The goal of diet therapy for weight loss is to achieve a daily energy deficit whereby energy intake is less than energy expenditure. Daily energy expenditure can be estimated based on age, gender, and activity level. Two thousand calories (kcals) per day is often used as a reference level, but different individuals’ requirements may vary by hundreds of calories per day. The goal of most weight-loss diets is to achieve an energy deficit of approximately 500 kcal/day. Since a pound of fat equals approximately 3500 kcal, this will result in approximately 1 lb of weight loss per week.
Meta-analysis of randomized studies of dietary therapy to achieve weight loss have demonstrated an average weight loss of 5–10% of the starting weight in patients followed for 3 months to 1 year. Follow-up for up to 5 years suggests average long-term weight loss of 2–4% of initial weight. Of note, all studies have a substantial variation of response. Some patients, approximately 20%, are able to lose substantially more weight and keep it off for longer periods of time. Studies of commercial weight-loss programs also demonstrate modest amounts of weight loss but with wide variations. Weight Watchers, for example, reports 3.2% weight reduction at 2 years.
The optimal composition for weight-loss diets has been the subject of numerous clinical trials and, of course, thousands of popular weight-loss books. Most nutrition guidelines such as the Dietary Guidelines for Americans, recommend a low-fat, high-fiber, “balanced” diet. This approach emphasizes consumption of a wide variety of foods to achieve a balanced mix of macronutrients, that is, 20–35% of total calories from fats, 45–65% of total calories from carbohydrates, and 10–35% of total calories from protein. These ranges are substantially wider than earlier federal guidelines and allow for flexibility in diet design. The DASH (Dietary Approaches to Stop Hypertension) Eating Plan, originally developed for treatment of hypertension, is an example of a balanced diet that meets these macronutrient ranges and is supported by federal guidelines. A Mediterranean-style diet can also be used to achieve an energy deficit for weight loss and remain within these ranges.
Numerous clinical trials have compared this balanced approach with low carbohydrate diets such as the Atkins Diet and South Beach Diet. These studies have consistently shown equivalent amount of weight loss independent of the macronutrient composition of the diet. Rather than nutrient composition, weight loss was predicted by adherence to the diet. Satiety, hunger, lipid levels, insulin levels, and other metabolic factors were also found to be equivalent despite large differences in macronutrient composition. Similar findings have been published with comparisons of the very low-fat Ornish diet, the Zone diet, and other diets with various combinations of macronutrients. Thus, patients can be reassured that almost any whole food diet can result in safe weight loss as long as the patient consistently follows the diet.
An important alternative in the dietary treatment of obesity is the use of safe and effective very low-calorie diets (VLCDs). Previously known as protein-sparing modified fasts and protein-formula liquid diets, these diets restrict calorie intake to less than or equal to 800 kcal/day. Patients ingest only prepackaged, often liquid, food that provides adequate protein, vitamins, and minerals. Additional intake is limited to 2–3 quarts of calorie-free beverages per day. The major advantage of these diets is the “complete removal of patients from the food environment” to facilitate short-term adherence. In addition, the significant energy deficit results in rapid weight loss, usually 2 lb/week, encouraging the patient to continue. Ongoing concerns about these diets include their cost, side effects, and long-term results. Clinical trials suggest that the use of 800-kcal VLCDs can lower cost and prevent most of the significant side effects associated with the lower-calorie (400–600) VLCDs, including gallstones and fluid and electrolyte disorders, with equal long-term efficacy.
As with standard diet therapy of obesity, VLCDs require adherence during the diet, and long-term nutritional and behavioral changes to maintain weight loss. Well-planned programs that combine VLCDs with nutrition education, behavior therapy, exercise, and social support report improved long-term results. For example, an average weight loss of 55 lb with 75% and 52% of the loss maintained at 1- and 2 1/2-year follow-up, respectively, and maintenance of an average of 24 lb after 2- to 3-year follow-up have been reported. Meta-analysis of published VLCD trials suggest that while initial weight loss is greater with VLCDs than standard diets, both approaches have equal long-term results. As with other approaches, however, there is a wide variation of response with VLCDs, and some patients can achieve long-term weight loss. Because of the quicker and greater amounts of weight loss, VLCDs may be particularly useful in situations in which rapid weight loss is clinically indicated such as preoperative treatment (joint replacement, transplant surgery, bariatric surgery) or for initial management of severe medical conditions (severe sleep apnea, poorly controlled diabetes, heart failure, or coronary artery disease).
Recent studies have helped elucidate predictors of long-term weight maintenance after diet therapy for weight loss. Most important has been information from the National Weight Control Registry. This cohort of individuals who have lost an average of 33 kg and maintained the loss for 5 years report consumption of low-calorie, low-fat diets averaging approximately 1400 cal/day; high levels of physical activity averaging 60 minutes/day; regular self-monitoring of body weight; eating breakfast daily; and maintaining consistent dietary patterns on weekdays and weekends.