If you eat at McDonald's or KFC or Burger King regularly, you are more likely to gain weight and become obese. Why? British researchers from the Medical Research Council Human Nutrition Center and the London School of Hygiene and Tropical Medicine have put forth what they call a "probable" explanation: Fast food not only contains many more calories than traditional food, but also is more likely to undermine normal appetite control systems, reports The London Times.

The hidden reason fast food makes us fat: It has a very high energy density--about 65 percent higher than a typical diet and twice as high as recommended healthy diets--which makes us eat more than we otherwise would. Energy density refers to the amount of calories an item of food contains in relation to its weight. Foods with a high energy density confuse the brain's control systems for appetite, which are based solely on portion size.

Eat a Big Mac and fries and you'll consume almost twice as many calories as you would if you ate the same weight of pasta and salad. "Fast food restaurants are feeding the obesity epidemic by tricking people into eating many more calories than they mean to," writes Times science correspondent Mark Henderson.

Study leaders Andrew Prentice and Susan Jebb, who analyzed the energy density of McDonald's, KFC, and Burger King menu items using nutritional data from the fast food restaurants' Web sites, found that when we eat high energy density foods, we don't reduce the portion size so we get a lot more calories than we need. "We all possess a weak innate ability to recognize foods with a high energy density," explained Prentice. "We tend to assess food intake by the size of the portion, yet a fast food meal contains many more calories than a similar-sized portion of a healthy meal." We're being fooled into eating too much food.

Prentice blames fast food for "accidental obesity." That is, people get fat eating regular portion sizes, but since the food has a high energy density, they gain weight. In evolutionary terms, the human appetite was designed for low energy density foods. In the parts of the world where these foods are still the dietary staples, obesity is virtually non-existent. "Our bodies were never designed to cope with the very energy-dense foods consumed in the West, and this is contributing to a major rise in obesity," Prentice said.

Do you know who is most at risk from that order of super-sized fries? Kids. They haven't yet learned the dietary restraint needed to remain slim. Co-author Susan Jebb insists, "Fast food companies could play a major part in halting the rise in obesity if they adopted a more positive attitude to healthy eating such as providing meals of lower energy density."

The study findings were published in Obesity Reviews.