Where does weight go?

DEAR DR. ROACH: When you lose weight through diet and exercise, where does the weight go? -- L.A.J.

ANSWER: Weight gain and weight loss in the very short term can be related to body fluid, but I think you are really talking about people who have made a long-term loss of, say, 10 pounds. Those 10 pounds can be fat or muscle. In people who are eating very few calories and not exercising, it will be a combination of both of these body components. Unfortunately, losing muscle is unhealthy, which is why it’s best to exercise in combination with changing the diet, so that the weight loss is all fat. I’ve had patients who have lost fat and gained muscle, who can be frustrated that their weight isn’t going down, but in reality they are much, much healthier.

From a physics standpoint, a pound of fat represents about 3,500 calories. Eating 3,500 more calories than you expend means a pound of fat gain, while the reverse means a pound of weight loss. In weight loss, the fat is used to provide energy for the body’s metabolic processes and is (almost literally) burned away into carbon dioxide.

However, the physics doesn’t properly address the complexity of weight gain and loss in humans. Simply eating less and exercising more doesn’t lead to the expected weight loss, as the body has ways of adapting to lower food intake. Also, as people lose weight, their energy needs decrease further, and the “3,500 calories per pound” rule becomes a very poor approximation. In some people, the body uses every trick it can, including overpowering hunger, to stay at the same weight. For these reasons, I don’t find simply telling people to eat less and move more to always be an effective therapy.

DEAR DR. ROACH: I have prostate cancer. He has given me two options: One is to have radioactive seeds implanted, and the other is to freeze the cancer. Which of these has the lower chance of erectile difficulty? -- H.K.E.

ANSWER: Radiation seed implants (called brachytherapy) and freezing (cryotherapy) are considered reasonable options for some men with localized prostate cancer. There probably are differences in effectiveness and in other side effects, which may make you consider how these treatments affect you and the cancer.

However, in terms of sexual, my understanding of the literature suggests that brachytherapy has less risk of poor function.