Research demonstrates that weight loss is associated with the length of the diet, pre-diet weight (people who are more overweight tend to lose more weight, more quickly than those who are only mildly overweight), and the number of calories consumed. Any diet that restricts caloric intake such that calories consumed are less than those expended will promote short-term weight loss. The key to weight loss through diet is adherence—if people do not stick to their diets, then they will not lose weight. As early as the 1860s, William Banting, describing the benefits of his low-carbohydrate diet, wrote that "The great charms and comfort of the system are that its effects are palpable within a week of trial and creates a natural stimulus to persevere for a few weeks more."

The successes achieved using regimens that restrict dieters to a single food or food group such as grapefruits, pineapples, or cabbage are probably in part attributable to the human hankering for variety. Limited to just one food, most dieters experience boredom—there is just no appeal to eating the same food at every meal, for days on end, so naturally, less food is consumed. In addition, these diets generally rely on low-calorie foods, so that even if dieters were inspired to consume ten grapefruits per day, their total daily caloric consumption would be about 1,200 calories, which is sufficient to produce weight loss for most people who are overweight. Similarly, diets that involve stringent portion control effectively reduce calories to produce weight loss.

Traditional dietary therapy for weight loss generally seeks to create a deficit of 500 to 1,000 calories per day TABLE 5.3 Low-calorie Step I diet "Table 4. Low-Calorie Step I Diet," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)

with the intent of promoting weight loss of between one to two pounds per week. Low-calorie diets (LCDs) for men usually range from 1,200 to 1,600 calories per day; for women LCDs contain between 1,000 and 1,200 calories per day. Table 5.3 is an example of the recommended percentages of nutrients in an LCD that aims to decrease risk factors for hypertension and high cholesterol as well as cause weight loss.

The most successful LCDs take individual food preferences into account to custom-tailor the diet. Table 5.4 and Table 5.5 show examples of how traditional American cuisine may be used to create a low-calorie diet containing 1,200 and 1,600 calories per day respectively. Table 5.6 incorporates regional Southern cuisine into a reduced-calorie diet. Table 5.7 illustrates how Asian-American cuisine may be adapted to 1,200- and 1,600-calorie-per-day diets, and Table 5.8 shows how Mexican-American cuisine may be adapted for low-calorie diets. Table 5.9 is a sample of a reduced-calorie TABLE 5.4 Sample reduced calorie menus, traditional American cuisine—1,200 calories "Appendix D. Traditional American Cuisine—1,200 Calories," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)diet that vegetarians who eat milk and eggs but no meat or fish can use to lose weight. Food exchanges, such as those shown in Table 5.10, enable dieters to enjoy a variety of foods in their reduced-calorie meals, which can prevent boredom and the tendency to abandon the diet.

Research reveals that reducing fat in the diet is an effective way to reduce calories, and that when low-calorie diets are combined with low-fat diets, better weight loss is achieved than through calorie reduction alone. Further, while very-low-calorie diets that provide about 500 calories per day have been demonstrated to produce greater initial weight loss than the low-calorie diets, the long-term weight loss is not different between the two regimens.

During 2004 and 2005 several rigorous research studies reported that low-carbohydrate diets were as effective, or even more effective, in producing short-term weight loss than low-fat diets. The low-carbohydrate diets owed much of their success to adherence—dieters were better able to stick with their diets, and as a result achieved better results. Another hypothesis about the success of low-carbohydrate regimens is that dieters do not feel as hungry as they do on other diets because protein is the most satisfying of the three macronutrients—carbohydrates, fats, and protein.

The scientific premise of low-carbohydrate diets is that consuming certain carbohydrates can cause surges in blood sugar and insulin that not only stimulate appetite TABLE 5.5 Sample reduced calorie menus, traditional American cuisine—1,600 calories "Appendix D. Traditional American Cuisine—1,600 Calories," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)and weight gain, but also may increase risk for diabetes and heart disease. The earliest low-carbohydrate diets viewed all carbohydrates as equally harmful. Increasingly, however, low-carbohydrate diets distinguish between simple and complex carbohydrates, which contain simple or complex sugars.

Examples of single sugars from foods include fructose, which is found in fruits, and galactose, which is found in milk products. Double sugars include lactose in dairy products; maltose, which is found in certain vegetables and in beer; and sucrose (table sugar). Examples of complex carbohydrates, often referred to as "starches" are breads, cereals, legumes, brown rice, and pastas. Simple carbohydrates occur naturally in fruits, milk products, and vegetables; at the same time, these foods also contain vitamins and minerals. The simple carbohydrates most nutritionists call "empty calories" are the processed and refined sugars found in candy, table sugar, and sodas, as well as such foods as white flour, sugar, and polished rice (unless they are labeled "enriched" or "fortified" with vitamins or other nutrients).

In addition to distinguishing between simple and complex carbohydrates, low-carbohydrate regimens rely on a measure known as the glycemic index (GI), which ranks foods based on how rapidly their consumption raises blood glucose levels. The GI measures how much TABLE 5.6 Sample reduced calorie menus, Southern cuisine "Appendix D. Southern Cuisine—Reduced Calorie," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)blood sugar increases over a period of two or three hours after a meal. Carbohydrate foods that break down quickly during digestion have the highest glycemic indices. The GI may be used to determine if a particular food will trigger the problematical "carbohydrate-blood sugar-insulin cascade." High-GI foods are those that are rapidly TABLE 5.7 Sample reduced calorie menus, Asian American cuisine "Appendix D. Asian American Cuisine—Reduced Calorie," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)digested and absorbed or transformed metabolically into glucose. These include refined starchy foods such as bread, cereal, pasta, and table sugar. In general, fiber-rich foods are low glycemic. Most vegetables, legumes, and fruits are low-GI foods.

Measurement of GI is a relatively recent practice. It began during the 1990s, following the discovery that specific carbohydrates such as potatoes and cornflakes raised blood sugar faster than others such as brown rice and oatmeal. Harvard University School of Public Health TABLE 5.8 Sample reduced calorie menus, Mexican American cuisine "Appendix D. Mexican American Cuisine—Reduced Calorie," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)

researchers used GI to calculate glycemic load—a measure that considers the food's GI and the amount of carbohydrate contained in a single serving. For example, many whole fruits, vegetables, and grains have low glycemic loads, which when consumed prompt a moderate rise in blood glucose and insulin. When the same fruits, vegetables, and grains are squeezed or pulverized into juice or flour, their glycemic load increases—effectively

TABLE 5.9 Sample reduced calorie menus, Lacto-ovo vegetarian cuisine "Appendix D. Lacto-Ovo Vegetarian Cuisine—Reduced Calorie," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, June 1998, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)

rendering them with the same high glycemic load of sugar water.

After consuming a meal with a high glycemic load, blood sugar rises higher and faster than it does after eating a meal with a low glycemic load. In an effort to recover TABLE 5.10 Food exchange list "Appendix E. Food Exchange List," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)from the resulting peaks and plummets, the brain transmits a hunger signal long before the next meal is due. Wildly fluctuating blood sugar and insulin may result in overeating, which in turn causes overweight. For people who are overweight or physically inactive, another potential danger of consuming foods with high glycemic loads is that they may already be insulin resistant, and overexertion of insulin-producing cells in the pancreas required to metabolize the high glycemic loads may ultimately exhaust their insulin-producing cells, leading to diabetes.

Proponents of low-carbohydrate diets observe that consuming foods with low glycemic loads stabilizes blood sugar and insulin to prevent the fluctuations that can cause overeating and may increase risk for diabetes. They also assert that reliance on low-fat diets inadvertently led to diets that were high in simple carbohydrates, and indirectly promoted the observed increase in overweight and diabetes in the United States.

Low-fat diets reduce caloric intake by reducing fat consumption. Fat has nine calories per gram, while protein and carbohydrates have four calories per gram. TABLE 5.11 Low calorie, lower fat alternatives "Appendix C. Instead of … Replace with …" in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12,2006)These diets rely on the high-fiber content of complex carbohydrates to satisfy dieters. High-fiber foods also slow absorption of carbohydrates, so they do not provoke a rapid rise in blood sugar and insulin.

Table 5.11 shows some of the food substitutions that may be made to reduce the dietary fat content. In addition to making substitutions, many fat-free or low-fat food products are available—from fat-free frozen desserts to TABLE 5.12 Fat free or reduced fat versus regular "Fat Free or Reduced Fat Versus Regular," in The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, October 2000, http://www.nhlbi.nih.gov/guidelines/obesity/practgde.htm (accessed January 12, 2006)reduced-fat peanut butter. However, dieters are often cautioned that fat-free or reduced-fat foods are not calorie-free, and their consumption will not result in weight loss when more of the reduced-fat foods are consumed than would be eaten of the full-fat versions. For example, eating twice as many baked tortilla chips would actually result in higher caloric intake than a single serving of regular tortilla chips. (See Table 5.12.)

In the absence of rigorous scientific research and studies demonstrating the long-term safety and effectiveness of low-carbohydrate and low-fat diets, many investigators and health professionals hesitate to proclaim one diet's superiority over all others. There is general consensus that while some diets may produce greater initial weight loss, most perform similarly over time.

In "Efficacy and Safety of Low-Carbohydrate Diets: A Systematic Review" (Journal of the American Medical Association, vol. 289, no. 14, April 2003), Dena Bravata and her colleagues from Stanford and Yale Universities reported the results of their analysis of data about diet-induced changes in weight, serum lipids, fasting serum glucose and fasting serum insulin levels, and blood pressure among adults using low-carbohydrate diets. The investigators stated that they undertook the research in response to concerns about low carbohydrates expressed by the American Dietetic Association (ADA) and the American Heart Association (AHA). Both organizations had warned that low-carbohydrate diets may lead to abnormal metabolic functioning that in turn may prompt serious medical consequences, particularly for participants with cardiovascular disease, Type 2 diabetes mellitus, hyperlipidemia, or hypertension. Specifically, it has been cautioned that low-carbohydrate diets cause the accumulation of ketones, which may result in abnormal metabolism of insulin, impaired liver and kidney function, and salt and water depletion that may cause postural hypotension (sudden drop in blood pressure when rising from sitting) as well as fatigue, constipation, and kidney stones. It has also been posited that excessive consumption of animal proteins and fats may promote hyperlipidemia, and that higher dietary protein loads that may impair kidney function.

Not surprisingly, the investigators found that diets that restricted calorie intake and were longer in duration were associated with weight loss. They also observed that when lower-carbohydrate diets resulted in weight loss, it was likely due to the restriction of calorie intake and longer duration rather than changes in carbohydrate intake. The investigators did, however, observe that at least in the short term, low-carbohydrate diets were not associated with the anticipated adverse effects on lipid levels, glucose levels, or blood pressure. Further, their findings suggested that people without diabetes tolerated a lower-carbohydrate diet better than higher-carbohydrate alternatives and that this diet may be an effective means of achieving short-term weight loss without significant adverse effects on serum lipid levels, glycemic control, or blood pressure. They cautioned, however, that there is still inadequate evidence to recommend or condemn the use of low-carbohydrate diets among people with diabetes or for long-term use.

Michael Dansinger and his colleagues at Tufts University, New England Medical Center, compared the effectiveness of four popular diets—Atkins (low carbohydrates), the Zone (moderate carbohydrates), Ornish (low-fat vegetarian), and Weight Watchers (moderate fat). They presented the results of their study, funded by a National Institutes of Health (NIH) grant, the Tufts-New England Medical Center, and the USDA Nutrition Research Center at Tufts, at the scientific sessions of the American Heart Association meeting in November 2003.

Study participants were asked to follow the diets they were given as best they could for two months, and they were given official diet cookbooks and assigned to small group classes for diet education. For the remaining ten months the participants were told to follow their assigned diets "to whatever extent they wanted." The investigators reported that nearly one-quarter of the participants (22%) had dropped out of each diet after just two months, and by twelve months half of the participants assigned to low-carbohydrate or low-fat vegetarian diets had dropped out, as had 35% of participants assigned to the moderate carbohydrates and moderate fat diets. For those participants who adhered, weight loss and reduction in cardiac risk scores as measured by reductions in low-density lipoprotein (LDL cholesterol) and insulin levels were comparable for participants on the low-carbohydrate, moderate-carbohydrate, and moderate-fat plans. According to the study, the Ornish diet (low-fat vegetarian) "was best for lowering 'bad' LDL cholesterol, while other diets were better at raising 'good' HDL cholesterol."

The investigators concluded that their research "demonstrated that all these diets work." They also reiterated the importance of tailoring the selection of a weight-loss diet to ensure adherence, asserting "That means that physicians can work with patients to select the diet that is best suited to the patient. For example, if you have a patient who likes meat, it is unlikely that he or she will comply with the Ornish diet" (American Heart Association Scientific Sessions, Abstract 3535, November 12, 2003).

In "Low-Carb, Low-Fat Diet Gurus Face Off" (Journal of the American Medical Association, vol. 289, no. 14, April 2003), author Joan Stephenson asked the champions of the opposing regimens to offer their opinions about weight-loss approaches and the obesity epidemic. The late Dr. Atkins observed that low-carbohydrate dieters experienced more weight loss and greater improvements in LDL cholesterol and HDL than people on low-fat diets. He explained that while people on his plan consume about 40% of their calories from fat, as opposed to the U.S. average of 30%, the increased fat consumption is not harmful because "When the carbohydrate content is low, the fat takes a different metabolic pathway and forms energy through ketones, which then gets burned up right away. So the ketones do not accumulate because they're being used for energy."

In the same article, Dr. Ornish asserted that the overall health benefits of a low-fat diet were greater because carbohydrates are less calorie dense than protein and fat and that this diet sharply limits consumption of disease-promoting substances such as saturated fat and dietary cholesterol while increasing intake of protective nutrients such as phytochemicals, carotenoids, and lycopene—substances found in fruits, vegetables, grains, beans, and soy products. Ornish also observed that while there is not yet any data indicating that low-carbohydrate diets can reverse coronary heart disease, low-fat diets have been found to not only produce weight loss but also to reverse heart disease as measured by quantitative arteriography (systematic examination of the arteries using an injected contrast material that can be seen using x-ray equipment) and other cardiac imaging studies.

In May 2004, two published studies reaffirmed the safety and efficacy of low-carbohydrate diets. In the first study, 120 study participants were assigned to a low-carbohydrate, high-protein diet or a low-fat, low-cholesterol, low-calorie diet. The low-carbohydrate group was allowed unlimited calories, animal foods (meat, fowl, fish, and shellfish), and eggs, as well as 4 ounces of hard cheese, two cups of salad vegetables (lettuce, spinach, or celery) and one cup of low-carbohydrate vegetables (broccoli, cauliflower, or squash). The low-fat, low-cholesterol, low-calorie group consumed less than 30% of daily caloric intake from fat, less than 10% of calories from saturated fat, and less than 300 mg cholesterol daily. After six months, weight loss was greater in the low-carbohydrate diet group than in the low-fat diet group. Compared with the low-fat diet group, the low-carbohydrate diet group had greater decreases in serum triglyceride levels and greater increases in HDL cholesterol levels (William Yancy Jr. et al., "A Low-Carbohydrate, Ketogenic Diet versus a Low-Fat Diet to Treat Obesity and Hyperlipidemia: A Randomized, Controlled Trial," Annals of Internal Medicine, vol. 140, no. 10, May 18, 2004).

In the second study, researchers assigned 132 obese adults to either restrict carbohydrate intake to less than 30 g per day (low-carbohydrate diet) or to restrict caloric intake by 500 calories per day with less than 30% of calories from fat (conventional diet). After one year, weight loss was greater in the low-carbohydrate diet group, and the researchers found that the low-carbohydrate diet group fared better in terms of greater decrease in triglyceride levels (Linda Stern et al., "A Low-Carbohydrate, Ketogenic Diet versus a Low-Fat Diet to Treat Obesity and Hyperlipidemia: A Randomized, Controlled Trial," Annals of Internal Medicine, vol. 140, no. 10, May 18, 2004).

In October 2004 yet another study confirmed that modified low carbohydrate diets produced greater weight loss than the U.S. National Cholesterol Education Program (NCEP) diet, which replaces saturated fat with carbohydrates (Y. W. Aude et al., "The National Cholesterol Education Program Diet vs. a Diet Lower in Carbohydrates and Higher in Protein and Monounsaturated Fat: A Randomized Trial," Archives of Internal Medicine, vol. 164, no. 19, October 25, 2004).

The following month, however, Suzanne Phelan, a researcher from Brown Medical School asserted that low-fat diets produce better long-term weight loss than low-carbohydrate diets at a meeting of the North American Association for the Study of Obesity. She and her colleagues studied 2,700 people who entered the National Weight Control Registry, which records successful efforts to lose at least thirty pounds and maintain the loss for at least one year. All of the subjects reported eating about 1,400 calories a day, but the portion derived from fat rose from 24% in 1995 to more than 29% in 2003, while the portion from carbohydrates fell, from 56% to 49% The number who were on low-carbohydrate diets (less than 90 grams a day) rose from 6% to 17% during the same period.

Although the type of diet—low-fat or low-carbohydrate—made no difference in how people lost weight initially, those who increased their fat intake over a year regained the most weight. The researchers concluded that they ate fewer carbohydrates, since the amount of protein in their diets remained the same. The researchers concluded that the minority of successful dieters use low-carbohydrate regimes.

While there is no single winner in the diet wars, further research has dispelled some fears about the safety and effectiveness of the low-carbohydrate diet. Low-carbohydrate diets appear to be safe and effective in the short term, but long-term outcomes still are unclear. Although some results suggest that higher protein and fat intakes lead to lower total caloric intake by producing earlier satiety, these diets have not been shown to alter fundamental eating behaviors nor have they demonstrated, as many of their proponents argue, the ability to modify caloric balance such that weight loss persists when more calories are consumed than expended.

Finally, research published in the Journal of the American Medical Association in January 2005 asserted that adherence to a diet for one year, rather than the specific type of diet, is the single most important determinant of weight loss and reduction of risk of cardiovascular disease. The investigators found the amount of weight lost was associated with the level of dietary adherence but not with diet type, and they concluded that, "One way to improve dietary adherence rates in clinical practice may be to use a broad spectrum of diet options, to better match individual patient food preferences, lifestyles, and cardiovascular risk profiles. Our findings challenge the concept that one type of diet is best for everybody and that alternative diets can be disregarded. Likewise, our findings do not support the notion that very low carbohydrate diets are better than standard diets, despite recent evidence to the contrary" (Michael Dansinger et al., "Comparison of the Atkins, Ornish, Weight Watchers, and Zone Diets for Weight Loss and Heart Disease Risk Reduction: A Randomized Trial," Journal of the American Medical Association, vol. 293, no.1, January 5, 2005).

In an accompanying editorial, Robert H. Eckel, a physician from the University of Colorado at Denver, commented that, "It seems plausible that for maintenance of reduced body mass, the right diet needs to be matched with the right patient" He advised a "low fad" approach for people dieting to lose weight ("The Dietary Approach to Obesity: Is It the Diet or the Disorder?" Journal of the American Medical Association, vol. 293, no.1, January 5, 2005).