Q. and A. - Are High-Protein, Low-Carb Diets Effective? - NYTimes.com Is a calorie really just a calorie? Do calories from a soda have the same effect on your waistline as an equivalent number from an apple or a piece of chicken? For decades the question has percolated among researchers - not to mention dieters. It gained new momentum with a study published last month in The Journal of the American Medical Association suggesting that after losing weight, people on a high-fat, high-protein diet burned more calories than those eating more carbohydrates. We asked Dr. Jules Hirsch, emeritus professor and emeritus physician in chief at Rockefeller University, who has been researching obesity for nearly 60 years, about the state of the research. Dr. Hirsch, who receives no money from pharmaceutical companies or the diet industry, wrote some of the classic papers describing why it is so hard to lose weight and why it usually comes back. The JAMA study has gotten a lot of attention. Should people stay on diets that are high in fat and protein if they want to keep the weight off? What they did in that study is they took 21 people and fed them a diet that made them lose about 10 to 20 percent of their weight. Then, after their weight had leveled off, they put the subjects on one of three different maintenance diets. One is very, very low in carbohydrates and high in fat, essentially the Atkins diet. Another is the opposite - high in carbohydrates, low in fat. The third is in between. Then they measured total energy expenditure - in calories burned - and resting energy expenditure. They report that people on the Atkins diet were burning off more calories. Ergo, the diet is a good thing. Such low-carbohydrate diets usually give a more rapid initial weight loss than diets with the same amount of calories but with more carbohydrates. But when carbohydrate levels are low in a diet and fat content is high, people lose water. That can confuse attempts to measure energy output. The usual measurement is calories per unit of lean body mass - the part of the body that is not made up of fat. When water is lost, lean body mass goes down, and so calories per unit of lean body mass go up. It's just arithmetic. There is no hocus-pocus, no advantage to the dieters. Only water, no fat, has been lost. The paper did not provide information to know how the calculations were done, but this is a likely explanation for the result. So the whole thing might have been an illusion? All that happened was the people temporarily lost water on the high-protein diets? Perhaps the most important illusion is the belief that a calorie is not a calorie but depends on how much carbohydrates a person eats. There is an inflexible law of physics - energy taken in must exactly equal the number of calories leaving the system when fat storage is unchanged. Calories leave the system when food is used to fuel the body. To lower fat content - reduce obesity - one must reduce calories taken in, or increase the output by increasing activity, or both. This is true whether calories come from pumpkins or peanuts or pâté de foie gras. To believe otherwise is to believe we can find a really good perpetual motion machine to solve our energy problems. It won't work, and neither will changing the source of calories permit us to disobey the laws of science. Did you ever ask whether people respond differently to diets of different compositions? Dr. Rudolph Leibel, now an obesity researcher at Columbia University, and I took people who were of normal weight and had them live in the hospital, where we diddled with the number of calories we fed them so we could keep their weights absolutely constant, which is no easy thing. This was done with liquid diets of exactly known calorie content. We kept the number of calories constant, always giving them the amount that should keep them at precisely the same weight. But we wildly changed the proportions of fats and carbohydrates. Some had practically no carbohydrates, and some had practically no fat. Did people unexpectedly gain or lose weight when they had the same amount of calories but in a diet of a different composition? There was zero difference between high-fat and low-fat diets. Why is it so hard for people to lose weight? What your body does is to sense the amount of energy it has available for emergencies and for daily use. The stored energy is the total amount of adipose tissue in your body. We now know that there are jillions of hormones that are always measuring the amount of fat you have. Your body guides you to eat more or less because of this sensing mechanism. But if we have such a sensing mechanism, why are people fatter now than they used to be? This wonderful sensing mechanism involves genetics and environmental factors, and it gets set early in life. It is not clear how much of the setting is done before birth and how much is done by food or other influences early in life. There are many possibilities, but we just don't know. So for many people, something happened early in life to set their sensing mechanism to demand more fat on their bodies? What would you tell someone who wanted to lose weight? I would have them eat a lower-calorie diet. They should eat whatever they normally eat, but eat less. You must carefully measure this. Eat as little as you can get away with, and try to exercise more. There is no magic diet, or even a moderately preferred diet? Some diets are better or worse for medical reasons, but not for weight control. People come up with new diets all the time - like, why not eat pistachios at midnight when the moon is full? We have gone through so many of these diet possibilities. And yet people are always coming up to me with another one.