Calories don't count ... equally.

Calories Don't Count...Equally

A calorie is a calorie. So goes the adage. But a growing body of evidence suggests that once a calorie enters the body, this simple rule of dieting dogma isn't always true.

Instead, fat calories may be particularly good at making fat bodies. And people who have lost weight may be especially "hungry" for the fatty foods that will do their diets in. What's worse, their bodies may be primed to store every bite of incoming fat, rather than burn it for energy.

Why some people gain weight and others stay slim is still a puzzle. It has no easy, and probably no single, solution. Yet many researchers now agree that if any diet is going to help take and keep weight off, it's got to be low in fat.

HANDLING COSTS NOT INCLUDED

Any nutritionist can rattle off the 4-7-9 rule. That is, a gram of protein or carbohydrate has 4 calories; a gram of alcohol has 7; and a gram of fat has 9. Already, fat looks bad.

But that's just the beginning. Researchers got the 4-7-9 rule by measuring the heat given off when they burned fat or protein or carbohydrate in a closed metal box. What happens in the body appears to be quite different.

"From a physical and chemical point of view, a fat calorie may equal a carbohydrate calorie, but not in humans," says Eric Jequier of the University of Lausanne in Switzerland. "There is a marked difference in the body's response to fat and carbohydrate feeding."

In fact, there are several differences. One is intuitively obvious: it's easier to turn food fat into body fat than it is to turn carbohydrate into body fat.

"In the case of carbohydrate, you pay a greater cost for handling and storage," explains Jean-Pierre Flatt of the University of Massachusetts Medical School in Worcester.

Flatt has estimated that the body's "handling" costs are: ] 3 percent to store fat as fat, ] 7 percent to store carbohydrate as carbohydrate (in a form called glycogen), and ] 23 percent to convert carbohydrate to fat.(1)

CONVERSION CATCH 22

Not only is it "expensive" to convert carbohydrates into fat, the body seems reluctant to do it.

Flatt, Jequier, and co-workers fed healthy young people 700 grams of carbohydrate--that's 2,800 calories. "The whole thing was accommodated as glycogen [carbohydrate] storage," explains Jequier. "We had to give an enormous amount of carbohydrate to see a net increase in fat deposition."

According to dieting dogma, if you eat too much carbohydrate, your body turns the excess into fat. But when they say "too much," they really mean it. Only when people eat massive amounts of carbohydrate does it get converted to fat.(2,3)

"Because we don't readily convert carbohydrate to fat, one has to worry separately about what happens to carbohydrate and fat," says Flatt.

That doesn't mean dieters can gorge themselves on sherbet, sodas, and other carbohydrate-rich foods and expect to lose weight. For one thing, many overweight people have problems handling carbohydrates, especially sugars. For another, you still have to eat fewer calories than you expend in order to lose weight.

CARBOHYDRATES UNDER CONTROL

The more carbohydrate you eat, the more carbohydrate you store--and the more you burn. But that's not true for fat.

Flatt fed young men a low-fat breakfast of bread, jam, and dried meat.(4) On another day, they were fed the same breakfast, with an extra 10 pats of margarine. But on the margarine day, their bodies didn't adjust by burning a greater proportion of fat.

"You have to watch out for fat calories more than carbohydrate calories, because the body maintains carbohydrate and protein balance automatically," Flatt explains. "But it doesn't control fat as well."

His co-worker, Eric Ravussin, is now at the National Institute of Diabetes and Digestive and Kidney Diseases in Phoenix. Ravussin has just completed a study on both lean and obese people that supports the earlier findings.(5)

"The body is able to cope with excess protein and carbohydrate, but not excess fat," says Ravussin.

PROGRAMMED TO BE PLUMP

Fat's tendency to stick to the ribs (and thighs and hips) doesn't entirely explain why some people are trim and others are plump. If it did, everyone eating a typical, fat-laden American diet would have a fat-laden body.

Animal studies illustrate this point. When albino mice are fed low-fat diets, almost all stay trim.(6) The more fat (and the less carbohydrate) they're fed, the less uniform they look. Some grow fat, others stay slim--just like humans.

Clearly, genetics seems to play a role in obesity. One way genes may act is by determining how well (or how poorly) your body handles fat.

Trudy Yost of the University of Colorado Health Sciences Center in Denver has been studying women who used to be fat, but who have stayed reasonably trim for at least a few months. The formerly-obese are critical to answering the $64,000 question in obesity research: Why are these people so likely to regain their lost weight?

THE FAT LOADER 

Yost and her co-workers have been looking closely at lipoprotein lipase (LPL), an enzyme that's responsible for (among other things) loading fat into adipose (fat) cells. Apparently, the LPL in adipose tissue behaves differently in formerly-obese women.

"If a normal person eats fat, the body's response is to [lower its LPL level]," says Yost. "The normal body attempts to use fat as energy before loading it into fat cells."

But when Yost gave fat to the formerly-obese women, their LPL soared far above already-high starting levels.(7) "If you present fat calories to a formerly-obese body, the first thing that happens is that the fat is packed into cells for storage," she explains. "These bodies are preferentially pulling fat calories out for storage."

Some (but not all) genetically obese animals have high LPL levels. Yost's goal is to find out what can go wrong with LPL and "fix" it. But in the meantime, she says, the only solution is to not provide the fat calories.

"We used to say that if you eat a 1,500-calorie-per-day diet, it doesn't matter if it's fat, protein, or carbohydrate. For the formerly-obese, that isn't true. Fat calories are going to be metabolized very differently than protein or carbohydrate calories."

A TASTE FOR FAT

"If it tastes good, it must be fattening." That observation may be more true for dieters than for others.

Recent studies suggest that the fatter people are, the more they prefer the taste of fat: ] When given a choice of milkshakes made with varying amounts of cream and sugar, overweight people chose fattier shakes than their lean counterparts.(8) ] Overweight people report eating no more calories than others, but more of those calories come from fat.(9)

These studies suggest that whatever makes people fat (or even the very act of trying to stay thin) also makes them crave fatty foods. But if they can fight the craving by thinking of fat--not just calories--as the enemy, it might help.

THE BOTTOM LINE

"I can tell you right now, the people in our clinic who are taught to keep fat out of their diet do better at keeping the weight off," says Elliot Danforth of the University of Vermont. "But no one will believe it unless we keep 100 people or so in a locked ward or dormitory for two-to-three years. That's a mega-expensive study."

Other researchers have made similar observations. Claude Bouchard and colleagues at Laval University in Quebec, Canada, had eight overweight women exercising five times a week for two years. The women lost weight, but then the losses leveled off--until his colleagues recommended a 20 percent low-fat diet.

"Their weight loss was spectacular," says Bouchard. "Some lost 50-to-60 pounds."

But Bouchard's women might have eaten fewer calories on the low-fat diet. The question is: will a 1,000-calorie low-fat diet lead to thinner bodies than a 1,000-calorie high-fat diet? So far, no one knows.

A low-fat diet is not a magic wand that will eradicate obesity. Clearly, genes influence your chances of tipping the scales, as does exercise. But eating less fat is also likely to help.

"If a formerly-obese woman eats 1,500 calories of fat, she's sunk," says Yost. "If she eats 200 calories of protein, 100 calories of fat, and 1,200 calories of carbohydrate, she has a chance." (1)Bray, G.A., Recent Advances in Obesity Research 2: 221, 1978. (2)Am. J. Physiol. 246: E62, 1984. (3)Am. J. Clin. Nutr. 48: 240, 1988. (4)J. Clin. Invest. 76: 1019, 1985. (5)Am J. Physiol. 255: E332, 1988. (6)Int. J. Obes. 9: 443, 1985. (7)J. Clin. Endocrinol. Metab. 67: 259, 1988. (8)Physiol. Behav. 35: 617, 1985. (9)Am. J. Clin. Nutr. 47: 406, 995, 1988.