Obesity in the United States is a socioeconomic issue. It is related to limited social and economic resources and may be linked to disparities in access to healthy foods. Added sugars and added fats are far more affordable than are the recommended "healthful" diets based on lean meats, whole grains, and fresh vegetables and fruit. There is an inverse relationship between energy density of foods (kJ/g) and energy cost ($/MJ), such that energy-dense grains, fats, and sweets represent the lowest-cost dietary options to the consumer. Good taste, high convenience, and the low cost of energy-dense foods, in conjunction with large portions and low satiating power, may be the principal reasons for overeating and weight gain. Financial disparities in access to healthier diets may help explain why the highest rates of obesity and diabetes are found among minorities and the working poor. If so, then encouraging low-income households to consume more costly foods is not an effective strategy for public health. What is needed is a comprehensive policy approach that takes behavioral nutrition and the economics of food choice into account.