Q: I seem to gain weight so easily and lose it so slowly. Could I have a slow metabolism? Or thyroid problems?
A: It's possible, but let's start by defining our terms. The word "metabolism" means the speed at which your body burns calories. Even at rest, your body uses up some calories to maintain your body temperature and keep your brain working, your heart beating, etc. When you start moving around, your metabolism speeds up to keep pace with your muscles.
The thyroid gland—a small organ at the base of your neck—produces hormones that influence your metabolism. When these hormones are in short supply, you have a condition called hypothyroidism. Fatigue, depression, dry skin, coarse and thinning hair and intolerance to cold can all be symptoms. However, there are several reasons why some people's metabolisms are slower than others, and thyroid problems are not usually to blame. Still, with some simple blood tests, your doctor can easily check your thyroid function and correct it, if necessary.
So what does account for a slow metabolism? Genetic factors are often at work here, so some people naturally have a more sluggish metabolism than others. And our metabolic rate tends to fall as we age, partly because of decreased activity.
Diets can lower your metabolism too. Very low-calorie diets—which typically restrict calories to 800 or fewer per day—slow your metabolic rate almost immediately and keep it low for weeks after the diet ends. These diets send your body into a starvation mode, in which your cells try to conserve energy as much as possible by holding on to calories.
Q: Can I change my metabolism?
A: Yes, at least short-term. To measure your metabolic rate, doctors use equipment that records how quickly your body uses oxygen and produces carbon dioxide. Both are indicators of how quickly your cells consume fuel—that is, burn calories. Using these techniques, we're able to see how various factors alter metabolism.
Here are the keys to faster calorie burning:
1. By far, the biggest increase in metabolism comes with physical activity, particularly aerobic exercise such as brisk walking, running, dancing or singles tennis. Your calorie burn is dramatically higher than at rest, and the effect may persist briefly after you stop.
2. Weight training helps, too, because it increases—or at least maintains—your muscle mass. Muscles burn calories more quickly than most other body tissues.
3. If you limit your calories, be careful. Even if you're eager to lose weight, very low-calorie diets tend to slow down your metabolism. Here's my rule of thumb: Take your ideal body weight, and multiply it by 10. This gives you your calorie minimum. For example, if I would like to weigh 150 pounds, my calorie minimum would be 1,500. I can eat more than this, but if I'm restricting calories, I cannot go below this number. If I do, I risk slowing down my metabolism.
4. Increase your after-meal burn. Your body has to work to digest the food you've just eaten, and, in the process, it burns some calories as body heat. This after-meal burn is sometimes called the thermic effect of food. It turns out that vegetarians seem to have a better after-meal burn than meat eaters do, as we here at the Physicians Committee for Responsible Medicine observed in lab experiments done in 2000 and 2001 at George Washington University.
We measured the after-meal burn in 59 overweight, postmenopausal women consuming the same liquid breakfast drink. Then half of the group went on a low-fat, vegetarian diet, while the others went on a low-fat diet that included modest amounts of animal products.
After 14 weeks, we brought each participant back into the lab and checked her after-meal calorie burn again. It turned out that members of the vegetarian group had significantly increased their after-meal burn while the others had not. We measured the effect for about three hours after the meal, but it likely persists a bit longer than that.
Here's at least part of what seems to be going on: The vegan diet improves insulin sensitivity, which means it's easier to turn the nutrients you eat into energy. Because the after-meal burn comes largely from carbohydrates, not fatty foods, vegetarian diets are ideal. They are generally rich in complex carbs and low in fat, so they generate a better after-meal burn naturally, which may be part of why most people lose weight when they go vegetarian.
Now, the after-meal burn is small, totaling about 10 percent of your calorie expenditure (about 200 calories each day). However, small changes can add up, so it may well be that plant-based diets help with weight loss because they are lower in calories to begin with, and they may also increase metabolism after meals.
So, yes, you can rev up your metabolism and burn calories faster. By using aerobic exercise and weight training, maintaining an adequate calorie intake and increasing your after-meal burn, you will give yourself an edge.