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Extra pounds are nothing to make light of. Just ask the 29 million Americans who became overweight overnight last June, when the government revised medical guidelines. "It is a problem—we're the fattest nation in the world," says the author of the guidelines, Dr. Xavier Pi-Sunyer, 64, chairman of the National Heart, Lung and Blood Institute task force of the National Institutes of Health. The new guidelines use a ratio of weight to height to determine a person's Body Mass Index (BMI)—divide your weight in pounds by your height in inches squared, then multiply by 704.5. Under the old guidelines, a BMI of 27.8 or higher for men and 27.3 or higher for women indicated a weight problem. The new rules lower the cutoff to 25 for both sexes. Why the change? After consulting health experts and reviewing exhaustive studies, "the task force found that there is a higher risk for illness beginning at a BMI of 25," explains Pi-Sunyer, citing as an example the increased likelihood that an overweight person has of developing diabetes. Still, not everyone is convinced. Judith S. Stern, professor of nutrition at the University of California at Davis and a member of the American Obesity Association, says that health risks aren't serious until one reaches a BMI of 27 and that the new charts fail to take into consideration muscle mass, which is heavy but not unhealthy. Personally, Pi-Sunyer, a six-foot Manhattan resident who is married (to Penelope, a social worker) and the father of three grown children, weighs in at 168 pounds and keeps his. BMI at about 23 by going light on snacks and exercising daily. He even walks 28 blocks to and from his office. "That's a 40-minute total walk, and walking is a good source of energy expenditure," he says. Pi-Sunyer spoke to PEOPLE contributor Frances Hong about the obesity guidelines.

What should someone do if he or she has a high BMI?

Those between 25 and 30 BMI should avoid gaining any more weight, and try to keep closer to 25. If the BMI is 30 or higher, the patient needs to try to lose 10 percent of his or her present weight—and not through a crash diet, which can be very hard on one's body. The safe amount of time that it takes to lose that 10 percent is six months, or no more than 4 pounds a month.

What are the potential health problems of being overweight?

Part of the problem is that Americans don't always associate obesity with health problems. But obesity can cause gall bladder problems, pulmonary osteoarthritis, heart disease, diabetes, hypertension and certain kinds of cancer. We're moving people into the obese class of 30-plus BMI at an astounding rate of 1 percent a year.

How can doctors help?

Being overweight is no different from having high cholesterol or high blood pressure. Doctors should tell patients that obesity is a risk factor for disease and convince them to get more exercise and eat less. People can add a significant number of years to their lives if their BMI remains under 25.

But critics say BMI fails to take muscle weight into consideration.

Yes, they say that people over 25 BMI are not "overfat" but "over-muscular." But athletic people who are very muscular represent a small percentage of the population. They also train and see their physicians regularly. But the huge majority of Americans are sedentary, and normally they gain half a pound to 1 pound each year from age 20 to age 60.

How much fatter is the average American now than, say, 20 or 40 years ago?

In a 1962 health examination survey by the government, for the 30-plus BMI group, 10 percent of men and 15 percent of women were obese. Compare that with 1994, when almost 20 percent of men and 25 percent of women hit that mark.

Why do Americans have weight problems?

Forty percent of the problem of obesity is genetic and 60 percent is environment. The environment comes with the availability and palatability of food—it's tasty and cheap. Generally we eat too much, and the portions are too large. Also, we eat all the time. We're a snacking nation. If you go around the world, you'll see that a lot of nations don't eat between meals, have less fast food available and often treat meals as a social affair.

Are there healthy snacks?

Apples, pears and oranges are good and provide vitamin A and carotene; potato sticks, pretzels and carrots are also good. Actually it is wrong to talk about good foods versus bad foods. Cheese, for example, is a good food and has a lot of calcium, which you need. Most foods, in fact, are good for you. But often we just need to cut back on portions. A little chocolate won't hurt you, but a pound of chocolate in one day is certainly bad.

Is dieting enough?

No. Americans are incredibly sedentary. We drive everywhere; we take the escalators, not the stairs; we have beautiful lawns, but we hire someone else to mow them. Today, 2 percent of Americans are farmers and the rest sit behind desks all day long. I recommend 30 to 40 minutes a day of walking or exercise. The very minimum that people at any age should do is walk.

How does the U.S. population compare with others?

Traditionally, most of the Asian nations are much leaner—Indonesia, Thailand, India, China and Japan, to name a few. They tend to have low-fat diets that are high in carbohydrates, particularly rice. Their populations also tend to be more active. For instance, they don't drive everywhere. They walk or take bicycles. But even there, there has been some change because those countries are becoming more prosperous. People can afford more food, sometimes cars and even televisions.

So prosperity can be a mixed blessing?

True. Some of the fatter nations are among the most wealthy, like Austria, Germany, Australia and the United Kingdom.

Do other countries rely on the same guidelines we do?

Absolutely. In fact, we've come into compliance with the rest of the world. Essentially all the countries have adopted the World Health Organization guidelines.

Is it safe to use drugs to lose weight?

I'm not for or against any kind of weight-loss programs or diets. Refer to the Food and Drug Administration—they're the ones who approve certain drugs for safe weight loss. But even then, you should remember that there is no drug on the market that has been tested for use longer than a year.

Is there such a thing as a dangerously low BMI?

Below 18.5 is abnormal. This is common with anorexic patients or smokers and can present problems such as lowered resistance to infectious agents, which can lead to a greater possibility of disease and a higher mortality rate.

In controlling your own BMI, do you have any weaknesses?

Nuts. I like peanuts, almonds, cashews. Yes, they are indeed high in fat content, but I control myself. I eat just a few at a time.

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