Section 18.5

Low-Carbohydrate Diets

Low-carbohydrate, high-protein, high-fat diets (such as the Atkins diet) have become very popular in recent years. These diets promise weight loss, weight maintenance, good health, and disease prevention. But do they work?

Unlike most diets, low-carb diets generally do not restrict total calorie intake. Instead, the dieter is encouraged to eat as much protein and fat as they desire, while avoiding most carbohydrates. Proponents of low-carb diets claim that the foods eaten in pursuit of high-protein and fat intake are more "nutrient-dense," and therefore satisfy hunger more quickly, reducing total calorie intake effectively as a side-effect of the diet.

The Food Guide Pyramid

In 1992, the U.S. Department of Agriculture released the Food Guide Pyramid to help Americans make safe and healthy dietary choices. The Food Guide Pyramid arranges foods into categories, such as "Breads, Cereals, Rice, and Pasta" or "Meat, Poultry, Fish, Dry Beans, Eggs, and Nuts." The categories are arranged on a pyramid to indicate the recommended dietary intake of each type of food—from 6-11 servings for "Breads, Cereals, Rice, and Pasta", to "use sparingly" for "Fats, Oils, & Sweets." The Food Guide Pyramid also recommends that fat be limited to no more than 30% of total caloric intake. The general message of the Food Guide Pyramid can be summarized as "Carbohydrates: Good, Fats: Bad."

Figure 1: The U.S. Department of Agriculture's Food Guide Pyramid

However, recent studies have found that the Food Guide Pyramid might be misleading. For example, saturated fats are generally considered bad, and have been shown to raise cholesterol levels in the blood; but unsaturated fats have been shown to actually reduce cholesterol.

Low-Carb vs. Low-Fat

Conventional diets based on the Food Guide Pyramid recommend reduced calories, high carbohydrate intake, and, above all, low levels of fat consumption. Nutritionists championing such diets have objected to low-carb diets such as the Atkins diet, claiming that they are unhealthy and based on "pseudo-science."

On the other side of the issue, proponents of low-carb diets claim that the conventional diets are unhealthy and impossible to follow. "After all," explains the Atkins Center website, "nothing is more difficult to endure for a lifetime than being constantly hungry."

Science Weighs In

A recent study published in the New England Journal of Medicine sought to evaluate the effectiveness of low-carb diets. In the study, researchers compared subjects following a low-carbohydrate, high-protein, high-fat diet to subjects following a conventional low-calorie, high-carbohydrate, low-fat diet. Many people assumed that low-carb, high-protein, high-fat diets—heavy in cholesterol-rich foods such as red meat—would result in high levels of LDLs, or so-called "bad cholesterol." LDLs are called "bad cholesterol" because they carry excess dietary cholesterol to tissues where it can accumulate and lead to heart disease (see the "Cholesterol" Interactive Animation for more details).

Overall weight loss in the year-long study was nearly identical among the two groups. Although LDL levels initially increased in the low-carb diet group, LDLs were not statistically different between the two groups after one year. Somewhat surprisingly, subjects following the low-carb diet showed a greater increase in HDLs ("good cholesterol"—the packages that transport excess cholesterol from the tissues to the liver, where the cholesterol is broken down and eliminated from the body) than subjects following the conventional diet, along with a greater decrease in triglyceride concentrations. Both LDLs and triglycerides have been identified as risk factors for heart disease, while HDLs have been linked to decreases in heart disease risk. Contrary to traditional wisdom, these results imply an overall reduction in risk factors for heart disease for the high-fat diet.

Animation 1: Results of the study comparing low-carb diets to conventional diets
(*s signify statistically significant differences between the two groups.)
Total Cholesterol: While total cholesterol initially increased for the low-carb group, total cholesterol was not significantly different between the two groups after one year.
LDL: LDLs also initially increased in the low-carb group, but was not significantly different after one year.
HDL: Subjects following the low-carb diet showed a greater increase in HDLs than subjects following the traditional diet.
Triglycerides: Subjects following the traditional diet showed and overall increase in triglycerides after one year. Triglycerides have been identified as risk factors for heart disease.

It Ain't Over...

While these results are promising news for proponents of low-carb diets, more research is necessary. The study had a small sample size to begin with (33 individuals for the low-carb diet, 30 for the conventional diet), and fewer still actually completed the study (20 individuals for the low-carb diet, 17 for the conventional diet). While the percentage of subjects who dropped out of the conventional diet was higher than that of the low-carb diet (43% versus 39%), the difference was not statistically significant. In addition, the study only lasted one year. Long-term health benefits or damages of low-carb diets have not yet been determined in a randomized, controlled study—and even if a long-term study showed that a low-carb diet was healthier than a traditional diet, such a result would not necessarily mean that low-carb diets are the healthiest alternative.