Weight control is far more complex than simply balancing calories consumed with energy expended. Here are 10 top (alternative) reasons why Americans are overweight – and some may surprise you.

Rates of obesity are higher than ever in the United States. According to the Centers for Disease Control, more than six in 10 American adults are overweight or obese -- and most adults are about 25 lbs heavier than people were in the 1960s. To make matters worse, more than half of all overweight people actually think they’re at a healthy weight, according to a recent Associated Press poll.

In theory, weight control is a simple matter of balancing energy intake (the calories supplied by food) with energy output (the calories expended by physical activity, the digestion of food, and the functioning of your body). To lose weight, you need to expend more energy than you take in. In practice, however, the task is not that simple. While the basic principle of energy balance remains true, several mechanisms -- genetic, metabolic, and environmental -- can affect how much you eat and how your body uses and stores energy.

A revealing study published in the International Journal of Obesity (online edition, June 27, 2006) highlighted 10 reasons why Americans are fat. The study explained that too much food and a lack of exercise are not the only factors contributing to obesity. They identified 10 other possibilities:

• Weight gain factor 1 -- Less sleep. We’ve gone from an average of nine to seven hours of sleep a night; sleep deprivation is linked to a more robust appetite.
• Weight gain factor 2 -- Pesticides and other chemicals in foods. These substances can change hormonal activity, which can boost body fat.
• Weight gain factor 3 -- Air conditioning and heating. We don’t sweat and shiver as much as our ancestors, so we don’t burn as many calories.
• Weight gain factor 4 -- Fewer smokers. Nicotine is an appetite suppressant.
• Weight gain factor 5 -- Greater use of medications, such as antidepressants and diabetes drugs, which have weight gain as a potential side effect.
• Weight gain factor 6 --Population changes. There are now more older people as well as more Hispanics in the United States, and these groups have higher rates of obesity.
• Weight gain factor 7 -- Older birth mothers, whose children are more prone to excess weight gain than the offspring of younger mothers.
• Weight gain factor 8 -- Genetics. Obese moms may pass the trait onto their children in utero.
• Weight gain factor 9 -- Higher body weight = greater fertility. Heavy people produce more offspring than thinner people.
• Weight gain factor 10 -- Assortative mating. Heavy people tend to mate with each other and produce heavy children.