It's almost bathing-suit season, the time of year when diet-book sales typically take off. But these days, with one out of two Americans overweight, diet books have taken up permanent residence on the bestseller list. The pursuit of weight loss has become a way of life.

Last February, the U.S. Department of Agriculture held a "Great Nutrition Debate" (it should have been called a "Great Dieting Debate," given that most of the speakers were diet-book authors, not nutrition experts). Sparks flew between Dr. Robert Atkins, The Zone author Barry Sears, Dean Ornish, John McDougall, Sugar Busters! co-author Morrison Bethea, and other panelists. What didn't fly was good research.

Ornish was the only speaker who has published studies comparing people randomly assigned to his eating plan versus a "control" group. The catch is that his Life Choice Program isn't just a diet. It also gets people to exercise, stop smoking, and participate in stress-reduction. So, chances are, diet alone--which Ornish hasn't studied--didn't account for his patients' entire weight loss (25 pounds after one year, which shrank to 13 pounds after five years).(n1)

Ornish aside, the research cupboard is largely bare. A handful of studies (mostly on normal-weight people) has found that when people eat less fat--without trying to cut calories--they lose about five pounds.(n2) But it could be the lower density of low-fat diets that the difference, not fat per se. Either way, that's not much evidence for a nation of dieters to go on...especially for people who want to keep weight off over the long haul.

The popular diets "all produce weight loss and they all do it the same way--they cut calories," said Keith-Thomas Ayoob of Albert Einstein College of Medicine in New York. "The problem isn't weight loss, but long-term weight management.'

The panelists agreed that there's a dearth of good data. When Dr. Atkins claimed lack of funding to explain his lack of evidence, Ayoob retorted, "Ten million books in print and you can't fund a study?"

It's not just Atkins who's to blame. One might ask a similar question of the USDA or the Department of Health and Human Services: Americans spend $50 billion a year on weight-loss regimens and you can't fund a study to compare how good--and how safe--those diets are?

The Million-Dollar Question
Safety aside, good diet studies ask one of two questions:

1. Is a calorie a calorie? Researchers randomly assign people to eat either the test diet or a control diet, and make sure that each has the same number of calories.

Dr. Atkins, The Zone, and Protein Power all claim that calorie for calorie, low-carbohydrate diets lead to more weight loss than high-carb diets. Though researchers haven't tested their specific diets, dozens of studies have found that if you cut any calories--from fat, protein, or carbs--you'll lose the same amount of weight.

"Any differences in how well calories are used by the body are trivial," says Susan Roberts, head of the Energy Metabolism Laboratory at the Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tuffs University in Boston.

2. Do some diets help people eat fewer calories? Little long-term research has tackled this question. But a few studies suggest that what you eat may affect how much you eat. For example, in the first decent study to compare how much food people eat on a high-protein versus a high-carbohydrate diet, more protein led to more pounds shed.

Arne Astrup and colleagues at the Royal Veterinary and Agricultural University in Copenhagen randomly assigned 60 overweight men and women to either a control group (whose members were told not to change their eating habits) or to one of two diets: high-protein (25 percent protein, 45 percent carbs) or high-carb (12 percent protein, 58 percent carbs).(n3) Both diets got 30 percent of their calories from fat.

After six months, the high-carb group lost an average of 11 pounds, while the high-protein group lost nearly 20 pounds. On average, the high-protein-eaters consumed about 450 fewer calories a day than the high-carb-eaters. (The control group didn't lose weight.)

"The main difference in the foods available to the two groups was that people on the high-protein diet were allowed to select fat-reduced dairy products and lean cuts of beef, pork, poultry, lamb, and fish,' says Astrup, "whereas the high-carbohydrate group ate a mainly vegetarian diet with more vegetables, fruits, breads, rice, and pasta, and were also allowed to cheer themselves up with chocolate and sweets."

Less opportunity to eat sweets may have helped the high-protein group. "High-protein diets restrict variety enormously," says Roberts. The fewer the choices, the less people eat.

Another explanation: "High-protein foods have a much higher satiety value than high-carbohydrate foods,' says Astrup. In other words, they're more likely to make people feel full.

And some studies suggest that when people cut carbs from their diets, they don't compensate by eating an equal amount of protein, even if they're allowed to.(n4)

Calories Per Pound
Other research suggests that what matters isn't protein vs. carbohydrates, but high vs. low calorie density--that is, calories per pound (or gram or any weight) of food. Books like Volumetrics and The Pritikin Principle argue that the key to losing weight and keeping it off is eating foods with few calories and lots of bulk. Translation: lots of vegetables, fruits, and only low-fat dairy, poultry, meat, salad dressing, and mayo...but not low-fat, high-calorie foods like fat-free cakes and ice cream.

"These books are low-fat, high-fiber diets packaged with a few bells and whistles like adding water or air to foods," says Roberts. "Studies show that lower-fat diets lead to modest weight loss--four or five pounds on average. Whether the additional effects of adding water or air are real is something we need to study further.'

Diet Safety
Which diets are safe?

When it comes to high-sat-fat, very-low-carb diets, the most obvious problem is LDL ("bad") cholesterol, which rose an average of 18 percent in a 1980 study of 24 people on the Atkins diet.(n4) Other studies suggest that a diet rich in saturated fat may not raise LDL, as long as you're cutting calories or losing weight.(n5) But dieters eventually stop doing both. What's more, a recent study found that a diet that's low in saturated fat cuts LDL about 20 percent more than one that's high in sat fat.(n5)

And it's not just LDL, but the risk of colon and prostate cancers, that could climb when people switch to a diet loaded with sat fat, or, more precisely, red meat.

"I'm not aware of a single, trustworthy piece of evidence that suggests that high-protein diets like Atkins's are a healthy way to eat," says Roberts. "The epidemiological studies point to fruits, vegetables, and whole grains as healthy foods.'

Earlier reports cautioned that excess protein might burden the kidneys.(n6) But if you look at the grams of protein eaten on an Atkins-type diet, depending on the serving sizes you choose, you may eat no more protein than you would on some higher-carb diets (see p.12). Other side effects(if the carbs are low enough) are bad breath (from the ketones in your blood) and constipation (from lack of fiber).(n7)

Atkins's diet--like many other weight-loss diets--also runs short on some nutrients. To play it safe, dieters should take a multivitamin-and-mineral supplement as well as calcium (see NAH, April 2000).

What about diets--like Astrup's--that limit sat fat and don't restrict carbs as much as Atkins's? They should be reasonably healthy as long as you get enough fruits and vegetables-which should help reduce the risk of cancer, high blood pressure, and possibly heart disease.

In separate studies, Astrup found no adverse effect on kidney function or bone loss in people on his higher-protein diet.(n8) "I think this diet is safe, but our findings should be confirmed by other studies before we allow them to influence dietary guidelines," he says.

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(n1) J. Amer. Med. Assoc. 280: 2001, 1998.

(n2) Nutrition Reviews 56: S29, 1998.

(n3) Int. J. Obesity 23: 528, 1999.

(n4) J. Amer. Diet. Assoc. 77: 264, 1980.

(n5) Amer. J. Clin. Nutr. 71: 706, 2000.

(n6) J. Amer. Med. Assoc. 224:1415, 1973.

(n7) Eric Westman, Durham VA Medical Center, unpublished data.

(n8) Int. J. Obesity Relat. Metab. Disord. 23: 1170, 1999, and personal communication.



By Bonnie Liebman

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