were kept constant) gained only about half as much weight as those who ate diets with either normal or high amounts of protein: nearly 7 lbs. for low-protein eaters, compared with 13-14 lbs. for the other two groups.

That sounds like a boon, right? Not quite. When researchers looked more closely at how that excess weight was materializing — using whole-body scanners to gauge changes in the participants’ body fat and lean mass — they found that all three groups had gained the same amount of fat, 7.7 lbs. That means that although the low-protein group had gained less weight overall, their gain consisted almost entirely of fat (they also lost about 1.5 lbs. of lean body mass like muscle — which is not a good thing) while the other two groups gained both fat and lean body mass.

“The composition of what you eat isn’t important for determining what happens to your fat stores — only the calories,” says Bray, but “one of the things this study shows is that our handling of protein and our handling of calories can be separated. [Protein] does some very different things than what the total calories do.”

Participants in the low-protein, high-fat group stored more than 90% of their extra calories as fat, and the lack of dietary protein caused their loss of lean body mass. “They were actually mobilizing some of their body’s proteins” — that is, using up the body’s existing lean mass — Bray notes. Those who ate normal- or high-protein diets, by contrast, stored only 50% of their extra calories as fat.

Bray also points out that based on the amount of protein consumed daily by participants eating the low-protein diet (48 g), overeaters would have to up their intake to 78 g to keep from losing lean body mass. Meanwhile, Americans are typically advised to eat a minimum of 56 g of protein a day “as the lower limit of normal,” Bray says, “suggesting that this criterion might need to be reconsidered.”

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