The Low-down on Low-Carbohydrate Diets
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The Low-down on Low-Carbohydrate Diets

Low carb image Low-carbohydrate diets—such as the Atkins diet and the South Beach Diet—are sweeping the nation almost as fast as obesity. Many people believe that these diets are effective for weight loss—a belief now supported by some evidence. However, the safety of “low-carb diets” remains debatable with respect to potential long-term risks.

What’s the Big Concern?

Most low-carb diets restrict carbohydrates below the minimum of 130 grams per day needed by the body, as recommended by the Institute of Medicine of the National Academies. This restriction is achieved by severely limiting whole grains, milk products, fruits and fruit juices, and many vegetables, which in the long run may lead to vitamin and mineral deficiencies. In addition, such restriction may allow for unlimited consumption of dietary fat and protein (most of which is obtained from animal products), which may be harmful to the cardiovascular system and the kidneys, respectively.

What Does the Science Suggest?

Low-carb diets appear to be effective for weight loss, but not for weight maintenance.

Several studies have shown that low-carb diets are generally more effective for weight loss than conventional, low-fat diets—in the short run. However, in the long run, low-carb dieters appear to gain back some of the weight lost. So that after a year, the total weight lost is similar between low-carb and low-fat dieters. This is likely because low-carb diets are not easily maintainable over time.

In another study, researchers looked at three diets: Mediterranean, low-carb, and low-fat diet. Dieters had more success with the Mediterranean diet or the low-carb diet, rather than the low-fat diet.

Low-carb diets may improve some lipid levels, but more research is needed.

Several weight-loss studies have also looked at lipid levels as an indication of heart disease risk, including total cholesterol, low-density-lipoprotein cholesterol (LDL)-“bad" cholesterol, high-density-lipoprotein (HDL)-“good” cholesterol, and triglycerides.

Several studies have shown that people on the low-carb diets had greater decreases in triglycerides. Some studies have also shown that low-carb dieters had greater increases in HDL cholesterol. At least one study seemed to show that lipid changes seeming to favor low-carb diets do not persist once weight loss is achieved. In this study LDL levels ended up higher, after weight loss, in participants on low-carb diets.

Overall, some of the changes in lipid levels that occur when following a low-carbohydrate diet suggest an improvement in heart disease risk, although more studies are needed before drawing definitive conclusions about the significance and longevity of any changes.

Low-carb diets may result in more efficient use of insulin in the body, but studies have measured different outcomes, making more research necessary.

Some studies have measured different variables that may be involved in the development of type 2 diabetes and metabolic syndrome, a lipid disorder particularly associated with abdominal fat and the inefficient use of insulin (ie, glucose intolerance). One study found that low-carb dieters had lower levels of circulating insulin, which may be an indication of lower risk of metabolic syndrome, type 2 diabetes , and heart disease. Another study measured insulin sensitivity and found that people on a low-carb diet had better improvements than people on a low-fat diet.

Much more research is needed to explore the complex parameters that may be involved in the development of metabolic syndrome and type 2 diabetes, including the degree of carbohydrate restriction that is necessary for benefit without harm.

What Are Some Limitations of the Studies?

Long-term studies are needed to determine conclusive results regarding weight loss, lipid profiles, insulin sensitivity, and most importantly, safety. Existing studies vary in terms of caloric restrictions; relative amounts of carbohydrate, protein, and fat; and other outcomes, making it difficult to compare results. Studies often include only severely obese patients, not those people who only need to lose a few pounds. Therefore, the results may not apply to all people.

Who Should Not Be on a Low-carbohydrate, High-Protein Diet?

  • People with impaired kidney function and/or diabetes, as it may lead to further decline in kidney function.
  • People with diabetes who take insulin require adequate carbohydrate intake to prevent hypoglycemia, or low blood sugar.
  • People with liver problems. Carbohydrate storage can be impaired in liver disease and by-products of protein metabolism can also accumulate in the body causing altered mental status.
  • Pregnant women should not be attempting to lose weight by any means.
  • Breastfeeding women should not attempt weight loss by restricting carbohydrate-rich foods, as they may deprive the baby of important nutrients. In addition, it’s unknown whether ketones (by-products of fat metabolism when carbohydrate is not present) pass through breast milk to cause potential harm to the baby.

What About Low-Carbohydrate Foods in the Grocery Store?

Many manufacturers have formulated “low-carb” variations of America’s favorite foods. However, the Food and Drug Administration has not yet defined a “low-carb food.” Reading labels is extremely important, since manufacturers are not held to any standards. On the positive side, low-carbohydrate products tend to be higher in fiber. But in general, low-carbohydrate foods also tend to be higher in fat (including saturated fat, a harmful type) and protein than their “regular” counterparts. As a result, they often have the same, if not more, calories. These products generally cost more as well.

The Bottom Line

There are many different ways to lose weight. Among them, low-carbohydrate, high-protein diets appear to promote weight loss in the short term. But in the long run, they appear to be no more effective than more balanced weight-loss diets, which include a variety of whole grains, dairy products, fruits, and vegetables. In addition, low-carbohydrate diets may not be maintainable for life, since they are restrictive and may cause side effects such as dehydration, constipation, and bad breath. Furthermore, certain people should not follow a low-carbohydrate diet under any circumstances. As a result, it is always best to consult a healthcare provider before beginning any diet for weight loss.


American Dietetic Association

The Nutrition Source
Harvard School of Public Health


Canadian Digestive Health Foundation

Dietitians of Canada


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*¹7/22/2008 DynaMed's Systematic Literature Surveillance DynaMed's Systematic Literature Surveillance: Shai I, Schwarzfuchs D, Henkin Y, et al. Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet. N Engl J Med. 2008;359:229-241.

Last reviewed June 2008 by Dianne Scheinberg MS, RD, LDN

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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