Introduction

Very small but chronic differences in daily energy balance can cause dramatic changes in body composition over a long period of time. In the United States, people gain an average of 13 to 14 kg of body weight between 25 and 55 years of age, or 1 lb of fat per year (1). This amount of weight gain represents a very small imbalance between the number of calories ingested and the number expended; consuming one Lifesaver (10 kcal) a day more than the calories expended every day will result in a gain of about 1 lb/yr. The reverse is also true; therefore, a small but daily negative energy imbalance can lead to a significant amount of weight loss. Moreover, modest weight loss has been associated with considerable medical benefits (2, 3). For example, a 2% to 5% weight loss in patients who have type 2 diabetes can improve insulin sensitivity (3).

Altering dietary habits is the cornerstone of weight-loss therapy for obese patients. Although many different diets have been shown to induce short-term weight loss, poor long-term compliance and weight regain are common. The optimum diet for effective, safe, and lasting weight loss is unknown. Several intrinsic factors within food may be involved in regulating energy intake. These factors include macronutrient composition, energy density, fiber content, fat and sugar substitutes, portion size, food variety, and the physical properties of food (e.g., taste and feel). In addition, extrinsic factors related to societal, social, and economic influences, including food cost, availability, variety, and marketing also affect food intake. Therefore, effective diet therapy may need to consider both intrinsic and extrinsic food factors to achieve successful long-term weight loss in obese patients.

Macronutrient Composition of Diets

The macronutrient composition of a diet can have significant clinical effects, independent of any changes in body weight. For example, switching from a weight-maintaining high-carbohydrate diet to a high-monounsaturated-fat diet has decreased serum triglyceride and increases serum high-density lipoprotein-cholesterol concentrations (4, 5, 6, 7), whereas a weight-maintaining diet that is high in saturated fat has often increased low-density lipoprotein-cholesterol concentrations. In addition, it is possible that a diet that has adverse metabolic effects in patients who are maintaining their body weight can result in metabolic benefits if it induces significant weight loss.

Traditionally, a low-fat, high-carbohydrate diet has been recommended to help obese patients lose weight (8). This approach makes sense because fat contains more than twice as many calories per gram than protein or carbohydrate, and fat tends to make food more palatable, which may increase intake. Data from epidemiological studies (9), meta-analysis of clinical trials that evaluated the effect of reduced fat intake on serum lipids (10), and randomized controlled trials on the effect of a low-fat diets on body weight in obese subjects (11) have supported the notion that limiting dietary fat intake causes weight loss. However, limiting fat intake may not be more effective than focusing on restricting total calorie intake. Data from a recent meta-analysis that evaluated randomized clinical trials that compared fat-restricted and calorie-restricted diets has found no difference in weight loss between diet groups (12).

Recently, low-carbohydrate diets have become increasingly popular. The first documentation of successful weight loss using a low-carbohydrate diet dates back to 1863, when William Banting published a Letter on Corpulence, which details his own loss of 50 lb and a decrease in BMI from 33.7 to 25.3 kg/m² by consuming a low-carbohydrate diet (13). Dr. Robert Atkins is probably the single person most responsible for the recent increase in low-carbohydrate dieting, and his diet books have been exceptionally popular (14, 15). The Atkins' low-carbohydrate diet approach is outlined in Table 1.

Table 1. - Atkins diet approach.

Full table

The results from four randomized controlled trials that compared the effect of a low-carbohydrate diet with a low-fat diet on body weight in adult obese subjects have recently been published (16, 17, 18, 19, 20). A consistent difference in weight loss at 6 months has been observed between groups across studies; subjects randomized to the low-carbohydrate diet lost 4 to 5 kg more weight than those randomized to the low-fat diet. However, weight loss was no different between groups at 1 year (18, 20). In addition, in subjects who had type 2 diabetes, there were greater improvements in fasting blood glucose concentrations and insulin sensitivity with a low-carbohydrate than with a low-fat diet (17, 20). The data from these studies has also found greater improvements in serum triglyceride and high-density lipoprotein-cholesterol concentrations, but not in serum low-density lipoprotein-cholesterol concentrations, in the low-carbohydrate than the low-fat group (Figure 1).

Figure 1.

Effect of low-carbohydrate and low-fat diets on serum lipids. Reproduced with permission from Foster et al. A randomized trial of low-carbohydrate diet for obesity. N Engl J Med. 2003;348:2082–90 ( 18) .

Full figure and legend (81K)

Surreptitious reduction in carbohydrate intake by switching foods containing sucrose with foods containing an intense sweetener can also result in weight loss (21). In one short-term (10-week) study, energy intake and body weight was lower in subjects who were given foods that contained an intense sweetener than those given foods that contained sucrose.

Energy Density

Energy density is defined as the energy (kilocalories) present in a certain weight (grams) of food. The energy density of foods can affect the total number of calories consumed during a meal, because the volume or weight of food may be involved in the regulation of food intake (22). Energy-dense foods are usually high in fat (e.g., butter) and/or are dry (e.g., pretzels). Therefore, the fat and water content of a food determines its energy density. In general, most energy-dense foods are high-fat foods, and most foods that contain a lot of water (e.g., fruits, vegetables, soups) have a low-energy density (Figure 2).

Figure 2.

Effects of fat and water content on energy density. Reproduced with permission from Rolls and Bell: Dietary approaches to the treatment of obesity. Med Clin North Am 2000;84:401–418 ( 22) .

Full figure and legend (80K)

Data from a series of studies has suggested that food intake is regulated, in part, by food weight, so energy intake is inversely correlated with food energy density (22, 23, 24). Moreover, the results of short-term studies have suggested that manipulating energy density might be a useful approach to noncognitively regulate total ad libitum energy intake. In fact, in a 6-month randomized controlled trial, obese subjects randomized to a low-energy-density diet (containing 25% calories as fat and 55% as carbohydrate) have lost more weight (1% to 2% weight loss) than those randomized to a medium-energy-density diet (containing 35% calories as fat and 45% as carbohydrate; no weight loss) (11).

Conclusions

A modern industrialized society has unhealthy effects on eating behavior. Many environmental factors, such as food marketing, convenient food acquisition and preparation, food variety, and supersizing, encourage overconsumption of calories. In addition, most people are genetically programmed to eat food and store excess ingested calories as fat. The combination of these genetic and environmental influences makes it difficult for obese persons to lose weight and maintain long-term weight loss by dieting. Successful dieters must be exceptionally vigilant or they will rapidly regain their lost weight. Therefore, additional research is needed to identify the factors in food, such as macronutrient composition and energy density, that may be involved in regulating food intake. Taking advantage of these noncognitive factors in food may provide a useful strategy to reduce energy intake without the need for constant cognitive monitoring that is difficult to maintain.

At present, it is unlikely that one diet is optimal for all overweight or obese persons, and dietary guidance should be individualized to allow for specific food preferences and individual approaches to reducing energy intake. Low-carbohydrate diets cause greater short-term (up to 6 months) weight loss than low-fat diets, but the long-term clinical safety and efficacy of these diets are not known and require additional investigation.

Question and Answer Period

Dr. Roth.

What we all notice is that when people start on a diet, they do well, and then their weight loss eases off. How about serial diets, by that I mean, you try a given diet for 3 months, and it has its novelty and then you switch the diet. Can you keep the freshness of the early weight loss?

Dr. Klein.

You present an interesting concept, but I am not aware of any studies that have directly addressed this issue. In general, many people do follow serial diets over long periods of time. They try one diet for a few months or 1 year and another diet the next year. It is possible that a structured serial diet approach could be successful, but we need data from randomized controlled trials to answer this question.

Dr. Roth.

That's what I'm suggesting, with cheerleading by the health professionals. Because what happens is that when people fail on a diet, they get discouraged, and there's depression associated with it. However, if it's done as a thoughtful upbeat set of serial diets, would that work?

Dr. Klein.

I agree that most diets cause short-term weight loss, but the lack of long-term dietary adherence results in weight regain. It is possible that the novelty of interval diets will improve adherence.

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