Weight loss diets
The primary determinant of weight loss is energy deficit. Short term weight loss has been achieved by energy reduction in diets of varied macronutrient composition. Obesity is a chronic and relapsing disease; hence, it is the long term efficacy of these dietary strategies in maintaining lowered weight (and minimizing the risk of diet related chronic diseases) that is of fundamental importance.
Types of dietary treatment
There are several dietary strategies available both in a clinical and commercial setting. These diets vary greatly in the degree of caloric restriction, relative amounts of macronutrients (protein, carbohydrate, fat), medical supervision, scientific basis, and cost. These diets can be broadly divided into:
- Low-calorie diets - 3400-6300 kJ (800-1500 kcal) a day
- Very low-calorie diets - less than 3400 kJ (800 kcal) a day
Traditionally, low-calorie diets that incorporate various methods for restricting food intake have been recommended for weight management.
Such treatment requires a period of supervision for at least 6 months. A review of 48 randomized control trials shows strong and consistent evidence that an average weight loss of 8% of the initial body weight can be obtained over 3-12 months with a lowcalorie diet (LCD) and this weight loss causes a decrease in abdominal fat, the adipose tissue deposition that is associated with the highest disease risk. Very low-calorie diets (VLCD) have been shown to reduce weight at a greater rate in the first 2-3 months compared to low-calorie diets but have not been associated with superior maintenance of lost weight after a year. A review of weight loss trials of LCD and VLCD with available follow-up during 2-7 years showed that long-term weight loss in most trials is in the range of 2-6 kg.
Low-fat, high-carbohydrate diets
Low-fat, high-carbohydrate diets have played a central role in the dietary management of overweight and obesity. Generally, these strategies aim to provide a macronutrient composition of 25-35% energy from fat, 45-60% from total carbohydrate, and 15-20% from protein, thereby moving individuals towards national dietary guidelines (COMA reports). A review of controlled clinical trials demonstrated that a 10% reduction of dietary fat leads to a ~3-4 kg weight loss in normal overweight subjects and ~5-6 kg weight loss in the obese. Evidence from a recent systematic review suggests that a low-fat diet is equally as effective in achieving long-term weight loss in overweight and obese subjects as alternative dietary strategies. Low-fat high-carbohydrate diets may have a role in weight maintenance. Combined with physical activity and behavioral strategies, the American Diabetes Prevention Program and the Finnish Diabetes Prevention Trial demonstrated maintenance of modest weight loss (3-4 kg) with a marked reduction in the risk of developing type 2 diabetes mellitus over a 4-year study period.
Low glycemic index diets
The glycemic index (GI) is a dietary concept originally developed for the therapy of diabetes, which has recently become popular despite scant evidence of its effectiveness in weight management. The GI is a property that describes the effect of carbohydrate from a given food on postprandial blood glucose. It is measured by comparing the blood glucose response of the test food with that of a reference food (usually white bread). Low-GI foods are more slowly absorbed leading to an attenuated and prolonged insulin and metabolic response to foods; it is suggested that more moderate blood glucose and metabolic response may sustain satiety and energy balance to a greater extent than larger metabolic shifts would.
Epidemiological analyses link low-GI load diets to a more favorable lipids profile and reduced incidence of type 2 diabetes mellitus and cardiovascular disease. Evidence from interventional studies supports the benefits of low-GI diets in reducing the risks of coronary heart disease and diabetes but there are no long-term studies that have evaluated its weight-loss efficacy. Therefore, it is appropriate to promote the constituents of a low-GI diet (increased legumes, wholegrain cereals, and fruit consumption) as part of a well balanced hypo-caloric diet for the long-term management of obesity and its metabolic complications.
High-protein, low-carbohydrate diets
High-protein diets have recently been popularized as a means of rapid weight loss despite the lack of objective evidence in long-term efficacy and safety. Typically, these diets offer wide latitude in protein food choices, and are restrictive in other food choices (mainly carbohydrate). Animal protein rather than plant protein is advocated leading to a higher intake of total fat - mainly saturated fat and cholesterol. Many of the popular high-protein diets promote protein intake of 28-64% of dietary energy, which exceeds established requirement of 10-15%, and severely limit carbohydrate dietary energy to 3-10%. A recent popular high-protein, low-carbohydrate diet, the Atkins diet, provides on average 27% energy from protein, 5% energy from carbohydrates, and 68% energy from fat. The diet results in the avoidance of important staple foods, such as bread, pasta, rice, potatoes, and cereals, as well as foods high in sugars. Consumption of fruits, vegetables, whole grains, and low-fat dairy products, foods associated with lowering blood pressure and protecting against cancer and heart disease, are all limited.
The initial weight loss in high-protein diets is high due to fluid and glycogen loss related to low carbohydrate intake, overall caloric restriction that is encouraged by structured eating plans, restricted range of foods allowed, and limited tolerance of high-protein foods. This often promotes a misconception about weight loss by suggesting that it is not related to total energy intake but is due to exclusion of certain foods.
A recent systematic review of the efficacy of low-carbohydrate, high-protein diets demonstrates that the amount of weight loss is principally associated with decreased caloric intake rather than reduced carbohydrate content. Researchers have yet to establish whether individuals can maintain long-term weight loss with a high-protein, low-carbohydrate diet because of the short duration of these studies, and long-term adverse effects are also unknown. Possible negative effects include increased risks of cardiovascular disease, renal disease, cancer, osteoporosis, and compromised vitamin and mineral status.
Energy prescribed diet
This dietary strategy determines the daily energy requirement for weight loss by calculating energy expenditure, adjusting for physical activity, and subtracting an energy deficit to induce weight loss - usually 2100-2520 kJ (500-600 kcal) for 0.05 kg weight loss. As a result the prescribed diet will often be in excess of 3400-6300 kJ (800-1500 kcal). The popularity of this approach relates to the findings of improved compliance in those advised on a 2520 kJ (600 kcal) deficit diet compared to a traditional fixed energy intake of 5040 kJ (1200 kcal) a day.
Formulas and meal replacements
Meal replacements are another category of calorie-controlled diets. These include nutritional fortified shakes, snack bars, and low-calorie frozen meals. An entire meal or snack is replaced with a portion controlled prepackaged meal or drink that provides approximately 840-1260 kJ (200-300 kcal), although formulations and nutrient content vary. Meal replacements are designed to be eaten with additions of conventional foods that supply dietary fiber, other nutrients, additional calories, and water. Most weight loss programs that use meal replacements recommend replacing two meals and one snack a day to lose weight and then replacing one meal per day to maintain weight loss. This strategy generally provides 5040-6729 kJ (1200-1600 kcal) a day and the regular meal should meet the recommendations of a healthy diet.
A recent meta-analysis that summarized the efficacy of this approach compared to conventional energy-restricted diets suggests that it is an effective weight-loss strategy both in the short and long term in a clinical trial setting. There is no information about the efficacy outside a clinical trial where meal replacement products need to be purchased, and are frequently discontinued at an early stage.
Very low-calorie diets
Very low-calorie diets are formula foods; they are designed to provide larger and more rapid weight loss than the standard low calorie diets. They are commonly given in liquid form to completely replace usual food and snack intake providing in the region of 1890-3400 kJ (450-800 kcal) a day. To reduce the potential risks from loss of lean body tissue, VLCDs are enriched in protein of high biologic value and also includes the full complement of recommended daily allowance for vitamins, minerals, electrolytes, and fatty acids. However, diets providing such low-energy intakes are often associated with a feeling of fatigue, constipation, nausea, and diarrhea. A most serious complication associated with VLCD is the development of symptomatic cholelithiasis associated with the rapid weight loss (1-2 kg a week).
Owing to the potential adverse effects of these diets, they are generally reserved for short-term treatment in individuals who are moderately to severely obese (BMI > 35) and who have failed at more conservative approach to weight loss, in particular in those with medical conditions that may respond to weight loss such as obstructive sleep apnea, type 2 diabetes mellitus, or prior to surgical procedure.
Weight regain is common with the reintroduction of food. Studies show that in the long term, VLCDs are no more effective than more modest dietary restriction.