Long-term reduction in energy intake in the diet (calorie restriction [CR]) extends the life of the laboratory rat by about 25%. However, in humans there are no life-long studies of CR, but only short-term trials which indicate that 20% CR acting over periods of 2-6 years is associated with reduced body weight, blood pressure, blood cholesterol, and blood glucose--risk factors for the major killer diseases of cardiovascular disease and diabetes. In addition, recent research has shown that CR for 6 months is able to improve biomarkers for longevity (deep body temperature and plasma insulin) and thus should increase life expectancy. The magnitude of the life-extension effect of CR in humans can only be estimated. The Okinawans, the longest-lived people on earth, consume 40% fewer calories than the Americans and live only 4 years longer. Similarly, women in United States consume 25% fewer calories than men and live 5 years longer. From the survival studies of overweight and obese people, it is estimated that long-term CR to prevent excessive weight gain could add only 3-13 years to life expectancy. Thus the effects of CR on human life extension are probably much smaller than those achieved by medical and public health interventions, which have extended life by about 30 years in developed countries in the 20th century, by greatly reducing deaths from infections, accidents, and cardiovascular disease.