Fast food bypasses our body's natural appetite control systems, argue British researchers, which is why they make us fat.

Professor Andrew Prentice of the London School of Hygiene and Tropical Medicine and colleague Dr Susan Jebb, report their study on how fast food can fool you, in the November issue of the journal Obesity Reviews.

It may seem obvious to say that eating takeaway food can make you fat, but researchers say we've evolved to eat a certain amount of food at mealtime, no matter what kind of food it is. The problem is that a regular size fast food meal contains one and a half times as many calories as a British meal, and two and a half times the calories of an African meal, they report.

So while we would consume a lot more calories in a fast food meal than say, a meal of steamed vegetables and rice, it would seem as if we had eaten the same amount of food.

"You'd need to eat well below the portion size offered to avoid greatly exceeding recommended energy and fat requirements," says Jebb.

"Fast foods are frequently linked to the epidemic of obesity, but there has been very little scientific appraisal of a possible causal role," say the researchers.

Prentice and Jebb reviewed studies which showed that energy density (the amount of calories different foods contain weight for weight) is a critical factor in regulating food intake. They say the evidence shows that food with a high energy density can cause people to accidentally eat more calories than they need - a phenomenon known as "passive over-consumption".

In one study conducted by the Medical Research Council, two groups of volunteers were fed seemingly identical diets. But in fact the diets had been altered to have a range of energy densities (calories). Those volunteers on the high energy diets gained fat at a remarkable rate - over 60g per day - despite the fact that they believed they were eating a normal healthy amount of food.

The researchers conclude that a diet high in fast foods would increase a person's risk of weight gain and obesity, even though they may feel that they are eating no more than an average meal.

Prentice and Jebb argue that this is probably because the human body is not used to regulating intake of fast food: "Since the dawn of agriculture, the systems regulating human appetite have evolved for the low energy diet still being consumed in rural areas of the developing world where obesity is almost 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."

Australian obesity expert, Professor Ian Caterson of the University of Sydney said further research was required to confirm how fast food might interfere with appetite regulation. He said volume was one factor, but other factors were neural sensors and hormonal feedback loops that tell someone to stop eating. Earlier this year, New Scientist reported that researchers had found that fast food triggered hormonal changes that could make it difficult to control eating.

"It's nice to see there's a review of published literature and it's nice to have an example of energy density versus diet," Caterson told ABC Science Online. "It's something we could probably work out in Australia, given our nutrition survey but to my knowledge no one's done it yet."

"There's always been argument about what in the diet causes obesity and fat is certainly associated with it but we've probably overemphasised that," he said. "But there are obviously other things in the diet that cause you to get fat and one is the total energy."

He said popular "reduced fat foods" were also a problem. "They tend to have more energy because when you take the fat out, to keep the consistency and feel you have to add something back in and they tend to have the same if not more energy than was in the original food."

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