Researchers at the Pennington Biomedical Research Center in Baton Rouge, LA, have shown that when people significantly reduce their calorie intake, they undergo a metabolic adaptation that results in a slower metabolic rate. The slower metabolic rate results in a behavioral adaptation in which individuals become less physically active.
The researchers examined data from 48 overweight people who followed 1 of 4 diet regimens for 6 months: a 25-percent calorie restriction (CR) diet, a low-calorie diet, a 12.5-percent CR diet plus 12.5-percent aerobic exercise program, or a normal diet. While the first three groups lost weight, after 6 months, the CR and low-calorie diet groups showed a reduction in their basal metabolic rate, accompanied by reduced physical activity. The CR plus physical activity group also lost weight but did not undergo a metabolic adaptation.
This is the first study to measure metabolic rate and energy expenditure precisely through the use of doubly labeled water and indirect calorimetry. The study also shows that long-term calorie reduction without increased exercise can result in a lower metabolic rate. The researchers note that the data “suggest potential mechanisms by which CR causes large inter-individual variability in the rates of weight loss and how exercise may influence weight loss and weight loss maintenance.”