Does metabolism matter in weight loss?

Do you know people who complain about having a slow metabolism and how they barely eat anything yet still gain weight? Or have you met people who complain about someone they know who can eat whatever he or she wants — including large portions of junk food — due to a fast metabolism and apparently never gain weight? In both cases the individual usually ends by saying, "It's not fair!" These scenarios raise several very good questions:

• What role exactly does metabolism play in weight gain or weight loss?
• Is your metabolic rate determined by your genes? If so, can you speed up a slow metabolism through exercise, drugs, or certain foods?
• Is the importance of metabolism just a myth? Is weight gain or loss purely due to "calories in and calories out?"

The answer to these questions involves a mix of nature (genetic make-up) and nurture (the environment).

What Is metabolism?

Metabolism, or metabolic rate, is defined as the series of chemical reactions in a living organism that create and break down energy necessary for life. More simply, it's the rate at which your body expends energy or burns calories.

Our bodies burn calories in several ways:

• through the energy required to keep the body functioning at rest; this is known as your basal metabolic rate (BMR). Your BMR is partly determined by the genes you inherit.
• through everyday activities
• through exercise.

Metabolism is partly genetic and largely outside of one's control. Changing it is a matter of considerable debate. Some people are just lucky. They inherited genes that promote a faster metabolism and can eat more than others without gaining weight. Others are not so lucky and end up with a slow metabolism.

One way to think about metabolism is to view your body as a car engine that is always running. When you're sitting still or sleeping, you're engine is idling like a car at a stop light. A certain amount of energy is being burned just to keep the engine running. Of course, for humans, the fuel source is not gasoline. It's the calories found in foods we eat and beverages we drink — energy that may be used right away or stored (especially in the form of fat) for use later.

How fast your body's "engine" runs on average, over time, determines how many calories you burn. If your metabolism is high (or fast), you will burn more calories at rest and during activity. A high metabolism means you'll need to take in more calories to maintain your weight. That's one reason why some people can eat more than others without gaining weight. A person with a low (or slow) metabolism will burn fewer calories at rest and during activity, and therefore has to eat less to avoid becoming overweight.

Weight control: Part nature, part nurture

It's part truth and part myth that metabolism is the key to weight. The rising tide of obesity in this country cannot be blamed entirely on an inherited tendency to have a slow metabolism. Genes do not change that quickly. Something environmental — particularly changes in diet and exercising too little — is a much more likely culprit. Age can be a factor, too, although new evidence suggests metabolism reaches a peak earlier in life and slows down much later than previously thought.

The reality is that for most people, excess weight is not all due to bad luck, thyroid trouble, or some other unexplained, uncontrollable external factor. For most of us, calories in/calories out has a strong influence on changes in weight over a lifetime.

Regardless of whether your metabolism is fast or slow, our bodies are designed to store excess energy in fat cells. So, if you eat and drink more calories (energy intake) than your body expends (energy output), you will gain weight. On the other hand, if you eat and drink fewer calories than are burned through everyday activities (including exercise, rest, and sleep), you'll lose weight. Our bodies are also programmed to sense a lack of food as starvation. In response, our BMR slows down, which means fewer calories burned over time. That's one reason why losing weight is often difficult.

Perhaps the most remarkable thing about all of this is how little our weight tends to change from day to day. In fact, only a few excess calories each day could lead to significant weight gain at the end of a year. For example, eating an extra apple each day could lead to a weight gain of nearly 9 pounds by the end of one year! Similarly, even a small reduction in calories each day could lead to remarkable weight loss. Eliminating dessert one day a week would lead to weight loss of nearly six pounds in a year.

Many theories exist to explain what controls the amount of food a person eats, when they feel full, and why they eat past the point of feeling full. These factors also play a role in determining one's ultimate weight. One theory is that each of us has a set point — a weight at which the body is "happy." If you lose weight, you'll feel hungry until you get back to your set point weight. That may be another reason it is so hard to lose excess weight. But how that set point is determined and whether there truly is such a mechanism remain uncertain.

The bottom line

When it comes to weight, metabolism is important and does have a genetic component. Whether you can change your metabolic rate, however, is a matter of considerable debate. Clearly, you can change how you balance the calories you take in against the calories you burn up through activity, which can change your weight.

Image: Alexey Yakovenko/Getty Images