Weight Loss and Metabolic Benefits With Diets of Varying Fat and Carbohydrate Content: Separating the Wheat From the Chaff

Summary and Introduction

Summary

With the rising prevalence of both obesity and diabetes, the contributing role of diet to the prevention and treatment of these conditions has become a major focus of research, clinical practice, and public policy. There has been intense debate over which dietary regimens might be most effective for weight loss, with interest centered on the potential for specific dietary macronutrients to affect body composition, metabolism, and overall health. This Review addresses two regimens with distinct macronutrient prescriptions that have been widely touted as being beneficial for weight loss and/or metabolic profile: diets low in carbohydrate and diets high in monounsaturated fat. Although data from recent randomized, controlled trials suggest these popular diets may be useful for weight control, cardiovascular health, and glycemic control, longer studies of the efficacy and safety of varying macronutrient content are needed to strengthen the evidence base for nutritional recommendations. Until more support for specific macronutrient combinations is available, practitioners can recommend an array of diets with moderate amounts of macronutrients, tailored to individual needs and preferences.

Introduction

The concurrent rise in the prevalence of obesity^[1] and diabetes^[2] over the past two decades is a major public health concern. Lifestyle behaviors, such as nutrient-poor diets and physical inactivity, have been implicated in the increasing occurrence of both obesity^[3] and diabetes.^[4] Large randomized, controlled trials (RCTs) have demonstrated long-term weight loss and reduced incidence of diabetes with lower-fat diets and increased physical activity in people at high risk.^[5,6] Lifestyle modification, therefore, remains the first clinical approach when patients present with obesity and attendant metabolic consequences.

With a pressing need for the prevention and treatment of obesity and its comorbidities, the therapeutic role of diet has become a major focus of research, clinical practice, and public policy. Consequently, there has been a surge of interest in the potential effects of diets with varying macronutrient content, with intense debate about the ideal combination of carbo hydrate, protein, and fat for effectively managing obesity and its metabolic consequences such as diabetes and other cardiovascular risks.

The Acceptable Macronutrient Distribution Ranges (AMDRs) established by the Institute of Medicine^[7] are widely used as dietary guidelines for healthy adults, as well as for persons with diabetes.^[4] The AMDRs are as follows: 45-65% of total calories derived from carbohydrate; 10-35% from protein; 20-35% from total fat. The overarching goals of these recommendations include reduction of cardiovascular risk and achievement of glycemic control, along with weight management. Specific dietary strategies to reach these goals must be sensitive to the needs, values, and preferences of the individual patient.^[4]

Similarly to many past and current nutritional recommendations for health promotion and disease treatment,^[8] the diet recommendations of expert bodies such as the Institute of Medicine,^[7] American Heart Association,^[9] American Diabetes Association,^[4] and European Association for the Study of Diabetes^[10] are based on the clinical expertise and distillation of available evidence by groups of leading practitioners. Although long-term dietary interventions are difficult to implement, there have been a number of recent randomized, controlled effectiveness studies that have strengthened the evidence on which to base clinical nutrition practice. These studies have provided additional, but not yet sufficient, evidence for modification of current nutritional guidelines. Despite a recent flurry of research, many fundamental questions related to dietary macronutrients and their effects on energy balance and metabolism remain unanswered.

The unceasing demand by patients and health-care providers for effective weight-loss methods has led to the use of a wide number of unproven strategies. It is, however, essential for clinicians to be aware of the latest research findings related to dietary macronutrients and the sometimes controversial interpretation and application of these findings. This Review, therefore, will address two dietary approaches with a focus on macronutrient composition rather than energy restriction to promote weight loss and improve cardiovascular risk: diets low in carbohydrate and diets high in monounsaturated fat (MUFA).