This time of year, thoughts turn from overloaded holiday tables to overweight bodies, the beach, and diet programs. Losing weight is not just a matter of looking good in a swimsuit. Packing on the pounds increases a person’s risk of heart disease, diabetes, high blood pressure, stroke, and some cancers. Recent surveys estimate that more than 50 percent of adults in the United States are overweight. As the U.S. public has gotten fatter, public health officials have been pushing diets low in fat. A variety of epidemiological data supports this advice, but it’s now being challenged as other types of weight-loss diets have gained support.
“As a country, our fat intake has decreased, but our calorie intake has increased, and obesity rates are going up,” says Bonnie J. Brehm of the University of Cincinnati. “Over the last 10 years, Americans have been so obsessed with low fat that people have forgotten that carbohydrates have calories, too. The pendulum may be swinging back a bit.”
Some recent studies—and provocative articles in the popular press—have suggested that low-carbohydrate diets, such as the Atkins diet, could be more effective for weight loss than low-fat diets are. However, the low-carb diets tend to be high in fat and protein. So, there are concerns about their potential health effects. Although scientists caution that these diets haven’t yet been studied over long periods, several new trials have shown them to have surprisingly positive short-term effects.
The idea behind cutting fat out of weight-loss diets was that fatty foods represent the densest source of calories that a person eats, says Jennie Brand-Miller of the University of Sydney in Australia. Dieters have been told to replace high-fat items with fruits, vegetables, and grains.
Various studies have shown that such diets can help people achieve and maintain a healthy weight. In 2001, the U.S.-based Diabetes Prevention Program showed that low-fat, low-calorie diets combined with exercise produced a 5 to 7 percent weight loss over 6 years (SN: 9/8/01, p. 159: Available to subscribers at Walking and eating for better health).
Last November at the American Heart Association meeting in Chicago, researchers reported that among 74,000 women, those who increased their fruit and vegetable intake over the 12-year study period were 26 percent less likely to become obese than were women who decreased their consumption of such foods.
However, some scientists argue that low-fat diets aren’t more effective than tracking calories. Early last year, an analysis of six studies that compared low-fat and fixed-calorie diets concluded that participants lost about the same amount of weight, 5 to 10 pounds. “The review suggests that fat-restricted diets are no better than calorie-restricted diets in achieving long-term weight loss in overweight or obese people,” concludes Sandi Pirozzo of the University of Queensland in Australia.
Furthermore, she notes, “the overall weight loss . . . in all studies was so small as to be clinically insignificant.”
One reason that nutritionists had thought that people would lose more weight on a low-fat diet than on other calorie-restricted diets was that traditionally low-fat foods have been bulkier and higher in fiber than fattier foods. The nutritionists reasoned that people feel fuller after eating low-fat foods than after dining on other foods.
Over the past decade, the food industry’s introduction of many low-fat choices has altered the relationship between fat, bulk, and fiber. Brand-Miller says, “New low-fat foods are not necessarily bulky. Nor are they low in calories because they often have added sugars.” That means that it’s become easier for people to eat low-fat meals and still add pounds.
Nevertheless, low-fat eating may have health benefits beyond any weight loss. Many epidemiological studies have shown that people who report eating diets low in fat and high in fruits and vegetables are less likely to develop heart disease and diabetes than people eating higher-fat diets are. The review of six studies concluded that participants in the low-fat group were slightly more likely to show a drop in cholesterol concentrations in their blood than were those in the fixed-calorie group.
In fact, one of the widely used low-fat diets was developed a decade ago to help people with heart disease reduce fatty buildup in their arteries. Dean Ornish, a professor of medicine at the University of California, San Francisco School of Medicine, developed a high-fiber diet in which less than 10 percent of the calories come from fat. That’s about a third of the fat of a typical U.S. diet.
Most national health organizations have weighed in on behalf of low-fat diets.
However, critics of these diets point out that the studies often encourage participants to not only change their diets but also increase exercise and learn stress-management strategies. Thus, in these tests, it’s difficult to tease out the effects of any diet on weight loss.
Critics also note that high carbohydrate consumption can result in overproduction of insulin and eventually in people’s becoming less sensitive to it (SN: 4/8/00, p. 236: The New GI Tracts). This condition, called insulin resistance, may eventually lead to diabetes.
“What’s becoming increasingly clear is that low-fat diets for people with certain biological predispositions may increase their risk of developing the insulin-resistance syndrome,” says endocrinologist David S. Ludwig of Children’s Hospital Boston. He speculates that replacing fats with processed sugars and starches played a role in the development of current epidemics of obesity and diabetes.
One of the most popular low-carbohydrate diets today was devised and has been promoted by Robert C. Atkins, a cardiologist in New York City. The diet restricts carbohydrate consumption to less than 10 percent of total calories eaten, whereas people in the United States often get more than 50 percent of their calories from carbohydrates such as bread, processed foods, starch in vegetables, and sugar in fruits. People on the Atkins diet tend to eat at least 40 percent of their calories in fat, while the average U.S. diet contains about 30 percent fat calories.
The body’s reaction to very low carbohydrate load is a condition called ketosis.
According to Atkins’ many books and magazine articles, people in ketosis preferentially burn stored body fat for energy—and burning fat takes more energy than burning carbohydrates does. Thus, he argues, dieters can lose weight while eating foods higher in calories than their previous choices.
Some researchers think there’s another factor at play. They speculate that any benefits of a low-carb diet stem not from ketosis but from the diet’s effects on blood sugar and insulin. A diet higher than average in protein and fat, which are digested more slowly than carbohydrates, might avoid carbohydrate-induced spikes of insulin in the blood that force blood sugar concentrations so low that the person feels hungry soon after eating, says Ludwig.
Critics of the Atkins diet say that it is likely to have dangerous side effects. Bone health is one concern about it and other low-carb diets.
“The huge load of animal protein ingested in such diets leaches calcium from the bones and sends it through the kidneys into the urine,” says Neal Barnard, president of the Washington, D.C.–based Physicians Committee for Responsible Medicine.
High protein intake increases the acidity of blood. In response, acid-neutralizing calcium gets pulled from bones. Also, excess urea from the protein pulls extra water into the kidneys, so dissolved calcium is expelled. “Over the long run, that can spell osteoporosis,” says Barnard.
A study in the August 2002 American Journal of Kidney Diseases showed that after 6 weeks on the Atkins diet, the 10 participants made urine containing 55 percent more calcium than it had at the start of the trial.
People on meat-heavy diets are also more prone to kidney stones, gout, colon cancer, and potentially cardiovascular problems, Barnard adds. A high-fat diet might also boost the cholesterol and triglycerides, or free fatty acids, in people’s blood. High cholesterol and fatty acid concentrations are linked to heart disease.
“Low-carb diets remain a serious health risk,” Barnard says. To investigate potential health effects, as well as the effectiveness of such a diet, Eric Westman of Duke University in Durham, N.C., undertook a study funded by the Robert C. Atkins Foundation. He tracked 60 overweight people following a diet with less than 30 percent of its calories from fat and 60 others following the Atkins diet. As part of the diet, the Atkins group took supplements of fish oil, borage oil, and flaxseed oil. Westman reports that participants were more likely to stick to the Atkins diet than to the low-fat regimen.
Over 6 months, the people in the Atkins group lost 31 pounds, compared with 20 pounds for the people in the low-fat group. The changes in blood characteristics associated with heart disease were more favorable in the Atkins group, Westman says. Low-density-lipoprotein-linked cholesterol (the bad cholesterol) didn’t change in blood samples from either group, while high-density-lipoprotein-linked cholesterol (the good cholesterol) went up slightly in the Atkins group but not in the other group.
Triglyceride concentrations dropped in the Atkins group members’ blood by almost twice as much as in the low-fat group’s members.
“The findings were unexpected,” Westman says, “but the results of several small studies seem to be consistent with ours.”
With funding from the American Heart Association, Brehm also compared two diets. She randomly assigned 53 women to either a low-carbohydrate or a moderately low-fat regimen. On the low-carb diet, women were permitted to eat as much as they wanted as long as they kept carbohydrate calories to less than 10 percent of the diet. On the low-fat diet, the women were asked to eat between 1,200 and 1,500 calories per day.
At the end of 6 months, the low-carbohydrate dieters had lost about 18.7 pounds, including 10.6 pounds of body fat, while the low-fat dieters had lost 8.6 pounds, of which 4.4 pounds was body fat, Brehm and her colleagues reported at meetings late last year. “According to food records, both groups took in about the same amount of calories,” Brehm says. Only the low-carb dieters showed signs of ketosis, she notes, so they burned more body fat for energy than the other group did.
Moreover, while blood concentrations of two markers of inflammation decreased in both sets of dieters, one marker decreased more in the low-carbohydrate group than in those on the low-fat diet. Inflammation has been linked with heart disease and diabetes (SN: 6/14/97, p. 374: http://www.sciencenews.org/sn_arc97/6_14_97/bob1.htm; SN: 8/31/02, p.
136: Inflammatory Ideas).
Blood pressure, cholesterol concentrations, and blood-sugar measurements weren’t significantly different in the two groups and didn’t change during the study. On the other hand, the amount of insulin in all the dieters’ blood—measured before a meal—decreased in both groups during the 6-month study. That change typically indicates that a person is becoming more sensitive to insulin, a positive health sign.
A third major trial of an Atkins-style regimen enrolled 60 overweight men and women.
Half followed a strict diet high in protein and low in carbohydrates, and the others adhered to a regimen lower in protein and higher in carbohydrates. Unlike the other trials, the researchers adjusted the volunteers’ protein intakes to keep the percentage of fat the same in the two diets, says Peter M. Clifton of Australia’s Commonwealth Scientific and Industrial Research Organisation in Adelaide. The two groups were matched for age and weight.
Over 16 weeks, the participants in both groups lost an average of about 18 pounds. Participants on the high-protein diet lost more fat and less muscle.
“There are subtle metabolic advantages for being on a high-protein diet, especially for women,” says Clifton. The researchers measured no difference in blood-cholesterol concentrations between the two diet groups, but people on the high-protein diet had greater reductions of triglycerides in their blood. Also, over the course of the study, their sensitivity to insulin improved more than that of the other participants.
Several other small studies have shown similar results. All the researchers say they aren’t prepared to recommend a low-carb diet ahead of other weight-loss plans, but they agree that the diet merits further investigation. The National Institutes of Health is funding a study that will track 360 participants at three universities for at least a year to compare the Atkins diet and a low-fat diet.
“We are trying to stay on top of the science here,” says Robert Bonow, president of the Dallas-based American Heart Association. “People should not change their eating patterns based on very small, short-term studies. Bottom line, the American Heart Association says that people who want to lose weight and keep it off need to make lifestyle changes for the long term—this means regular exercise and a balanced diet including lots of fruits and vegetables.”
Weighing the issues
There’s certainly room for improvement in the typical Western diet, says Brand-Miller. “There are good and bad high-protein diets, and there are good and bad low-fat diets,” she says.
Although proponents of the Atkins diet argue that low carbohydrate intake has specific metabolic effects, some researchers still hold that all dieting is basically a matter of eating less. “People will lose weight on any diet, like the Atkins diet, that cuts out major groups of food because people get bored of eating the same thing day after day. But, in my experience, people find these diets very difficult to stick to,” says Brand-Miller.
Though difficult in execution, dieting is simple in concept, says endocrinologist Gerald Reaven of Stanford University. “If you do carefully controlled studies, a calorie is a calorie is a calorie, and if you lower your calorie intake you lose weight. So, you can lose weight on any diet,” he says.
The short-term dietary changes needed for weight loss are unlikely to have negative effects on health, he argues, especially given the benefits of weight loss.
However, he takes a different view of the diets that people use over the long term to maintain their lowered weight. For example, a low-fat diet may be bad for people who are resistant to the effects of insulin, he says. Likewise, he argues, a low-carb diet may be bad for people with high cholesterol.
Reaven argues that the best long-term diet is one that contains moderate amounts of both fat and carbohydrates. Dietary recommendations released last year by the National Academy of Sciences suggest a diet of 10 to 35 percent protein, 45 to 65 percent carbohydrates, and 20 to 35 percent fat. Compared with previous guidelines, this new recommendation lowers the amounts of carbohydrates that people are told to eat and increases the permissible fat and protein, but it still rules out at least the initial phase of the Atkins diet, in which fruits or vegetables are strictly limited.
No matter how people do it, losing weight and keeping it off is a crucial public health issue. The federal government estimates that in 2000 the cost of obesity in the United States was more than $117 billion. Researchers agree that the rising numbers of overweight and obese people ensure that studies of diets and weight loss will be a burgeoning field for years to come.
|Average U.S. diet||15 %||51-53 %||32-34%|
|Most low-fat diets||Varies||Varies||20-30 %|
|N.A.S. recommendation||10-35 %||45-65%||20-35%|
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