One in three Americans is obese because the United States produces too much food, according to National Institute of Diabetes and Digestive and Kidney Diseases investigator Carson C. Chow. National agriculture policy changed in the 1970s from paying farmers to hold back production to encouraging farmers to grow as much food as possible. Food prices fell during this time, and Americans consumed an average of 1,000 more calories a day. The average American gained 20 pounds between 1975 and 2005, and the obesity rate increased from 20 percent to 30 percent. Lower food prices also led to people eating more frequently at restaurants, especially fast-food restaurants, while their levels of physical activity decreased. Chow also challenges the common belief that to lose one pound requires a 3,500 deficit. In fact, he shows that as the body loses weight, its calorie requirements decrease, so a greater deficit is needed to maintain weight loss. Using sophisticated mathematical models, Chow shows that most people would lose 10 pounds in three years by consistently consuming 100 few calories a day than they presently consume. Because it takes a long time for practical diets to show results, Chow advises that it is important to stick with the plan for at least a year to see lasting results. For understanding how many calories are needed to produce results over time, people can use Chow's interactive model at bwsimulator.niddk.nih.gov.