Energy-dense fast foods and convenience meals increase the risk of obesity by encouraging unintentional over-eating, report scientists from the UK's Medical Research Council (MRC).
They called for more healthy options in fast food restaurants and better nutritional profiles of foods offered in supermarkets to convey the message to consumers.
Professor Andrew Prentice and Dr Susan Jebb combined knowledge gained from meticulous studies on volunteers in the UK and Africa with information on the composition of foods obtained from fast food company websites.
Their research, published in the November issue of the journal Obesity Reviews, investigated data showing that energy density (the amount of calories different foods contain weight for weight) is a critical factor in regulating food intake.
Foods with a high energy density can cause people to accidentally eat more calories than they need, explained the team. A typical fast food meal has a very high energy density. It is more than one and a half times higher than an average traditional British meal and two and a half times higher than a traditional African meal.
The researchers concluded that a diet high in fast foods will increase a person's risk of weight gain and obesity, even though they may feel that they are eating no more than they would if they ate an average meal.
Professor Andrew Prentice, head of the MRC International Nutrition Group at the London School of Hygiene and Tropical Medicine, said: "We all possess a weak innate ability to recognise foods with a high energy density. 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.
"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," he added.
He also emphasised the consequences of a diet high in fast foods for children: "Children have not yet developed any of the learned dietary restraint that needs to be exerted by anyone wishing to remain slim in the modern environment."
"It's surely a stark paradox that the strategy used to achieve rapid weight gain in malnourished children in Africa - the frequent offering of energy-dense foods - has now become the norm for many overweight children in affluent societies."
Dr Susan Jebb, head of Nutrition and Health Research at the MRC Human Nutrition Research Centre, said that it was virtually impossible to select a combination of items with even a moderate energy density in many fast food outlets.
"You'd need to eat well below the portion size offered to avoid greatly exceeding recommended energy and fat requirements," she noted.
However, she welcomed the practice by some fast food companies of introducing healthier options and encouraged others to do more. "Fast food is here to stay, but our analysis shows that it is possible to offer healthier options. 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, appropriately marketed and with point-of-sale nutrition labelling," she said.
And other sectors also need to support the healthy eating message. "Many supermarket ready meals and convenience foods are also very energy dense. If we're going to stem the tide of obesity, it's important that we don't just swap one unhealthy meal for another. Research has shown time and again that to maintain a healthy weight, we need to eat foods with less fat and added sugars and to take more exercise," said Dr Jebb.
The researchers also pointed to the need to do more research on fast food eating habits to help inform dietary advice. Research into who consumes fast food and how much they eat will enable healthy eating messages to be better targeted.
Obesity Reviews fast-tracked publication of this research in light of the escalating obesity epidemic. The World Health Organisation has identified obesity as the leading global cause of diet-related disease, with an estimated 300 million obese people worldwide, and rising. In England obesity rates have trebled in the past 25 years. Current statistics show that without urgent and effective action, three in ten adults in the UK will be obese by the end of the decade.