Even though the experts say most Americas are obsessed with being thin, if you look around you'll see that most Americans are overweight. In fact, over one third of the adult U.S. population is obese--that's a 10% increase from the number of people who were obese in 1980. And the number continues to climb.

So how do we reduce our nation's fat, and your waist line? Exercise is certainly a sure- fire way to fight the bulge. But if you don't eat a low-fat diet in conjunction with your regular exercise program you may be fighting a losing battle.

Many health professionals say you have to go on a low- calorie diet. They say "A calorie is a calorie is a calorie" and "All excess calories will be stored as fat." Be cautious of these warnings. They are only half-truths, and are not the main issue when dealing with weight control. You do not eat calories, you eat carbohydrate, fat, and protein. And each of these are used and stored differently by the body.

What is a Calorie?

A kilocalorie, or what we commonly refer to as a calorie, is a measure of heat energy. It represents the amount of heat needed to raise the temperature of one kilogram of water (slightly more than a quart) by one degree Celsius. For example, a can of chicken noodle soup with 90 calories per serving has enough chemical energy in one serving to raise the temperature of 90 quarts of water by one degree Celsius. But if the can of soup is actually chemical energy that produces heat, what happens when you eat it?

Where Does It All Go?

The protein in the soup (coming mostly from the chicken), which equals four calories per gram, is broken down and then reassembled to replace protein in your body lost by routine cell turnover, especially in the muscles. Some protein is also used to make enzymes and other key chemicals needed to make your metabolism work.

Suppose you add up all the protein in your daily diet, and it comes to more than your body needs. What happens? The calorie counters say it's turned into fat. "But this would call for some monumental biochemical processes, and there is no evidence in the scientific literature to support this," reports Trevor Smith, a retired research chemist and editor of Running & FitNews. The fact is that extra protein is oxidized, which means it is burned off and converted to compounds that are eliminated from the body.

What happens to the fat in your soup, and other foods you eat? Some replaces lost tissues such as cell membranes and certain cells in your nervous system. The rest is used for energy in your body. Fat is a very high-energy source, providing nine calories per gram, and if you eat more fat than your body needs during the day, the rest is stored for later use. The trouble with this is that the body's capacity to store fat seems limitless.

People who eat more fat than they can burn keep storing the stuff day after day. And they get fatter and fatter. The fate of the carbohydrate in your soup (coming from the vegetables and pasta), and in the rest of your daily foods, is more interesting. A little carbohydrate is used in cell turnover but the majority is used for energy.

Although it produces only four calories per gram, when carbohydrate is the main part of your daily diet you have plenty of energy. What if you eat too much carbohydrate? The calorie counters say it's turned into fat. However, the scientific literature tells a different story. "First, some extra carbohydrate can be stored as glycogen, which is the breakdown product of carbohydrate. Normally you store glycogen in your muscles, with a smaller amount in your liver," says Susan Kalish, executive director of the American Medical Athletic Association. "The muscle store of glycogen can increase, which is the principle of carbo loading. If you use up muscle glycogen, say with a long, exhausting run, and then quickly begin replacing it by eating high carbohydrate foods (bananas, bagels, pasta), your muscle store can increase by up to 2.5 times."

If you eat more carbohydrate than your muscles can store then the rate of oxidation increases. In other words, your body "turns up the heat" and burns carbohydrate faster. Only when your body has filled all possible stores and turned up the burners full blast can it begin converting some of the extra carbohydrates into fat. And by this time, you would be eating truly massive amounts of food.

"Results from clinical nutrition studies show that the conversion of carbohydrate to fat in healthy people is minor, compared to storage of dietary fat. You only convert carbohydrate to fat if your body is deprived of fat, or if your body needs extra fat, as it does in the third trimester of pregnancy," says Smith.

What Should You Do?

"The evidence that counting grams of fat is the key to weight control is well documented, there is a mechanism for the process that makes sense, and the scientific literature supports it. The evidence that counting calories is the way to go is inconsistent, seldom corrected for the known influence of fat, often flawed, and there is no mechanism to explain how those excess calories contribute to weight (except for those from fat), that is consistent with human biochemistry," says Smith.

It is very hard to overeat on high carbohydrate foods because they are bulky and often contain a lot of water. If you are in good health, have a normal metabolism, and exercise regularly, "Eat one gram of fat for each kilogram (2.2 pounds) of body weight [or target body weight]," suggests Jack Daniels, Ph.D., exercise physiologist and coach at Arizona State University.

"It's never fun to cut out your favorite foods to lose weight. That's why so many diets fail," says Kalish. "The best way to lose weight and keep it off is to cut the fatty foods in your diet. Select the low-fat alternatives when you can. And load up on fresh fruits, vegetables, breads and pastas (without high-fat spreads or sauces). You really can eat until you're full--if you eat low-fat."

For a free brochure on assessing body fat and managing weight control, send a business-size, self-addressed, stamped (55c) envelope to "You and Your Body Fat," c/o American Running Association , 4405 East West Highway, Suite 405, Bethesda, MD 20814.