Risks of a Low-Carb Diet

Medically Reviewed

Low-carbohydrate (high protein) diets are appealing because they often result in rapid, seemingly effortless weight loss. However, it’s mostly due to the loss of body water—not body fat. And the diets come with some risks. Here’s what you should know before you consider one.

Low-carb diets were last popular in the 1960s, and they’re still promoting the same basic idea: Eat high-protein foods (such as meat and eggs) and restrict carbohydrate-rich foods (such as potatoes, pasta, fruits, and certain vegetables).

Proponents of low-carb diets say that reducing carbohydrate intake decreases the body’s production of insulin and thus slows the conversion of food to body fat. They also claim that carbohydrates are less filling than other foods, causing you to consume more calories in an effort to satisfy hunger.

The evidence

Studies show that a low-carb diet, once relegated to the realm of quackery, may work to help people lose an average of 5 to 10 pounds. In fact, in several studies, overweight and obese people who were placed on a very-low-carb diet lost more weight over a six- to 12-month period than people who followed other diets that reduced calories and fat (although the latter dieters often caught up later). People following the low-carb diet also reduced their triglyceride levels and increased their HDL cholesterol levels. While these changes normally occur with weight loss, they weren’t expected with low-carb diets because of their high fat ­content. In another study, a low-carb diet (the Atkins plan) was compared with three other popular diets (Weight Watchers, a calorie-restricted plan that follows national recommendations; The Zone, a moderate-carb, moderate-protein diet; and the low-fat Ornish diet). After one year, each of the diets reduced body weight and several heart disease risk factors (cholesterol levels, blood pressure and glucose levels) to a similar—though modest—degree.

Some concerns

Despite these studies, weight loss experts remain unconvinced about the value of low-carb diets and are still concerned about their potential risks. For instance, more research is needed to determine whether dieters following a low-carb diet can maintain their weight loss or continue to lose weight over a longer period than one year—or whether dieters will regain weight if they return to eating more carbs. (Most studies show that no matter what the diet, lost weight tends to be regained unless dieters adopt healthy new eating and exercise habits for life.) Furthermore, the long-term effects of low-carb diets (which are typically heavy on meat and saturated fat) on heart disease are currently unknown.

Also, experts still have concerns about the heavy emphasis on protein in these diets. Consuming too much protein places extra stress on the liver and kidneys because they have to metabolize and excrete more than normal amounts of waste products. Kidney stones can be caused or aggravated by high uric acid levels created by high-protein foods. And for those with chronic kidney disease, high-protein diets may speed the progression of kidney damage, even if the diet is followed for a short time. Furthermore, some studies suggest that eating too much protein causes excessive calcium loss, which can contribute to osteoporosis.

Restricting carbohydrate intake is unhealthy as well. That’s because drastically reducing carbohydrate consumption increases the metabolism of fatty acids and may cause ketosis. This condition results when excessive amounts of acidic substances known as ketone bodies are released into the bloodstream. Ketosis can be particularly dangerous for people with heart disease, diabetes or kidney problems.

In addition, restricting carbohydrates can lead to deficiencies in both vitamins and minerals. Healthy, carbohydrate-rich foods, such as whole grains, fruits and vegetables, provide essential nutrients as well as fiber and phytochemicals that work together to help prevent disease and promote good health. In fact, one of the basic underlying problems with most high-protein diets is their failure to promote a balanced diet and to teach long-term healthful eating habits.

Our advice

If you choose a low-carb, high-protein diet, it’s best to use it on a short-term basis and under the supervision of your doctor. But remember that there’s currently not enough evidence to support the safety and effectiveness of diets that promote unlimited consumption of protein or fat. The latest Dietary Guidelines recommend a diet that consists of 45 to 65 percent carbohydrates, 10 to 35 percent protein and 20 to 35 percent fat. To lose weight, simply cut back on calories.