The Truth About Metabolism

Your metabolic rate is strongly influenced by your body composition. People with more muscle and less fat generally have a faster metabolic rate, while people with more fat and less muscle generally have a slower metabolic rate.

Studies show that the body requires more energy to process a high-protein diet, which means that as your body digests high-protein foods, your metabolic rate rises.

Men tend to have a higher metabolic rate than women because women have a higher proportion of fat.

"Metabolism" is how your body converts or uses energy for activities such as breathing, digestion, building muscle, storing fat, and circulating blood -- things you need to do to live.

Your "metabolic rate" is the rate at which you burn calories. It's not the same as your metabolism.

Strictly speaking, "metabolic rate" is the rate at which you burn calories while your body is at rest -- something that is more properly referred to as your "resting metabolic rate," or, measured a slightly different way, your "basal metabolic rate."

Several things affect your metabolism and metabolic rate. Genes play a very important role. They're one reason that some people stay thin no matter how much they eat, while others find it harder to lose weight. Your lifestyle, particularly diet and exercise, also matters. So does your age: Your metabolism is likely to slow down as you get older.

Your thyroid regulates your metabolism. It does that by making hormones that affect almost every aspect of how your body performs: how fast or slow you burn calories, when or whether you build proteins or store energy as fat, and how your body responds to other hormones.

Not surprisingly, disorders of the thyroid can cause problems for your metabolism. An underactive thyroid ("hypothyroidism") produces fewer-than-normal thyroid hormones, causing a slower metabolism, and, consequently often causes weight gain. An overactive thyroid ("hyperthyroidism"), on the other hand, can cause the metabolism to speed up, often resulting in weight loss.

Strength training can help you lose weight by changing your metabolic rate. Exactly how that happens isn't fully understood.

Some claim that by building muscle and trimming fat, strength training boosts your resting metabolic rate, helping you burn more calories just sitting still.  But according to the American College of Sports Medicine, for most people, this won’t result in much weight loss. Instead, they say the main weight loss benefit is due to its effect on calories burned while doing strength training, or as a direct result, as your body burns calories building muscle.

As you age, you tend to lose muscle. This makes your metabolic rate start slowing as early as your 20s by about 2% per decade.

Diets backfire if you cut calories too much. Studies show that people who eat less than 1,200 calories per day tend to have a slower metabolic rate -- which can make it more difficult to burn calories and lose weight. Even going too long between meals can slow metabolic rate. That’s why experts recommend eating a small healthy meal or snack every 2 to 3 hours. 

Caffeine is a stimulant, so it raises your metabolic rate. This is why it is often an ingredient in weight-loss diet supplements. Studies have shown that one cup of American coffee can increase metabolism by about 3% to 4% for a short time.

Some spicy foods can raise your metabolic rate. In particular, the capsaicin in hot peppers can help burn calories, but probably not enough to help you lose weight. 

Many dietary supplements claim they can help you lose weight by boosting your metabolism. Buyer beware. Ephedra, a stimulant, is banned by the FDA due to serious health risks. Also, there's little evidence that other metabolism booster supplements such as L-carnitine or bitter orange can help you lose weight. 

Most doctors, nutritionists, and fitness trainers recommend that you boost your metabolism the old-fashioned way: through diet and exercise.

Your body has to work hard to maintain its optimal natural temperature. In cold weather, your metabolism must speed up to keep your body warm. And in hot weather, your metabolism has to speed up to keep you cool. People living in tropical climates have a resting metabolic rate that's 5% to 20% higher than people living in more temperate regions, estimates show.

Your body burns more calories to keep you alive than for physical activity. Still, being active can torch up to 30% of the calories you burn in a day, and it's one of the best ways to boost your metabolic rate.