In the last 30 years, the prevalence of obesity has doubled (from 14.5% to 30.9%). This is due to a variety of factors, but primarily unhealthy diets and an increase in sedentary behavior. New data from CDC also shows that we are simply eating more calories than we used to. From 1971 to 2000, men increased their energy intake from 2450 calories per day to 2618 calories per day, an increase of 168 calories per day. That doesn't seem like much, but 168 calories per day above the body's needs is equivalent to 17 pounds of extra fat in one year! Obviously the body is able to correct somewhat for this increase in calories, but not enough to prevent a significant increase in obesity.

During the same time period, the average energy intake for women increased from 1542 to 1877 calories per day, an increase of 335 calories per day. This increase in calories eaten comes primarily from an increase in carbohydrates eaten, an additional 62.4 grams per day for women and an additional 67.7 grams per day for men.

Further analysis shows that this increase in calories comes primarily from increased energy intake eaten away from home: fast food and regular restaurants, snack foods (salty snacks such as chips, soft drinks, and pizza), and from increased portion sizes (super sized restaurant meals and larger snack portions of chips, soft drinks, etc.).

Source: “Trends in Intake of Macronutrient” US 1971-2000, MMWR 2004, JAMA Mar 10, 2004