Weighing the diet books - Cover Story

Nutrition Action Healthletter, Jan-Feb, 2004 by Bonnie Liebman

And then there were three.

For years, Dr. Atkins' New Diet Revolution has dominated best-seller lists. But by last November, Atkins had company. Dr. Phil's The Ultimate Weight Solution had cracked the top ten. And The South Beach Diet sat comfortably at number one. Where it should be.

South Beach isn't perfect. Its rationale has some flaws. And just because people buy a self-help book, you can't assume that they'll follow its advice (or we'd all be trim, sculpted, happily married, wealthy, and living a purpose-driven life).

But for the first time in a long time, one of the most popular weight-loss books is recommending a healthy diet. Whether it helps those unwanted pounds disappear any better than other diets is another question.

After all these years, publishers know what sells. And, with a few exceptions, what's selling now are books about "good carbs."

Books like The South Beach Diet, The Zone, and Good Carbs, Bad Carbs argue that "bad" carbs are making us fat. Even the Atkins diet, which has urged dieters to limit all carbs since the 1970s, has modeled its recent advice (especially for Phase 2) after the "good-carb" books. In a nutshell, here's what they claim:

1. Bad carbs cause a quick rise in blood sugar.

2. High blood sugar raises blood insulin levels.

3. Insulin leads to weight gain (either by making the body store fat or by lowering blood sugar levels so much that it causes hunger).

The solution? Simple, say the books. All dieters have to do is eat "good" carbs (like whole grains, vegetables, and beans) instead of "bad" carbs (like sugar, white bread, and potatoes).

Yet most obesity experts, including those who believe in that advice, agree that the research cupboard is bare. "It's amazing how few good studies have looked at how different carbohydrates affect weight loss," says Walter Willett, chair of the nutrition department at the Harvard School of Public Health.

"So far, the long-term evidence on weight loss is meager. We need bigger and longer randomized trials."

Glycemic Confusion

The South Beach Diet calls them "slow sugar" and "fast sugar." To Good Carbs, Bad Carbs, they're "tricklers" and "gushers."

But the message is the same: "As far as obesity is concerned," says South Beach, "fast sugar is worse for you; slower is better." How do you know which foods are which?

"In the early 1980s, Dr. David Jenkins led a team of Canadian researchers who devised a scale to measure the rapidity and degree with which a fixed quantity of food increases your blood sugar," writes South Beach author Arthur Agatston. "They called it the glycemic index."

In fact, the index is much more complicated than most books pretend.

"People think that a food has a definitive glycemic index, but it depends on how the food is processed, stored, ripened, cut, and cooked," says Xavier Pi-Sunyer, an obesity expert at Columbia University College of Physicians and Surgeons in New York.

Furthermore, diet books imply that "good" carbs like whole grains have a low glycemic index (GI, for short), while "bad" carbs like sugars, white flour, and other refined grains have a high GI. In fact:

* Bread is typically high-GI, whether whole-wheat or white, because it's made from finely ground flour.

* Pasta is low-GI, whether whole-wheat or white, but there are variations. "Thin linguine has a higher GI than thick linguine," notes Pi-Sunyer. "How would we advise the public about this major difference?"

* Rice ranges from high-GI (instant white) to low-GI (Uncle Ben's converted white), with brown and long-grain white rice in the middle. "Are we going to specify for the public which kind of rice they should eat and which kind they shouldn't?" asks Pi-Sunyer.

* Sugars range from high-GI (glucose) to low-GI (fructose). Sucrose (table sugar) is smack in the middle. What's more, "researchers have found no relation between the sugar content of foods and their glycemic index," says Pi-Sunyer.

And it's not even clear that the rise in blood sugar that comes from eating high-GI foods leads to high blood insulin levels, or that higher insulin leads people to overeat, says Pi-Sunyer. (1)

"The glycemic index may account for less than a quarter of the insulin response to a food," he suggests. "And there is no evidence that the typical post-meal levels of insulin increase food intake or body weight."

Which leads to the question: how good is the evidence that low-glycemic-index foods promote weight loss?

No Magic Bullet

You'll find a glycemic-index ranking of foods in The Zone; Dr. Atkins; Good Carbs, Bad Carbs; and The New Glucose Revolution.

But unlike the other books, The New Glucose Revolution was written by scientists who have actually studied the glycemic index. In fact, one of its authors, Thomas Wolever of the University of Toronto, was one of the researchers who helped devise the scale.

The glycemic index is no magic bullet for dieters, says Wolever.

"I've yet to see evidence that a low-GI diet aids weight loss," he explains. "One or two studies show it and a number of others don't."