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 answers to these questions involves a mix of nature (genetic makeup) 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. 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, your 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. It's also why people with an overactive thyroid gland - a condition that speeds up metabolism - tend to lose weight unintentionally.

Meanwhile, 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 gaining excess weight.

This raises the question of whether you can increase your metabolism to lose excess weight or avoid gaining it in the first place. While there are ways to "speed up" your metabolism (such as high-intensity exercise workouts, increasing muscle mass, and perhaps through diet), it's usually quite challenging to meaningfully alter your metabolic rate.

Did you know...?

Lean people tend to be more active during everyday activities than people who are overweight. How? They may "fidget" more - that is, they tend to be in motion even when engaged in non-exercise activities. Whether this tendency to move more or less is genetically programmed or learned remains uncertain. But it can add or subtract hundreds of calories each day.

Obese people expend more calories, on average, than lean people during most activities, in part because it takes more effort to move around. But they tend to be more sedentary, which makes it harder to get rid of body fat.

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 - are much more likely culprits.

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'll likely 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 probably 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 excess 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 a year. Similarly, even a small reduction in calories each day could lead to remarkable weight loss: eliminating dessert one day a week could 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 hungry or 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, how it may be altered, and whether there truly is such a mechanism remain uncertain.

The bottom line

When it comes to weight, metabolism is important. But it has a major genetic component, and changing it is rarely easy and not always effective. If you're trying to avoid or lose excess weight, a more reliable first step is to improve the types of foods you eat (which can affect how hungry you feel) and to increase your level of physical activity.

There are more options to manage excess weight than ever before, so while your metabolism may be playing a role, ask your doctor what other factors might be at play and what treatments are best for you.

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