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Low-Carbohydrate Diets
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     1.Medical College of Wisconsin,2.Racine Family Medicine Residency Program, Racine, Wisconsin

    3.University of Pittsburgh Medical Center,

    Americans spend $33 billion annually on weight loss products and services, and a large portion of this money is spent on low-carbohydrate diets. Because of their higher protein and fat content and lower fiber and carbohydrate content, concerns have been raised about the potential health consequences of low-carbohydrate diets. Published long-term data are lacking. Short-term studies comparing traditional low-fat diets with low-carbohydrate diets found lower triglyceride levels, higher high-density lipoprotein cholesterol levels, similar low-density lipoprotein cholesterol levels, and lower A1C levels in persons on low-carbohydrate diets. These diets induce greater weight loss at three and six months than traditional low-fat diets; however, by one year there is no significant difference in maintained weight loss. Weight loss is directly related to calorie content and the ability to maintain caloric restriction; the proportions of nutrients in the diet are irrelevant. Low-carbohydrate diets had lower dropout rates than low-fat diets in several studies, possibly because of the high protein content and low glycemic index, which can be appetite suppressing. Data indicate that low-carbohydrate diets are a safe, reasonable alternative to low-fat diets for weight loss. Additional studies are needed to investigate the long-term safety and effectiveness of these and other approaches to weight loss. (Am Fam Physician 2006;73:1942-8, 1951. Copyright ? 2006 American Academy of Family Physicians.)

    Obesity is a major public health risk in the United States, where 65 percent of adults are overweight (i.e., they have a body mass index [BMI] of 25 kg per m2 or greater).1 The prevalence of obesity in the United States was 14.5 percent from 1976 to 19802 and has since risen to 30.5 percent.1 The percentage of children who are overweight (i.e., BMI in the 95th percentile or greater for age and sex) is at an all-time high: 10.4 percent of two- to five-year-olds, 15.3 percent of six- to 11-year-olds, and 15.5 percent of 12- to 19-year-olds,3 based on growth charts from 1979.4 Black and Hispanic children are more likely to be overweight than white children (21.5, 21.8, and 12.3 percent, respectively).5

    A = consistent, good-quality patient-oriented evidence; B = inconsistent or limited-quality patient-oriented evidence; C = consensus, disease-oriented evidence, usual practice, expert opinion, or case series. For information about the SORT evidence rating system, see page 1874 or http://www.aafp.org/afpsort.xml.

    Approximately 365,000 U.S. deaths in 2000 were attributed to poor diet and physical inactivity (15.2 percent of total deaths), the second leading cause of death for that year.6 Obesity-related medical expenditures were estimated to be $75 billion in 2003, approximately one half of which was financed by Medicare and Medicaid.7

    At any given time, 45 percent of women and 30 percent of men in the United States are trying to lose weight.8 However, less than 25 percent of these persons actually reduce their caloric intake and increase their activity level.8 Consumers spend $33 billion annually on weight loss products, diets, and services,8 and a significant proportion of this money is spent on low-carbohydrate diets.

    Definition of Low-Carbohydrate Diet

    Low-carbohydrate diets restrict caloric intake by reducing the consumption of carbohydrates to 20 to 60 g per day (typically less than 20 percent of the daily caloric intake). The consumption of protein and fat is increased to compensate for part of the calories that formerly came from carbohydrates (Table 1).9 The Atkins Diet10 is the prototypical low-carbohydrate diet. This type of diet differs from ketogenic diets used for seizure prevention, which substitute fat for protein and carbohydrates.

    note: Nutrient breakdowns are based on approximate average daily intake and therefore may not total 100 percent of daily calories.

    Adapted with permission from Freedman MR, King J, Kennedy E. Popular diets: a scientific review. Obes Res 2001; 9(suppl 1):3S,11S,12S.

    Diets such as the Zone Diet,11 Carbohydrate Addict's Diet,12 and later phases of the South Beach Diet13 restrict carbohydrates to 40 percent of calories or less, and they focus more on the glycemic index of foods than the Atkins Diet.

    Glycemic Index

    The glycemic index is a rating system for foods based on the extent to which they raise blood sugar levels in the two hours after they are eaten (Table 214,15). The reference point is pure glucose or white bread, which is arbitrarily scored as 100. The higher the glycemic index, the more rapidly that carbohydrate is released into the bloodstream as glucose.14

    Information from reference 36.

    Obesity results from the interplay between genes and environment; their contributions vary with each person. It is therefore reasonable to suspect that different types of diets (e.g., low-carbohydrate, low-fat, very low-fat) may be of varying benefit in different persons. Part of the solution may be to pair the most appropriate diet with each patient based on eating habits, patterns, and desires.

    Part of the genetic component of obesity is insulin resistance, the probable common pathway for metabolic syndrome (Table 336). Low-carbohydrate diets may be particularly helpful in patients with metabolic syndrome.

    The physiologic effects of consuming low-glycemic-index foods (e.g., lower insulin levels, less hunger)14 may explain why persons who successfully lose weight with low-carbohydrate diets take in fewer calories.23 Scant evidence exists that low-carbohydrate diets result in weight loss because of increased calorie use via ketogenesis. However, if low-carbohydrate diets safely modify insulin response and glucose metabolism and decrease caloric consumption,14 they are a reasonable alternative for persons who are willing to adhere to these diets.

    Adherence is the key to long-term success for safe, effective, lasting weight loss. There is no way to predict which patients will benefit most from certain types of diets, and attempts to create an optimal patient-diet dyad can be only hypothesized based on each patient's medical and diet histories and laboratory findings. An individualized approach probably is the best solution. Caloric intake must be less than caloric expenditure to achieve weight loss. A low-carbohydrate diet combined with an exercise program can help selected patients safely achieve weight loss and improve their biochemical profiles.

    Cautions

    None of the diets discussed in this article have been studied in a controlled clinical trial for longer than one year, and no study has measured clinical outcomes such as disease-specific mortality, cardiovascular events, and all-cause mortality. A low-carbohydrate diet that limits the intake of fruits, vegetables, and legumes cannot be endorsed, and physicians should counsel patients to exercise regularly as part of any health maintenance program.

    references

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