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:
The answer to these questions involves a mix of nature (genetic make-up) and nurture (the environment).
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:
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.
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 — eating too much and exercising too little — must be to blame.
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. It's simple accounting involving calories in and calories out that determines changes in weight over a lifetime.
Regardless of how fast or slow your metabolism is, 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. Our BMRs slow down, which mean 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 would 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 6 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.
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.
Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.