Carbohydrates without fear

Low-carb diets may not be as harmful as once feared, but there’s a healthier way to lose weight and help your heart.

Three diet books that promise weight loss to anyone willing to chow down hamburgers but give up the buns are on The New York Times’ best-seller list of advice titles. Nearly 13 percent of U.S. adults have embraced the low-carbohydrate high-protein life, according to a recent study. They’re snapping up products ranging from beef jerky, whose sales have soared 20 percent so far this year, to low-carb breakfast bars, pancakes, ice cream, and even beer.

Is carbohydrate control the long-sought key to weight loss? It may well be, though probably not in the form espoused by such popular diet titles as Robert C. Atkins’ Diet Revolution, Body for Life, The South Beach Diet, Sugar Busters, and The Zone. Fortunately there’s a science-based nutritional plan that offers the benefits of the low-carbohydrate diet without its nutritional drawbacks and potential health risks.


THE ATKINS STUDIES

The low-carb diet made headlines in the spring with the publication of three independent randomized controlled trials that, for the first time, compared the standard low-fat reducing diet with the recently deceased Dr. Atkins’ “diet revolution”--the original and still dominant low-carb diet. Surprising obesity and heart experts, people on the Atkins or a similar diet lost more weight over six months than people in the low-fat group. Moreover, despite consuming unlimited meat, eggs, and full-fat dairy products, containing dangerous-seeming quantities of saturated fats and cholesterol, their blood lipids not only showed no ill effects, but also in some cases actually improved.

A less-publicized result of the studies is that the effectiveness of the Atkins diet seems to dwindle with time. While Atkins dieters lost more weight over six months, in the one study that continued for a year both low-fat and low-carb dieters regained some of the lost weight. The Atkins group had still lost more than the low-fat dieters, but regained enough so that the difference between the two groups was no longer significant. “This suggests that it’s increasingly difficult for people to follow the diet over time because of its severely restricted nature,” says David Ludwig, M.D., director of obesity programs at Children’s Hospital in Boston.

In addition, Atkins-type regimens do not magically allow you to eat all you want and still slim down. The studies showed that low-carb dieters lost their weight the old-fashioned way: by ingesting fewer total calories.

Nor is it clear that a low-carb diet high in saturated fat and protein is safe for longer-term use. Scientists have long known that high-protein diets stress the kidneys; in fact, a standard treatment for serious kidney disease is to severely restrict protein intake. Since many overweight people suffer from undiagnosed diabetes--itself a major risk factor for kidney disease--the risk of possible kidney damage is real. This aspect of high-protein weight-loss diets has not been adequately studied.


THE BLOOD-GLUCOSE CONNECTION

Yet the apparent ability of the low-carb diet to help people lose weight somewhat more readily than low-fat diets has some important implications. For many Americans, a typical day’s menu includes heavy doses of highly refined carbohydrates, such as white bread, cookies, chips, soft drinks, and sweets. Because those foods are rapidly digested, they can cause sharp increases in blood sugar after eating. The resulting surge of insulin causes a drop in blood sugar to relatively low levels several hours later. That creates a sensation of hunger and may stimulate overeating. “When blood sugar is low, in the battle between mind and metabolism, metabolism usually wins,” says Ludwig. In addition, chronic spikes in blood sugar after meals may eventually cause vulnerable people to develop insulin resistance, a prediabetic condition strongly associated with an increased risk of cardiovascular disease.

The longstanding advice to cut back on fat may have had the unintended effect of increasing the glycemic content of the American diet. “If you’re going to eat a low-fat diet, you’ll have to eat a high-carbohydrate diet” in order to replace the calories you used to get from fat, explains Gerald Reaven, M.D., professor of medicine at Stanford University. “At that point, you have a problem.”

A growing number of researchers are studying a nutritional plan that offers the benefits of steadier-state blood sugar without the drawbacks of either the low-carbohydrate or low-fat diet: the low-glycemic diet. The low-glycemic diet is based on the scientific measurement of the impact of various foods on blood sugar. Scientists have calculated the “glycemic load” of various foods by testing blood-glucose levels of volunteers two hours after consumption of a single serving.

The glycemic load ranges from essentially zero for meats, fats, cheese, and nuts to 30 for a baked potato or candy bar. The glycemic load of carbohydrates depends mainly on fiber content, because fiber slows the passage of foods through the digestive system. Methods of preparation also matter; ounce for ounce, wheat-based pasta has a relatively low glycemic load because it’s dense and takes a while to break down in the digestive system, while fluffy white bread made from wheat “almost digests itself,” notes David Jenkins, M.D., research chair of cardiovascular biology at the University of Toronto.

A low-glycemic diet differs significantly from a low-carb one because it encourages the consumption of many types of carbohydrates, provided they have a low glycemic load. Those include whole grains, legumes, fruits, and practically any vegetable except potatoes. As a consequence, the diet provides an abundance of the vitamins and minerals from those sources that are missing from the Atkins regime (which prescribes vitamin supplements to make up for that deficiency). Finally, the low-glycemic diet allows significantly more fat than a standard low-fat diet, with strong emphasis on poly- and monounsaturated fats from vegetable sources, such as olive oil, nuts, and avocados. In addition to their known favorable effects on blood lipids, those fats help reduce the glycemic load of meals still further by slowing digestion.


HEALTH BENEFITS OF THE LOW-GLYCEMIC DIET

Though now being touted for weight loss, the low-glycemic diet first caught researchers’ attention because of its powerful effects on the risk of cardiovascular disease and diabetes. There’s compelling evidence from both epidemiologic studies and short-term studies of the effect of switching volunteers to a low-glycemic diet: the lower the glycemic load of the diet, the lower the risk of heart disease and diabetes (see our list of low- and high-glycemic foods).

To cite but three examples:

The Survey of British Adults, which measured a variety of dietary and metabolic factors in 1,420 people ages 16 to 64, found that the glycemic load was the only dietary factor that predicted a favorable HDL (“good”) cholesterol level.

The Nurses’ Health Study found that among 75,000 women followed since 1984, those who consumed the highest-glycemic diets had double the incidence of heart disease, and 40 percent more cases of diabetes, than those who ate the lowest-glycemic diets.

Experimental studies back up those findings. For example, researchers at the University of Toronto compared a low-glycemic diet that included plentiful plant-based fats, soy protein, nuts, and fiber with both a standard low-fat diet and the same low-fat diet augmented by a cholesterol-lowering statin drug. The subjects were slightly overweight middle-aged men and women who started out with high cholesterol counts. At the end of the study period of just one month, the low-glycemic diet worked as well as the statin in lowering “bad” cholesterol and C-reactive protein, a likely coronary risk factor that indicates blood vessel inflammation. The low-fat diet by itself had very little effect.


BUT WILL YOU LOSE WEIGHT?

The newest, and most controversial, application of the low-glycemic eating style is to weight loss. “One can obtain the advantages of both the low-fat diet and the Atkins diet on a low-GI diet,” argues Ludwig, a leading researcher in the field, who has yet to author a popular diet book. “Compared to Atkins, it’s like using a chisel instead of a bludgeon.” Other diet books promote their own versions of low-glycemic diets, but their advice is often contradictory: The Zone, Atkins, and South Beach diets strictly prohibit potatoes, for instance, while the Body for Life plan encourages their consumption. Body for Life and The Zone ban eggs yolks, while the other two plans say they’re fine. It’s important to note that most of these diets have never been studied systematically.

Short-term human studies support the theory that the low-glycemic diet reduces weight by avoiding the hunger that results from the spike-and-crash glucose levels that follow high-glycemic meals. In addition, the high fiber and slow digestion of a low-glycemic meal makes you feel physically full longer. Of 16 one-day feeding studies in which researchers gave volunteers low- or high-glycemic meals and then allowed them to eat as much as they wanted later, 15 found that subjects felt less hungry, and therefore ate less, after the low-glycemic “preload.” A handful of small, short-term outpatient studies, the longest of which lasted just four months, have also shown better weight loss on low-GI diets compared with low-fat ones.

In August, the first yearlong study to back up the low-glycemic weight-loss concept was published in the Archives of Pediatric and Adolescent Medicine. The study involved 16 obese adolescents who were randomly assigned for a year to either a standard low-fat diet or a low-glycemic diet that allowed them to eat as much as they wanted.

“In the low-fat group, the body mass index (BMI) basically stabilized, whereas you’d expect it to go up by one or two points over the year,” says Ludwig, who conducted the study. “But in the low-glycemic group, BMI decreased by two units, and all the difference--11 pounds--was body fat. Granted, it’s a small study, but when you see such big effects in a small study, it often means the effect is quite vigorous.” The study was a pilot for a larger, federally funded study involving 100 teenagers that’s just now getting under way.


SUMMING UP

Independent studies have shown that after six months and without unfavorable effects on cholesterol levels, people on low-carbohydrate, high-fat, high-protein diets lose significantly more weight than those on a low-fat, high-carbohydrate diet. But the difference seems to narrow after a year, and questions remain about the long-term safety of a diet so high in saturated fats and protein, and low in fruits and fiber.

For those who need to control their weight, we recommend a related, but more healthful, alternative: the low-glycemic diet. Like low-carb diets, this eating plan reduces post-meal blood sugar surges that can lead to hunger pangs a few hours later. But a low-glycemic diet includes abundant carbohydrates in the form of whole grains, pasta, legumes, nuts, fruits, and pretty much any vegetable except potatoes.

Also, numerous studies have shown that long-term reduction of surges in post-meal blood sugar protects against heart disease and diabetes.

The evidence for weight loss on a low-glycemic diet is less extensive, but growing. Eating low-glycemic meals has been shown to dampen hunger later in the day. And a recent yearlong study of obese adolescents showed a significantly greater weight loss when they were allowed to eat as much as they wanted of a low-glycemic diet, compared with a more restrictive conventional low-fat diet.

The usefulness of low-glycemic diets for weight loss needs to be confirmed with larger trials. But we agree with Susan Roberts, Ph.D., professor of nutrition at Tufts University, who says: “Low-glycemic diets are definitely not unhealthy, so they’re a safe choice. For somebody with heart disease or diabetes in the family, I’d definitely recommend them as worth considering.”

If you eat the low-glycemic way, you can have your cake, improve your health risks, and possibly lose weight, too.

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