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It is still not clear what effect diet composition has on the body’s response to overeating.
To find out, researchers conducted a small study of 25 people who were randomly assigned to follow a low protein (5% of energy), normal protein (15% of energy), and high protein diet (25% of energy) while being overfed.
They were given approximately 954 extra calories a day for eight weeks. Weight gain was less in the low protein group, but the increase in body fat was similar in all three groups.
However 90% of the extra calories was stored as fat in the low protein group compared to about 50% in the normal and high protein diets.
Lean body mass and calorie expenditure increased in the normal and high protein groups but not in the low protein group.
The researchers note that obesity is not simply overweight. Overweight can be caused by both increased fat and increased muscle. Obesity refers to an increase in body fat.
They note that while less weight was gained on the low protein diet, most of that weight was gained as fat rather than lean body mass. They also note that muscle weighs more than fat.
A major goal for weight management should be fat reduction rather than simply weight loss. Clinicians should consider assessing overall fatness rather than simply measuring weight or BMI.
*Please note, this is a small study of short duration. Larger, longer studies are needed to more accurately assess the effect of protein levels on body composition under conditions of overeating.