Human nutrition is the provision to obtain the materials necessary to support life. In general, people can survive for two to eight weeks without food, depending on stored body fat and muscle mass. Survival without water is usually limited to three or four days. Lack of food remains a serious problem, with about 36 million people dying every year from causes directly or indirectly related to hunger. Childhood malnutrition is also common and contributes to the global burden of disease. However global food distribution is not equal, and obesity among some human populations has increased to almost epidemic proportions, leading to health complications and increased mortality in some developed, and a few developing countries. Obesity is caused by consuming more calories than are expended, with many attributing excessive weight gain to a combination of overeating of "unhealthy" (high fat, high sugar, high carbohydrate) foods and insufficient exercise. A major risk of obesity is becoming a type 2 diabetic.