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Topic Overview

Low-carbohydrate diets are based on the idea that avoiding foods high in carbohydrate, such as pasta, bread, rice, cereal, fruits, and starchy vegetables, causes weight loss. These diets are usually high in protein and fat.

The appeal of low-carbohydrate diets, such as the Atkins diet, is rapid weight loss in the first few days. But most of the initial weight loss is water. As soon as you add carbohydrate back into your diet, you will regain the water weight.

Over the long term, though, low-carbohydrate diets also result in a more gradual weight loss because they contain fewer calories. Recent research on low-carbohydrate diets shows that it isn't the reduction in carbohydrate that causes the weight loss. Instead, it is due to a decrease in calories.¹

Two studies confirm those findings, suggesting that:^{2, 3}

• People on a low-carbohydrate diet may eat fewer calories because the high-fat, high-protein foods allowed in the diet are better at satisfying hunger.
• A low-carbohydrate diet may be easier to follow than a low-fat diet.

Both studies also found that the low-carbohydrate diets may have a positive effect on levels of certain fats in the blood—triglycerides and high-density lipoprotein (HDL, "good") cholesterol. Some people participating in the studies did have an increase in low-density lipoprotein (LDL, "bad") cholesterol levels, though.

One of the studies lasted 6 months, and the other lasted 1 year. People in the 6-month study were either mildly or moderately obese and had high levels of LDL cholesterol or triglycerides but were otherwise healthy.

Findings of the 6-month study included:²

• In the first 2 weeks of the study, people on the low-carbohydrate diet lost more weight than those on the low-fat diet and the loss was mainly from water weight.
• At the end of the 6-month period, those following the low-carbohydrate diet lost more body weight and body fat than those on the low-fat diet.
• People in the low-carbohydrate group had lower triglyceride levels and higher HDL cholesterol levels than those on the low-fat diet. HDL is considered the good cholesterol.
• About a third of the people in the low-carbohydrate group had a 10% increase in their levels of LDL cholesterol by the end of the study. LDL is considered the bad cholesterol.
• People in the low-carbohydrate diet group were more likely to stay with the study for the 6-month period than those in the low-fat group.
• People on the low-carbohydrate diet had more side effects than the low-fat group, including constipation, headache, bad breath, muscle cramps, diarrhea, general weakness, and rash.
• Results of the study may have been affected by vitamins and other nutritional supplements taken by the low-carbohydrate group. This group took essential oils supplements containing fish oils, which have been shown to decrease triglyceride levels and raise HDL levels and may have prevented some side effects of the diet, such as kidney stones.

The 1-year study of low-carbohydrate versus low-fat diets included people who were severely obese. Most had diabetes or metabolic syndrome. Findings included:³

• The total amount of weight loss between the groups was similar after 1 year.
• Changes in total cholesterol and LDL ("bad") cholesterol were not significantly different between the two groups.
• Those in the low-carbohydrate group had more favorable levels of triglyceride and HDL. Low-carbohydrate diets may positively affect blood sugar levels in those with diabetes.
• This study was influenced by a high dropout rate and the fact that most people didn't closely follow the requirements of their diets: less than 30 grams of carbohydrate a day in the low-carbohydrate group; and for the low-fat group, eating 500 fewer calories a day with less than 30% of calories from fat.

Low-carbohydrate diets are not recommended

The American Dietetic Association and the American Heart Association do not recommend low-carbohydrate diets. People who have serious medical conditions, such as kidney disease, heart disease, type 2 diabetes, high cholesterol, or high blood pressure, should talk to a doctor before starting a low-carbohydrate diet.

Because low-carbohydrate diets cause the accumulation of ketones in your blood, they may cause the abnormal metabolism of insulin, impaired liver and kidney function, and salt and water depletion. Also, low-carbohydrate diets usually are high in fat and protein and low in fiber, which can lead to impaired kidney function, constipation, and fatigue.¹

Also, the American Heart Association does not recommend low-carbohydrate diets, because these diets often restrict healthful foods, such as fruits and vegetables, and do not provide essential vitamins, minerals, and fiber.

Although research shows that these diets may not be harmful for a short time (1 year), researchers believe more studies are needed. But there are long-term studies that show the health benefits of eating plant foods that are high in carbohydrate.

The American Dietetic Association recommends including whole grains, vegetables, and beans, because these foods provide a large variety of nutrients and fiber. High-sugar foods, such as candy and soda, are high in calories, provide few nutrients, and should be limited in any diet.

If you are pregnant, do not go on a low-carbohydrate diet, because it may not be safe for your fetus.