REVIEW
GEORGE L. BLACKBURN, MD, PhD JUDY C.C. PHILLIPS, MS, RD SUSAN MORREALE, CHES
Associate Professor of Surgery and Nutrition, S. Daniel Program Manager, Center for Nutritional Research Project Manager, Center for Nutritional Research
Abraham Chair in Nutrition Medicine, Associate Director Charitable Trust; Content Manager, Centers for Obesity Charitable Trust; Program Coordinator, Centers for Obesity
of Nutrition, Division of Nutrition, Harvard Medical Research and Education, Wellesley, MA Research and Education, Wellesley, MA
School; Director, Nutrition Support Service, Director,
Center for the Study of Nutrition Medicine; Program
Director, Surgical Therapy for Severe Obesity; Chief,
Nutrition Metabolism Laboratory, Beth Israel Deaconess
Medical Center, Boston
Physician’s guide to popular
low-carbohydrate weight-loss diets
s A B S T R AC T P OPULAR LOW-CARBOHYDRATE diets such
as those described in such best-selling
Low-carbohydrate weight-loss diets are very popular, but books as The Zone and Dr. Atkins’ New Diet
the recommendations of many of these diets are Revolution can turn weight loss into a double-
diametrically opposed to those put forth by the US edged sword. These plans produce fast results
Department of Agriculture, the American Heart Association, relatively easily, without restricting intake of
and other national organizations. Their focus on foods high proteins and fats, but they can jeopardize
in protein, fat, and cholesterol has potentially serious health health in a variety of ways.
Physicians treating patients for obesity-
implications. Physicians need to be knowledgeable about
related conditions have a unique opportunity
the efficacy of these programs and to talk to overweight to influence patients’ food choices by provid-
patients about weight loss. ing reliable, objective information about the
safety and efficacy of low-carbohydrate diets.
s KEY POINTS
See related commentary, pages 777-781.
Low-carbohydrate diets fail because, like all fad diets, they
do not deal with the underlying issues of being overweight, This paper addresses common claims made
nor do they teach better lifelong eating habits. by proponents of low-carbohydrate diets and
discusses what to tell patients who are already
An important first step in advising patients who are already on such a diet or may be thinking of trying one.
on a low-carbohydrate diet is to assess their readiness to
question the merits of such diets. s THE ’SUPER-SIZING’ OF AMERICA
In 1980, 46% of US adults age 20 and older
Questions remain about the possible association of low-
were overweight or obese; by 1999, the num-
carbohydrate diets with the risk of colon cancer, heart ber had increased to 60%.1 This dramatic
disease, diabetes, and hypertriglyceridemia. increase has coincided with several trends:
• Higher energy intake from larger portions
Each pound of body fat contains 3,500 kcal; therefore, a at home and at restaurants (“super-sizing”)
person who consumes 500 kcal less than he or she expends • Greater consumption of high-fat foods
per day can lose only 1 lb of fat in 1 week. Any higher • Widespread availability of low-cost, good-
initial weight loss with ketogenic diets is therefore due to tasting, energy-dense foods
more severe caloric restriction or water loss rather than to • Decreased physical activity at work, at
fat loss. 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 $33
PATIENT INFORMATION billion a year on weight-loss products, pro-
What you should know about low-carbohydrate diets, page 775 grams, and pills.3
CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 68 • NUMBER 9 SEPTEMBER 2001 761
LOW-CARBOHYDRATE DIETS BLACKBURN AND COLLEAGUES
Not available for online publication.
See print version of the
Cleveland Clinic Jour nal of Medicine
Books on low-carbohydrate diets far out- Atkins Induction Diet to 170 g/day in the
sell others books on weight loss.3 The two Zone diet (TABLE 1). This is in stark contrast to
books already mentioned seem to be the most the American Heart Association’s recommen- 60% of adult
popular; others include Sugar Busters, Protein dation that carbohydrates should account for
Power, Suzanne Somers’ Get Skinny on Fabulous 55% to 60% of total daily caloric intake: 275
Americans are
Food, The Doctor’s Quick Weight Loss Diet (aka g/day for a diet of 2,000 kcal and 300 g/day for overweight
the “Stillman diet”), and The Carbohydrate a diet of 2,500 kcal.5
Addict’s Diet.3
or obese
CLAIM 2
s FALSE CLAIMS A state of perpetual ketosis causes weight
OF LOW-CARBOHYDRATE DIETS loss, regardless of calories consumed
CLAIM 1 Low-carbohydrate, high-protein diets are
The main cause of obesity called “ketogenic” because they cause the
is the shift from foods that contain fat to body to eventually burn fat for energy. Ketosis
processed foods that replace fat with sugar is the accumulation in the blood of ketones,
byproducts of fat oxidation, and it represents
Proponents of low-carbohydrate diets claim the body’s adaptation to fasting or starvation.
that the main cause of accelerated weight gain The theory behind low-carbohydrate diets is
in the United States is the shift from foods that that inducing perpetual ketosis causes the per-
contain fat to foods that replace fat with sugar son to lose weight (fat) regardless of how many
(ie, processed foods), a substitution they say calories from protein and fat are consumed.
leads to high insulin levels and fat accumula- However, weight loss can occur only if
tion.3 Not so: excessive energy intake—not caloric expenditure exceeds caloric intake.
diet composition—is the cause of weight gain.4 Furthermore, the level of carbohydrates need-
Low-carbohydrate diets restrict carbohy- ed to maintain ketosis is much less than either
drate intake to anywhere from 20 g/day in the the 275 g/day consumed by Americans on
CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 68 • NUMBER 9 SEPTEMBER 2001 765
LOW-CARBOHYDRATE DIETS BLACKBURN AND COLLEAGUES
average4 or the American Heart Association’s gerated hyperinsulinemia and glucose
recommended 220 g/day ( TABLE 1). The intolerance.
Ongoing Weight Loss Stage of the Atkins The glycemic index—a measure of the
diet, for instance, limits carbohydrate intake rise in blood glucose over a specified period of
to 20 to 40 g/day while allowing unlimited time (usually 2 hours) vs the response to an
amounts of meat, cheese, poultry, fish, eggs, equal amount of carbohydrate in a standard
salt, and fats, a recommendation that over- food (often white bread)—is a more pertinent
looks the total (or almost total) inability of way to assess how much insulin the body
the human body to convert fatty acids to glu- secretes in response to various foods.10–12
cose, the primary source of energy for the
human brain.6 CLAIM 6
Low-carbohydrate diets
CLAIM 3 have specific cardiovascular benefits
Low-carbohydrate diets are new
Dr. Atkins claims that those who follow his
Most low-carbohydrate diets are touted as regimen appear to have lower cardiovascular
new, but they are not. English surgeon risk, lower blood pressure, and significantly
William Harvey prescribed such diets for the lower triglyceride levels.13,14 No long-term
treatment of obesity in 1872.7 studies substantiate this claim. In fact, any
clinically significant weight loss (5% to 10%
CLAIM 4 of initial body weight) can have these
Ketogenic diets are safe effects.
Furthermore, animal and dairy products,
Ketosis from prolonged fasting in healthy peo- the main sources of protein in low-carbohy-
ple increases insulin resistance and glucose drate diets, usually contain fat. Even though
intolerance.8 Insulin resistance—a state in some of the fat can be removed, as with skim
which a given concentration of insulin is less milk, low-carbohydrate diets tend to be high
Key sources of effective both at stimulating glucose uptake by in fat overall. The intake of fat with low-car-
skeletal muscle and at restraining hepatic glu- bohydrate diets, particularly saturated fat,
protein in low- cose production—plays a central role in many increases to 56% to 66% of total calories—
carbohydrate disease states (eg, insulin resistance/metabolic twice the 30% or less recommended in current
syndrome, type 2 diabetes, hypertension, car- national dietary guidelines. Excessive intake of
diets usually diovascular disease, atherosclerotic cardiovas- dietary cholesterol and, to a greater extent, sat-
contain fat cular disease) and is a major risk factor for the urated fat increases levels of low-density
development of coronary artery disease, the lipoprotein (LDL) cholesterol and the risk of
chief cause of morbidity and mortality in heart disease and some types of cancer.15
patients with type 2 diabetes. Glucose intoler- Consumption of large amounts of meat may
ance has been linked to hypertension and dys- also contribute to cardiovascular disease.16
lipidemia.5,9
CLAIM 7
CLAIM 5 Low-carbohydrate diets are high
Eating carbohydrates leads to overeating in protein and therefore are healthier
Promoters of low-carbohydrate diets main- Low-carbohydrate diets are not necessarily
tain that carbohydrates raise insulin levels high in protein, as claimed. A comparison of
more than other foods do, thereby causing dietary intake among persons who consumed
the overeating that leads to obesity. In fact, a low-carbohydrate diet vs those who con-
insulin is secreted in reaction to all foods, sumed a typical American diet4 found scant
not only those containing carbohydrates. difference in protein intake (91 g/day vs 83
However, some responses are physiologic g/day). The low-carbohydrate group, however,
while others are pathologic; overeating consumed only about two thirds as many calo-
contributes to the latter by causing exag- ries as the group eating a typical American
766 CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 68 • NUMBER 9 SEPTEMBER 2001
LOW-CARBOHYDRATE DIETS BLACKBURN AND COLLEAGUES
diet (1,450 kcal vs 2,200 kcal), indicating that acid, indoles, isothiocyanates)19 and fiber, for
weight loss was due to reduced caloric intake, example, can only be obtained from foods.
not to high protein consumption.4 Due to poor intake of high-fiber breads, cere-
On the other hand, no direct link has yet als, and vegetables, dieters need to take fiber
been found between consumption of animal supplements or eat fiber-fortified foods to
protein and chronic disease. Though critics of avoid constipation and concentration of bile
high-protein ketogenic diets claim that the salts and chemicals that cause colon and
diets increase the risk of gout, osteoporosis, breast cancer.
and renal disease, they have no evidence to Complex carbohydrates. Carbohydrates
back those claims.17 In fact, obese persons are generally classified as simple (sugars) or
with diabetes may benefit from high-protein, complex (starches). Simple carbohydrates
low-calorie diets. In a study that compared either occur naturally or are refined and added
the effects of high-carbohydrate (low-pro- to foods during or after processing. Foods high
tein) vs high-protein (low-carbohydrate) low- in complex carbohydrates (whole grains, veg-
calorie diets for hyperinsulinemic obese etables, beans, fruits) are rich in fiber and
patients, high-protein diets proved more other nutrients and are relatively low in calo-
effective at lowering insulin levels and body ries. Processed foods based on refined starch
weight.18 and simple sugars (sugar, soft drinks, cookies,
donuts, cakes, sweetened cereals, white bread,
s COMPOSITION OF STANDARD pretzels) are generally high in calories and low
VS LOW-CARBOHYDRATE DIETS in fiber and other nutrients.
Diets deficient in complex carbohy-
Compared with national guidelines for drates are likely to be nutrient-poor. Weight-
healthy eating and weight loss, low-carbohy- loss plans that restrict high-carbohydrate
drate diets contain excessive amounts of cho- foods can lead to cravings for foods that are
lesterol, saturated fat, and animal protein. The high in sugar and fat.13 Diets high in simple
Atkins and Protein Power diets are particular- carbohydrates can lead to hypertriglyc-
Obesity-related ly high in fat. TABLE 1 shows how the macronu- eridemia.20
trient composition of the leading low-carbo-
conditions hydrate diets differs from the American s POTENTIAL ADVERSE EFFECTS
improve with Diabetes Association recommendations and OF KETOGENIC DIETS
the American Heart Association’s dietary
a weight guidelines for the year 2000.5 Ketogenesis may cause the following condi-
loss of tions:
Nutrients missing • Mild dehydration, which can cause dizzi-
only 5% to 10% from low-carbohydrate diets ness, headaches, confusion, nausea, fatigue,
Micronutrients. Cutting back on entire sleep problems, irritability, bad breath, and
food groups or restricting variety can lead to worsening of gout symptoms and existing kid-
deficiencies in vitamins, minerals, and other ney problems
essential micronutrients.3 Carbohydrate-rich • Poor athletic performance from the
foods can be excellent sources of fiber, vita- depletion of stored glycogen: insulin is
mins (B, C, and E), carotenoids, and other required for protein synthesis, and without
beneficial phytochemicals. They also provide insulin, muscle protein synthesis after exercise
calcium, potassium, and the majority of trace is impaired7
minerals. Supplements can replace some but • Increased risk of osteoporosis from calci-
not all of these. um loss if protein intake remains high and cal-
Fiber. Low intake of fiber can cause con- cium intake is low21; a high ratio of animal to
stipation and may contribute to the develop- vegetable protein intake may increase bone
ment of hemorrhoids, diverticulosis, polyps, loss and the risk of hip fracture in elderly
colon cancer, heart disease, diabetes, and obe- women22
sity. The health benefits of phytochemicals • Nausea may at first suppress the appetite,
(eg, carotenoids, lycopenes, flavonoids, phytic but the effect might not be long-term23
768 CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 68 • NUMBER 9 SEPTEMBER 2001
• Inability to maintain weight loss due to If the average intake for an obese person
cravings and boredom with the lack of variety who weighs 300 lbs is about 4,500 kcal/day,28
of foods; if the dieter “cheats,” a surge of cutting back to anything below 2,000 kcal/day
insulin can cause sodium and water retention is a severe restriction (semi-starvation, in
and subsequent weight gain fact) and is very hard to maintain. A low-car-
• Rising blood pressure with age due to the bohydrate diet may actually help control
deficit of high-carbohydrate, high-fiber foods hunger in this situation, owing to its high pro-
that protect against high blood pressure24 tein content, induced ketosis, or both.
• Orthostatic hypotension due to rapid Dr. Atkins claims that caloric intake is
weight loss25; this can put older patients who not important for weight loss or weight gain,
are already at risk for falls at an increased risk and that a “high insulin level directly lowers
of injury. energy needs.”14 His theory that calories are
unimportant and that “you can, in fact, sneak
s WHAT ACCOUNTS FOR THE WEIGHT LOSS them out of your body unused, or dissipated as
IN KETOGENIC DIETS? heat” has not been proven.14 Studies have
shown that there is indeed a greater weight
Weight loss can occur only if caloric expendi- loss on a ketogenic diet than with a mixed
ture exceeds caloric intake. Caloric intake is (balanced) diet, but that the loss was almost
reduced only by limiting the intake of one or entirely due to fluid, not fat.29–34
more of the macronutrients (protein, carbohy- In subsequent weeks of a ketogenic diet,
drates, fat, and alcohol).26 most of the weight loss is from body fat and
Low-carbohydrate diets provide an aver- averages 1 to 2 lbs per week. This weekly aver-
age of 1,450 kcal/day.4 Experts agree that the age, however, is similar to other types of low-
safest minimum caloric intake for people on calorie diets. Caloric intake drops because most
medically unsupervised diets is 1,500 kcal/day of the high-carbohydrate foods eliminated are
for men and 1,200 kcal/day for women.5,9 also very high in calories: cake, cookies, bread,
Since each pound of body fat represents 3,500 chips, fries, sweetened cereal, candy. In other
kcal, a man who takes in 500 kcal less per day words, low-carbohydrate diets work because Monitor
than he expends can lose only 1 lb in a week. overall caloric intake is decreased, and perhaps
A man who weighs 200 lb requires approxi- also because high fat intake or ketosis depresses
vital signs
mately 3,000 kcal/day to maintain his current the appetite.30 Nausea, however, may accom- and weight
weight, assuming a caloric intake requirement pany ketogenic appetite suppression.28
of 15 kcal per lb of body weight. If a man with
at least
moderate physical activity who weighs 200 lbs s MONITORING FOR ADVERSE EFFECTS monthly in
were to eat absolutely nothing for a week, the OF LOW-CARBOHYDRATE DIETS
most weight that he could lose from fat would
patients on
be 6 lbs. Patients on a low-carbohydrate diet should be low-
The initial, rapid weight loss experienced monitored for orthostatic hypotension
on low-carbohydrate diets, therefore, cannot (supine blood pressure vs standing blood pres-
carbohydrate
come from fat alone. Instead, it comes from sure), dizziness, headaches, fatigue, irritability, diets
the loss of water and electrolytes produced by gout, and kidney failure. Laboratory work
natriuresis (not by ketosis), which results from includes routine blood tests (glucose, blood
a decline in insulin. Water loss also results urea nitrogen, sodium, potassium, chloride,
from the breakdown of liver glycogen (stored and bicarbonate), urinalysis (specific gravity,
carbohydrates).27 In the absence of dietary pH, protein, and acetone), and a lipid profile.
carbohydrates, glycogen is converted to glu- Vital signs and the rate of weight loss
cose to maintain blood sugar levels. Glycogen should be monitored at least monthly during
contains a large number of water molecules, a low-carbohydrate weight-loss program.
and water that is the byproduct of the conver- Dosages of medications being taken for obe-
sion of glycogen to glucose is excreted in sity-related comorbidities (hypertension,
quantities sufficient to contribute to the high diabetes, coagulopathies, gout) may need to
initial weight loss. be adjusted.
CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 68 • NUMBER 9 SEPTEMBER 2001 769
LOW-CARBOHYDRATE DIETS BLACKBURN AND COLLEAGUES
s WHAT TO TELL PATIENTS Encourage patients to see permanent
ON A LOW-CARBOHYDRATE DIET weight loss as their goal.
Follow-up visits are a good opportunity for s NATIONAL NUTRITION SURVEYS
physicians and dietitians to educate patients
about realistic weight management and safe A common perception is that the diets that
and unsafe dietary practices. Have get the most press coverage are the most pop-
brochures and charts (eg, the USDA Food ular. But national nutrition surveys such as the
Guide Pyramid) on hand to give to patients. National Health and Nutrition Examination
More information on how to advise patients Survey (NHANES) and the USDA’s
who are ready to try a safer and more effec- Continuing Survey of Food Intakes by
tive weight-loss program can be found in Individuals (CSFSII) indicate that, if we
“How to help your patient lose weight” in define “most popular” as most widely liked and
the Cleveland Clinic Journal of Medicine.26 most prevalent, then the most popular diets
(Also, see the Patient Information page at are in fact balanced diets.
the end of this article: “What you should It appears that a combination of a low-fat,
know about low-carbohydrate diets,” page low-energy diet along with increased energy
775.) expenditure is the most successful method for
An important first step in advising maintaining weight loss in the long term. The
patients is to assess their readiness to question National Weight Control Registry (NWCR),
the merits of low-carbohydrate diets. When which tracks people who lose weight success-
discussing low-carbohydrate diets with fully (loss of 30 lbs or more, maintained for at
patients, stress these points: least 1 year), echoes this finding. The partici-
• Initial “fast” weight lost is water, not fat pants lost weight and maintained their weight
• These diets are deficient in nutrients that loss by voluntarily consuming a high-fiber, low-
cannot be replaced by supplements and fat diet and by exercising regularly. Walking
are excessive in nutrients that may was the most frequently cited physical activity.
increase the risk of mortality and chronic A study of initial enrollees in the NWCR See patient
disease revealed that the average caloric intake was information
• These diets are difficult to adhere to about 1,400 kcal/day, with 24% of calories from
because they lack variety and increase fat and 56% from carbohydrates.31 The foods page 775
the desire to consume high-carbohy- consumed by these dieters are the same as those
drate, high-fat foods. It is very difficult to in the 2000 American Heart Association
stay on a diet that includes less than 100 dietary guidelines.5 There are no reports of such
g of carbohydrate per day in the long success with low-carbohydrate diets.31
term, considering that the typical
American diet contains about 275 g/day s CONCLUSIONS AND RECOMMENDATIONS
• Ketogenic diets are associated with
adverse effects There is no evidence that low-carbohydrate
• A diet low in fruits, vegetables, and whole diets are effective for long-term weight man-
grains increases the risk of heart disease, agement, and their long-term safety is ques-
cancer, and stroke tionable and unproven. Long-term compli-
• Adherence to official dietary guidelines, ance also needs to be investigated; humans
such as those of the American Heart desire a variety of foods, and therefore diets
Association,5 provides a basis for healthy that restrict variety are destined to fail.
living and weight loss Low-carbohydrate diets fail because, like
• Obesity-related conditions improve with a all fad diets, they do not deal with the under-
weight loss of only 5% to 10%, even though lying issues of being overweight, nor do they
a weight loss of 30% may be needed to teach better lifelong habits. As Denise Bruner,
reach the ideal body weight. A 5% reduc- MD, stated at the USDA debate on nutrition,
tion in weight maintained for 1 year is con- in February 2000, “Weight reduction must
sidered successful long-term weight loss.9 focus on the whole life-style—not solely the
CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 68 • NUMBER 9 SEPTEMBER 2001 773
LOW-CARBOHYDRATE DIETS BLACKBURN AND COLLEAGUES
diet.”35 Ultimately, there is no escaping the American Heart Association dietary guide-
fact that weight loss boils down to eating less lines and an individualized plan that takes
and moving more. the patient’s food preferences into consider-
Given these facts, we recommend refer- ation.
ring patients to a registered dietitian who can Acknowledgment. We acknowledge Rita Buckley for her edito-
provide guidance in accordance with the rial assistance.
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