We hear a lot about metabolism, and we often blame a ‘slow metabolism’ for our inability to keep our weight under control. Find out what factors affect your metabolic rate and what can you do to change it.
When patients tell me their weight problems are due to a “slow metabolism,” I think they truly believe that their bodies simply burn calories at a slower pace than other people’s bodies. In their minds if they only could speed up the process, their weight problems would be solved. But what is metabolism, exactly? And, more importantly, is there anything you can do to change it?
In truth, your body weight and your metabolic rate are linked, but perhaps not in the way you might think. Simply stated, the term metabolism refers to all the chemical processes that your body undergoes every day in order to keep you alive. When your body converts the calories in your food into energy, or manufactures specialized chemicals that your cells need in order to do their job, those are metabolic processes. These hundreds of processes your body performs every day make up your metabolism.
I can see how the term metabolic rate could confuse people. When you hear the word “rate,” you might think “speed” or “tempo.” So, it’s reasonable to assume that metabolic rate can only mean how fast (or slow) your body performs its work.
In reality, though, your metabolic rate—or, more accurately, your resting metabolic rate—refers to the number of calories you burn during a 24-hour period just to keep your body’s most basic processes going. These are processes like pumping blood or breathing.
Of course, this isn’t all the calories you burn in a day, but your resting metabolic rate accounts for a sizable chunk. About 75% of total calories you use up every day are used simply to keep your body ticking. Most of the remaining calories get used up during your daily activities and bouts of exercise.
Some people’s bodies use up more calories to perform these basic metabolic processes than others. You might think they have a “fast” metabolism. And for those who seem to require very few calories, you might think their metabolism is “slow.” But now that you know that your metabolic rate isn’t really about how quickly you burn calories (it’s really the number of calories you burn each day), you can’t technically make your body burn calories any faster.
Let’s look at what affects your metabolic rate in the first place: it will give you a better sense of what you can and can’t do to change it.
Larger people have higher metabolic rates than smaller people do, and this is due largely to the simple fact that they just have more cells—each of which is doing some metabolic work. That’s one reason that men usually have higher metabolic rates then women; they tend to have bigger bodies overall.
A very important factor in determining your metabolic rate is the amount of lean body mass you have. Imagine that your body is divided into two parts: one part is your fat, and the other part is your lean body mass. In other words, everything else that isn’t fat like bone, fluids, organs and muscles. This lean body mass determines your metabolic rate, because every pound of lean mass you have burns about 14 calories per day, or about 30 calories per kilogram. A pound of fat only uses up about two. Muscle cells have a lot more machinery that converts calories into energy than your fat cells do. So, it makes sense that as your muscle mass increases so would your metabolic rate, since, metabolically speaking, your muscle cells are very active.
As you age, there is a tendency to lose some muscle mass. There are a couple of reasons this happens. For one thing, natural hormonal changes can contribute to some loss of muscle mass. And the muscle damage that results from everyday wear and tear isn’t repaired quite as quickly as you age. And that can contribute to some muscle loss, too. With fewer muscle cells overall, you can’t help but burn fewer calories over the course of the day.
Men have higher rates than women do for two simple reasons. They tend to be larger overall, and they tend to have more muscle mass than women do.
It’s true that when you cut your calorie intake too much your metabolic rate can take a dip. This makes sense if you think about it: your body is just trying to do the same metabolic work with fewer calories in order to keep you alive. In general, these decreases are relatively small, especially if you make modest—rather than dramatic—decreases in your calorie intake as you attempt to lose.
Now that you know what factors affect your metabolic rate, what can you do about it?