George L. Blackburn, M.D., Ph.D.

Physicians’ Guide to Popular Low-Carbohydrate Weight-Loss Diets
Cleveland Clinic Journal of Medicine 68(2001):751
by George L. Blackburn, MD, PhD, Judy C.C. Phillips, MS, RD, and Susan Morreale, CHES

George L. Blackburn, M.D., Ph.D., is an internationally renowned expert in the fields of obesity and clinical nutrition. Dr. Blackburn currently holds the following appointments and positions at Harvard Medical School: S. Daniel Abraham Chair of Nutrition Medicine, Associate Professor of Surgery, and Associate Director of Nutrition, Division of Nutrition.

He is also Director of Nutrition Support Services, Chief of the Nutritional/Metabolism Laboratory, and Director of the Center for the Study of Nutrition and Medicine at the Beth Israel Deaconess Medical Center.

Dr. Blackburn is President of the North American Association for the Study of Obesity, and immediate Past President of the American Society for Clinical Nutrition.

A Fellow in the American College of Surgery, he chairs the Massachusetts Medical Society Committee on Nutrition, and the Scientific Advisory Committee for Shape Up America! (Koop Foundation).


POPULAR LOW-CARBOHYDRATE diets such as those described in such best-selling books as The Zone and Dr. Atkins’ New Diet Revolution can turn weight loss into a double-edged sword. These plans produce fast results relatively easily, without restricting intake in proteins and fats, but they can jeopardize health in a variety of ways.

Physicians treating patients for obesity related conditions have a unique opportunity to influence patients’ food choices by provoking reliable, objective information about the safety and efficacy of low-carbohydrate diets.

This paper addresses common claims made by proponents of low-carbohydrate diets and discusses what to tell patients who are already on such a diet or may be thinking of trying one.


In 1980, 46% of US adults age 20 and older were overweight or obese; by 1999, the number had increased to 60%.1 This dramatic increase has coincided with several trends:

• Higher energy intake from larger portion at home and at restaurants (“super-sizing”)

• Greater consumption of high-fat foods

• Widespread availability of low-cost, good-tasting, energy-dense foods

• Decreased physical activity at work, at home, and during leisure time.

A growing national preoccupation with weight loss has accompanied these trends. At any given time, 44% of women and 29% of men are dieting,2 and Americans spend $3 billion a year on weight-loss products, programs, and pills.3

Books on low-carbohydrate diets far out-sell others books on weight loss.3 The two books already mentioned seem to be the most popular; others include Sugar Busters, Protein Power, Suzanne Somers Get Skinny on Fabulous Food, The Doctor’s Quick Weight Loss Diet (aka the “Stillman diet”), and The Carbohydrate Addict’s Diet.


Compared with national guidelines for healthy eating and weight loss, low-carbohydrate diets contain excessive amounts of cholesterol, saturated fat, and animal protein. The Atkins and Protein Power diets are particularly high in fat. TABLE 1 shows how the macronutrient composition of the leading low-carbohydrate diets differs from the American Diabetes Association recommendations and the American Heart Association’s dietary guidelines for the year 2000.5

Nutrients missing from low-carbohydrate diets

Micronutrients. Cutting back on entire food groups or restricting variety can lead to deficiencies in vitamins, minerals, and other essential micronutrients.3 Carbohydrate-rich foods can be excellent sources of fiber, vitamins (B, C, and E), carotenoids, and other beneficial phytochemicals. They also provide calcium, potassium, and the majority of trace minerals. Supplements can replace some, but not all of these.

Fiber. Low intake of fiber can cause constipation and may contribute to the development of hemorrhoids, diverticulosis, polyps, colon cancer, heart disease, diabetes, and obesity. The health benefits of phytochemicals (eg, carotenoids, lycopenes, flavonoids, phytic acid, indoles, isothiocyanates)19 and fiber, for example, can only be obtained from foods. Due to poor intake of high-fiber breads, cereals, and vegetables, dieters need to take fiber supplements or eat fiber-fortified foods to avoid constipation and concentration of bile salts and chemicals that cause colon and breast cancer.

Complex carbohydrates. Carbohydrates are generally classified as simple (sugars) or complex (starches). Simple carbohydrates either occur naturally or are refined and added to foods during or after processing. Foods high in complex carbohydrates (whole grains, vegetables, beans, fruits) are rich in fiber and other nutrients and are relatively low in calories. Processed foods based on refined starch and simple sugars (sugar, soft drinks, cookies, donuts, cakes, sweetened cereals, white bread, pretzels) are generally high in calories and low in fiber and other nutrients.

Diets deficient in complex carbohydrates are likely to be nutrient-poor. Weight- loss plans that restrict high-carbohydrate foods can lead to cravings for foods that are high in sugar and fat.13 Diets high in simple carbohydrates can lead to hypertriglyceridemia.20


Ketogenesis may cause the following conditions:

• Mild dehydration, which can cause dizziness, headaches, confusion, nausea, fatigue, sleep problems, irritability, bad breath, and worsening of gout symptoms and existing kidney problems

• Poor athletic performance from the depletion of stored glycogen: insulin is required for protein synthesis, and without insulin, muscle protein synthesis after exercise is impaired7

• Increased risk of osteoporosis from calcium loss if protein intake remains high and calcium intake is low21; a high ratio of animal to vegetable protein intake may increase bone loss and the risk of hip fracture in elderly women22

• Nausea may at first suppress the appetite, but the effect might not be long-term23

• Inability to maintain weight loss due to cravings and boredom with the lack of variety of foods; if the dieter “cheats,” a surge of insulin can cause sodium and water retention and subsequent weight gain

• Rising blood pressure with age due to the deficit of high-carbohydrate, high-fiber foods that protect against high blood pressure2

• Orthostatic hypotension due to rapid weight loss25; this can put older patients who are already at risk for falls at an increased risk of injury.


Weight loss can occur only if caloric expenditure exceeds caloric intake. Caloric intake is reduced only by limiting the intake of one or more of the macronutrients (protein, carbohydrates, fat, and alcohol).26

Low-carbohydrate diets provide an average of 1,450 kcal/day. 4 Experts agree that the safest minimum caloric intake for people on medically unsupervised diets is 1,500 kcal/day for men and 1,200 kcal/day for women.5’9

Since each pound of body fat represents 3,500 kcal, a man who takes in 500 kcal less per day than he expends can lose only 1 Ib in a week. A man who weighs 200 Ib requires approximately 3,000 kcal/day to maintain his current weight, assuming a caloric intake requirement of 15 kcal per Ib of body weight. If a man with moderate physical activity who weighs 200 Ibs were to eat absolutely nothing for a week, the most weight that he could lose from fat would be 6 Ibs.

The initial, rapid weight loss experienced on low-carbohydrate diets, therefore, cannot come from fat alone. Instead, it comes from the loss of water and electrolytes produced by natriuresis (not by ketosis), which results from a decline in insulin. Water loss also results from the breakdown of liver glycogen (stored carbohydrates).27 In the absence of dietary carbohydrates, glycogen is converted to glucose to maintain blood sugar levels. Glycogen contains a large number of water molecules, and water that is the byproduct of the conversion of glycogen to glucose is excreted in quantities sufficient to contribute to the high initial weight loss.

If the average intake for an obese person who weighs 300 Ibs is about 4,500 kcal/day,28 cutting back to anything below 2,000 kcal/day is a severe restriction (semi-starvation, in fact) and is very hard to maintain. A low-carbohydrate diet may actually help control hunger in this situation, owing to its high protein content, induced ketosis, or both.

Dr. Atkins claims that caloric intake is not important for weight loss or weight gain, and that a “high insulin level directly lowers energy needs. “His theory that calories are unimportant and that “you can, in fact, sneak them out of your body unused, or dissipated as heat” has not been proven.14 Studies have shown that there is indeed a greater weight loss on a ketogenic diet than with a mixed (balanced) diet, but that the loss was almost entirely due to fluid, not fat.29-3

In subsequent weeks of a ketogenic diet, most of the weight loss is from body fat and averages 1 to 2 Ibs per week. This weekly average, however, is similar to other types of low-calorie diets. Caloric intake drops because most of the high-carbohydrate foods eliminated are also very high in calories: cake, cookies, bread, chips, fries, sweetened cereal, candy. In other words, low-carbohydrate diets work because overall caloric intake is decreased, and perhaps also because high fat intake or ketosis depresses the appetite.30 Nausea, however, may accompany ketogenic appetite suppression.28


Patients on a low-carbohydrate diet should be monitored for orthostatic hypotension (supine blood pressure vs. standing blood pressure), dizziness, headaches, fatigue, irritability, gout, and kidney failure. Laboratory work includes routine blood tests (glucose, blood urea nitrogen, sodium, potassium, chloride, and bicarbonate), urinalysis (specific gravity, pH, protein, and acetone), and a lipid profile.

Vital signs and the rate of weight loss should be monitored at least monthly during a low-carbohydrate weight-loss program. Dosages of medications being taken for obesity-related comorbidities (hypertension, diabetes, coagulopathies, gout) may need to be adjusted.

Monitor vital signs and weight at least monthly in patients on low-carbohydrate diets.


Follow-up visits are a good opportunity for physicians and dietitians to educate patients about realistic weight management and safe and unsafe dietary practices. Have brochures and charts (eg, the USDA Food Guide Pyramid) on hand to give to patients.

More information on how to advise patients who are ready to try a safer and more effective weight-loss program can be found in “How to help your patient lose weight” in the Cleveland Clinic Journal of Medicine.26

An important first step in advising patients is to assess their readiness to question the merits of low-carbohydrate diets. When discussing low-carbohydrate diets with patients, stress these points:

• Initial “fast” weight lost is water, not fat

• These diets are deficient in nutrients that cannot be replaced by supplements and are excessive in nutrients that may increase the risk of mortality and chronic disease

• These diets are difficult to adhere to because they lack variety and increase the desire to consume high-carbohydrate, high-fat foods. It is very difficult to stay on a diet that includes less than 100g of carbohydrate per day in the long term, considering that the typical American diet contains about 275 g/day

• Ketogenic diets are associated with adverse effects • A diet low in fruits, vegetables, and whole grains increases the risk of heart disease, cancer, and stroke

• Adherence to official dietary guidelines, such as those of the American Heart Association,5 provides a basis for healthy living and weight loss

• Obesity-related conditions improve with a weight loss of only 5% to 10%, even though a weight loss of 30% may be needed to reach the ideal body weight. A 5% reduc tion in weight maintained for 1 year is considered successful long-term weight loss.9 Encourage patients to see permanent weight loss as their goal.


A common perception is that the diets that get the most press coverage are the most popular. But national nutrition surveys such as the National Health and Nutrition Examination Survey (NHANES) and the USDA’s Continuing Survey of Food Intakes by Individuals (CSFSII) indicate that, if we define “most popular” as most widely liked and most prevalent, then the most popular diets are in fact balanced diets.

It appears that a combination of a low-fat, low-energy diet along with increased energy expenditure is the most successful method for maintaining weight loss in the long term. The National Weight Control Registry (NWCR), which tracks people who lose weight successfully (loss of 30 lbs or more, maintained for at least 1 year), echoes this finding. The participants lost weight and maintained their weight loss by voluntarily consuming a high-fiber, low-fat diet and by exercising regularly. Walking was the most frequently cited physical activity.

A study of initial enrollees in the NWCR revealed that the average caloric intake was about 1,400 kcal/day, with 24% of calories from fat and 56% from carbohydrates.31 The foods consumed by these dieters are the same as those in the 2000 American Heart Association dietary guidelines.5 There are no reports of such success with low-carbohydrate diets.31 •


There is no evidence that low-carbohydrate diets are effective for long-term weight management, and their long-term safety is questionable and unproven. Long-term compliance also needs to be investigated; humans desire a variety of foods, and therefore diets that restrict variety are destined to fail.

Low-carbohydrate diets fail because, like all fad diets, they do not deal with the under lying issues of being overweight, nor do they teach better lifelong habits. As Denise Bruner, MD, stated at the USDA debate on nutrition, in February 2000, “Weight reduction must focus on the whole life-style-not solely diet.“35 Ultimately, there is no escaping the fact that weight loss boils down to eating less and moving more.

Given these facts, we recommend refer ring patients to a registered dietitian who can provide guidance in accordance with the American Heart Association dietary guide lines and an individualized plan that takes the patient’s food preferences into consider ation. K Acknowledgment. We acknowledge Rita Buckley for her edito rial assistance.


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