Q: I'm disturbed by a report I heard on the radio. The announcer said a study just published shows that fatness is inherited. My father is fat. Does this mean I'm doomed to be fat as well?

A: Hold on. Things aren't as bad as you might think. Let's see what that study, published in the January 23, 2003, issue of The New England Journal of Medicine, actually found.

The researchers compared a sample of 540 Danish adoptees with their natural parents and their adoptive parents. They found a strong correlation between the fatness of the adoptees and their biological parents, but no relation between the fatness of the adoptees and their adoptive parents. The researchers concluded: "Genetic influences have an important role in determining human fatness in adults, whereas the family environment alone has no apparent effect."

In one way the results of this study are not surprising. An earlier study of twins separated at birth showed that fatness is strongly influenced by inheritance. Moreover, research has shown that 80% of the offspring of two obese parents become obese, as compared with no more than 14% of the offspring of two parents of normal weight. The surprising finding was that, as far as researchers could tell, the adoptive family seemed to have no impact at all.

That's the bad news. The good news is that the researchers cautioned against reading too much into their results. "These findings do not mean that fatness, including obesity, is determined at conception and that, as is the case of determination of eye color, the environment has no effect," they wrote. "We do not know, for example," they continued, "how genetic predisposition to fatness may be affected by environmental factors."

In other words, this study doesn't mean that there is nothing you can do to counteract an inherited tendency to be fat. As the study's director, Dr. Albert J. Stuckard of the University of Pennsylvania, noted in a subsequent interview, lots of people with two fat parents (the ones who probably face the highest genetic risk of obesity)take weight off and keep it off. The study simply confirms that some people have to work harder than others to stay lean.

If one of your parents died at an early age from a heart attack, you wouldn't close your eyes to the controllable risk factors (smoking, high-fat diet, etc.) in heart disease. On the contrary, because heart disease appears to run in your family, you'd do everything possible to prevent having a heart attack yourself. By the same token, if you believe you have a genetic tendency to be fat, you should redouble your efforts to avoid it. If you work at it, chances are that you can become lean and stay lean; overcoming any tendency you may have toward fatness.

To show there's hope for those with fat genes, Dr. Stuckard and his fellow researchers cite a study involving genetically obese mice. Simply increasing activity, with no dietary intervention, prevented obesity in 50% of the mice, and greatly limited it in the other half.

Actually, this adoption study may merely demonstrate that body mechanisms which allowed us to survive in former times work against us in a modern world where there's a McDonald's on every corner and our cupboards are almost never bare. The fat our ancestors put on in the summer when food was comparatively plentiful was burned off during the lean winter months. Back then, those with the genetic tendency to store fat were the ones that survived the hard times when food was scarce. The others perished. Today, with an abundance of high-calorie food and machines to do our work, our beautiful adaptive mechanisms simply make us fat. It's the same phenomenon that causes animals, who stay extremely lean in the wild, to become obese in captivity and many professional athletes to blow up like balloons after they retire.

As you have probably guessed by now, one of the most effective things you can do to avoid becoming like your father is to stay physically active. As suggested by the experiment involving obese mice, the weight of people who begin to exercise regularly usually drops even if they make no attempt to diet. How lean you are depends to a large extent on the amount of exercise you do. Perhaps the best examples are endurance athletes: marathon runners, cross-country skiers and competitive cyclists. They are usually lean even when they eat whatever they want.

Bodybuilders also have an advantage in terms of the ability to stay lean. That's because your muscles burn most of the food you eat. Your muscles burn calories even when you're asleep. In addition to the fact that they burn extra calories lifting weights, bodybuilders find it easier to stay lean because they have developed more calorie-burning muscle tissue. In contrast, people who are inactive have less muscle tissue and therefore burn fewer calories and have a greater tendency to get fat. If you've ever had an arm or a leg placed in a cast, you know how inactivity causes muscles to atrophy.

The kind of food you eat also affects how well you control a tendency to get fat. We seem to have an inclination to pig out on high-calorie, high-fat, high-sugar foods. Again, in the days when famines were common, this tendency protected us against starvation. The best way to counteract it today is to eat plenty of whole grains, fruits and vegetables, and steer clear of refined and fatty foods. Bulky whole grains, fruits and vegetables provide lots of chewing, tasting and stomach-filling satisfaction without giving you too many calories. Refined and fatty foods, on the other hand, pack calories into a small volume and give you too many calories long before you've eaten your fill.

So to make a long story short, some people are born with the right genes for our modern world of inactivity and fast foods and don't have to worry about their weight. Others (you may be one of them) get fat if they don't exercise and eat correctly. Nevertheless, the bottom line is that most people with fat genes can control their tendency to be overweight.

Now, stop worrying and go work out.