Bigger, heavier, more muscular people generally have higher metabolic rates. But while you can’t get taller and don’t want to get fatter, getting more muscular is both desirable and well within reach, thanks to the power of exercise.

"Fat is inactive tissue, where muscle is a more metabolically active tissue that bums more calories," says Chris Rosenbloom. "One of the big pluses of exercise is you can put on more muscle mass and so you can burn more calories throughout the day."

Even at rest, muscle tissue is busy burning calories. Studies have found that increases in muscle mass from as few as three months of a moderate weight-training or resistance-training program can boost your resting metabolic rate. And even a small boost of your resting metabolic rate can sig¬nificantly increase the rate at which calories are burned during activity.

Physical activity and exercise are the keys to the greatest change in energy metabolism by far. In just a few seconds of intense activity, muscles will use an amazing amount of energy.

Sixty min¬utes of brisk walking or slow jogging can increase your energy expenditure ten-fold for that hour. High-intensity aerobic interval training has a major effect on your calorie-burning rate.
If that’s not enough to convince you, there’s a bonus called afterburn: a metabolic rate that stays elevated for as long as several hours after moder¬ate to vigorous exercise.

(Note: Afterburn follow¬ing aerobic exercise does not last as long as afterburn following resistance training.)

Now What Is Metabolism? 

Every minute we live and breathe, chemical reacations are going on in every cell of the body. Some of them use energy, others release it. The rate at which the body uses energy is what we mean when we talk about metabolism.

That energy is measured in what scientists call kilocalories, and what we call calories. In other words, calories are not evil little things con¬tained in the foods we eat, but good measures of how much energy it takes to process the food.

The minimum amount of energy (or calories) you need to keep functioning when you’re awake Illness and infection. The work of fighting infec¬tion and illness can increase resting energy expen¬diture by 10 to 40 percent.

"When you run a fever, your basal metabolic rate will increase by 15 percent for each degree Celsius rise in body temperature.

- Pregnancy. Having a baby is a lot of work. The energy cost of pregnancy has been estimated at an average of 300 calories per day.

- Eating and digestion. Breaking down, digesting, and absorbing food takes energy. A healthy break¬fast can temporarily boost resting metabolism by 10 percent or more.

- Gender and age. Women, who have more built-in body fat and less lean tissue than men, tend to have metabolisms 5 to 10 percent lower. The elderly, with decreasing levels of muscle mass, due in part to lower levels of activity, will experience a steady decline in metabolic rate.