A well-controlled, head-to-head comparison of low fat and low carbohydrate diets reveals (to no one s amazement) that there isn t much difference between them when it comes to weight loss. However, when loss of body fat per se is desired, a low fat diet has the edge.
Led by Dr. Kevin D. Hall, senior investigator at the NIH, the researchers studied the effect of reducing either dietary carbs or fat on the weight lost by 19 obese individuals 10 men and 9 women. The study s strength was that the participants lived on a metabolic ward at the NIH; their diets were strictly controlled, and their metabolism was measured, i.e. the rate at which they utilized body fat as a fuel on the different diets.
Initially, the participants (whose average BMI was 36) ate a balanced diet 50 percent carbs, 35 percent fat, and 16 percent protein. Then, for 9 days they consumed a diet that was reduced by about 800 calories per day (a 30 percent calorie restriction) by decreasing either the fat or the carbohydrate content of the diet. After a break, they returned to the metabolic ward and again had 5 days of the balanced diet, followed by the reduced diet they hadn t had previously.
When they consumed the low carb diet, the participants actually used more fat for fuel than during the balanced diet. In contrast, this wasn t true for the low fat diet. Importantly, cutting the fat intake resulted in a significantly greater loss of body fat in the low fat condition (-89 grams) than did cutting carbohydrates (-53 grams). This was true in spite of the fact that on the lower carb diet participants lost slightly more weight than they did on the lower fat diet (4 VS 3 pounds).
Dr. Ruth Kava, ACSH senior nutrition fellow commented This is a really well-controlled study, in which the participants diets and metabolism were tightly monitored. The fact that reducing the carbohydrate content resulted in slightly more weight loss is most likely due to the fact that carb restriction induces more water loss than would a fat-restricted diet. But it s fat loss that is the real goal of dietary restriction. Still, both restricted diets did reduce body fat, indicating that it s really the calorie restriction that is the key to fat and weight loss. No surprise there.