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 have shown 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 increases.

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

"Metabolism" refers to your body's systems for using or converting energy.  This includes breathing, digestion, the building of muscle, and the storage of fat, circulation of blood -- anything that's part of your body's systems for converting energy into an activity you need to live. Metabolism involves two distinct types of processes: "anabolic" reactions, which involve building cellular structures and the storing of energy; and "catabolic" reactions, which involve breaking down molecules for energy.

Metabolism is often confused with metabolic rate, and the two terms are often used interchangeably. More accurately, "metabolic rate" is the speed of your metabolism, or the rate at which you burn calories.

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."

Your metabolism and metabolic rate are determined by a number of factors.  Genes play an extremely important role; they're one reason why some people can remain thin no matter how much they eat, while others have a tougher time shedding pounds. Lifestyle -- particularly diet and exercise -- can also play an important role. No matter what sort of metabolism you were born with, or how well you have been maintaining it, it's likely to slow down as you grow older.

Your thyroid regulates your metabolism, producing hormones that influence 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 indeed help you lose weight by changing your metabolic rate. But there’s disagreement about how this works.

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 results in a metabolic rate that begins declining as early as your twenties -- about 2% per decade.

Diets that are too severe can prove self-defeating, in part because of their effect on metabolic rate, and dangerous. Studies have shown that people who eat less than 1,200 calories per day are likely to wind up with a slower metabolic rate -- which, ironically, 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. Plus, eating helps boost calorie burn as the body works to digest foods.

Caffeine is a stimulant -- and so by definition, it increases 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 period of time.

Some spicy foods can indeed raise your metabolic rate. In particular, hot peppers containing capsaicin are particularly stimulating to a metabolic process called thermogenesis, in which the body produces heat, thereby burning calories. But this effect is unlikely to be strong enough to help you lose weight. If that’s your goal, head to the gym -- not the spice rack.

Many dietary supplements claim they can help you lose weight by boosting your metabolism. Buyer beware. Ephedra, a stimulant that can increase your blood pressure and heart rate, was banned by the FDA in 2004 because of mounting evidence of potentially fatal dangers. And other studies have found little evidence to support the claim 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 is forced to speed up to keep your body warm.  And in hot weather, your metabolism has to speed up to keep you cool. It has been estimated that people living in tropical climates have a resting metabolic rate that is 5% to 20% higher than people living in more temperate regions.

You burn more calories just maintaining your body's vital processes than you do through physical exertion. In fact, 65% to 75% of the calories you burn in a day are burned by your metabolism in the process of maintaining your body's basic vital processes. Physical activity contributes up to 30% of your total daily calorie burn and is still one of the best ways to boost your metabolic rate.