Читать книгу Nutrition For Dummies - Carol Ann Rinzler - Страница 73

A gland is an organ that secretes hormones — chemical substances that can change the function and sometimes the structure of other body parts. Hormones secreted by three glands — the pituitary, the thyroid, and the adrenals — influence how much energy you use when your body’s at rest.

Your pituitary gland, a small structure in the center of your brain, stimulates your thyroid gland (which sits at the front of your throat) to secrete hormones that impact the rate at which your tissues burn nutrients to produce energy.

When your thyroid gland doesn’t secrete enough hormones (a condition known as hypothyroidism), you burn food more slowly, and your REE drops. When your thyroid secretes excess amounts of hormones (a condition known as hyperthyroidism), you burn food faster, and your REE is higher.

When you’re frightened or excited, your adrenal glands (two small glands, one on top of each kidney) release adrenaline, the hormone that serves as your body’s wake-up call. When you release adrenaline, your heartbeat increases. You breathe faster. Your muscles clench. And you burn food faster, converting it as fast as possible to the energy you need for the reaction commonly known as fight or flight. But these effects are temporary. The effects of your sex glands, on the other hand, last as long as you live.

If you’re a woman, you know that your appetite may rise and fall in tune with your menstrual cycle. In fact, this fluctuation parallels what’s happening to your REE, which goes up just before or at the time of ovulation. Your appetite is highest when menstrual bleeding starts and then falls sharply. Being a man (and making lots of testosterone) makes satisfying your nutritional needs on a normal American diet easier. Your male bones are naturally denser, so you’re less dependent on dietary or supplemental calcium to prevent osteoporosis (severe loss of bone tissue) later in life. You don’t lose blood through menstruation, so you need only less than half as much iron (8 milligrams for an adult male; 18 milligrams for a premenopausal woman who is neither pregnant nor nursing). Best of all, you can consume about 10 percent more calories than a woman of the same weight without adding pounds.

It is no accident that while teenage boys are developing wide shoulders and biceps, teenage girls are getting hips. Testosterone, the male hormone, promotes the growth of muscle and bone. Estrogen gives you fatty tissue. As a result, the average male body has proportionally more muscle; the average female body, proportionally more fat.

Muscle is active tissue. It expands and contracts. It works. And when a muscle works, it uses more energy than fat (which insulates the body and provides a source of stored energy but does not move an inch on its own). What this muscle versus fat battle means is that the average man’s REE is about 10 percent higher than the average woman’s. In practical terms, that means a 140-pound man can hold his weight steady while eating about 10 percent more than a 140-pound woman who is the same age and performs the same amount of physical work.

No amount of dieting changes this unfair situation. A woman who exercises strenuously may reduce her body fat so dramatically that she no longer menstruates, an occupational hazard for some professional athletes. But she’ll still have proportionately more body fat than an adult man of the same weight. If she eats what he does, and they perform the same amount of physical work, she still must take in fewer calories than he to hold her weight steady.