If your body were a machine, like a car, metabolism would be everything happening in the engine as it runs. Human metabolism is more complex. It's the set of biochemical processes that keeps us alive. Your metabolic rate is the rate at which your body uses energy, measured as the number of calories you burn over a given amount of time. It is made up of three components:
1. Calories needed for basic bodily functions, such as breathing, blood circulation (including the pumping of the heart), maintaining body temperature, and just existing. This is your basal metabolic rate (BMR). In most people, this accounts for 50 to 60 percent of calories expended.
2. Calories needed for eating, digesting, absorbing, and storing food. This is called the thermic effect of food (TEF). This uses another 10 to 15 percent of daily calories.
3. Calories you use for physical activities. Called activity-related energy expenditure, this accounts for the rest of the calories you burn.
Your metabolic rate increases to process what you eat and, of course, to provide the power for you to move around. These increases vary from person to person.
What many of us yearn to know is how we can boost our BMR so we’ll burn more calories faster, thus (perhaps) compensating for overconsumption of food and under-exertion.
Metabolic rates: some complexities
Your gender, age, hormone levels, and body size and composition all affect your metabolic rate. Women tend to have lower BMRs than men, and older people lower rates than younger. In part that’s because muscle burns more calories than fatty tissue does, and thus helps boost BMR.
Climate also affects BMR, which rises in hot temperatures, though sometimes also in cold. Fever increases it, while fasting and malnutrition reduce it. Your genes also help determine your metabolic rate.
Another factor: People who fidget or move around a lot unconsciously (this seems to be genetically controlled, in large part) tend to have a higher metabolic rate. It takes calories to fidget, so fidgeters may be leaner. This seems unfair—if you are not a fidgeter, you can hardly start training to be one.
Doomed to be fat?
Many people blame their low BMR for obesity, but that’s not all there is to it. A low BMR may predispose people to weight gain, of course. But BMR is usually higher in obese people, because they need more calories to fuel their larger bodies (they tend to be more sedentary, however, and thus expend less energy in activity).
Ironically, if you weigh 200 pounds and diet down to 150, your BMR will be lower than that of a person who has weighed 150 all along. Thus, it will be easier for you to gain your weight back. Another example of "no fair!"
Beating the system: is protein the way?
Though promoters of the Atkins and South Beach diets would say yes, the answer is not clear. It's true that it takes your body a little more energy to process protein than carbs. But that doesn't mean you'll lose weight on a high-protein diet—it depends, ultimately, on how many calories you consume and how many you burn. Eating more protein has a very small effect on your metabolic rate.
Other factors that are supposed to increase metabolic rate include (1) eating frequent small meals instead of two or three larger ones, (2) green tea supplements, and (3) getting enough sleep. The "grazing" theory has never proved out for weight loss—studies are few, and most have found no significant boost in metabolic rate. Green tea extract, often promoted as a weight-loss aid, might help in theory because of certain phytochemicals (including caffeine) in tea, but studies have yielded inconsistent results, and any such effect is likely to be small and brief. Several studies have linked inadequate sleep with weight gain and, possibly, reduced metabolic rate, but the effect of sleep loss has not been directly measured. Obviously, if you're exhausted from lack of sleep, you're less likely to exercise.
Parting thoughts: There is, alas, no neat trick for boosting your basal metabolic rate. But it's easy to pump up another part of the metabolic equation—your activity-related energy expenditure—simply by exercising more. It's sometimes claimed that strength training can greatly increase BMR. But average gym-goers don't build enough muscle to achieve a meaningful calorie-burning after-effect. More significant are the calories you burn during the workout. Aerobic exercise such as running, biking, swimming, or brisk walking burns more calories than strength training and is more likely to significantly boost your metabolic rate afterwards—at least briefly. A balanced workout should include both aerobic and strength-building activities.