To the Editor:

Re “In Dieting, Magic Isn’t a Substitute for Science” (July 10): Dr. Jules Hirsch states that to lose body fat, one must reduce calories taken in, or increase the output by increasing activity, or both, and we agree. Our findings do not conflict with any basic law of physics. Rather, we show that reducing consumption of processed carbohydrate may boost metabolism after weight loss, and this effect might make weight control easier over the long term.

Dr. Hirsch attributes the 300-calorie difference in energy expenditure among diets in our study to changes in body water. However, we measured calorie expenditure using two state-of-the-art methods after fluid shifts had stabilized, and our analytic methods would not be affected by changes in lean body mass. Ultimately, controlled feeding studies of at least 6 to 12 months duration will be needed to answer this fundamental question in nutrition. David Ludwig, M.D.

Cara Ebbeling

Boston