Professor Claude Bouchard, the outgoing president of the International Association for the Study of Obesity (IASO), described the concept of serious weight gain being the result of just a small imbalance between energy intake and expenditure of as little as 15 kcals as defective.
"This idea that obesity is the result of a tiny energy surplus accumulated over the years is quite misleading," Bouchard, a geneticist and executive director of Pennington Biomedical Research Center in the US, told this week's International Congress on Obesity in Sydney, Australia.
"I'm concerned when I hear those working in the health field and even some scientists coming out with this kind of notion, which is fundamentally flawed. Clearly if it truly were that simple, we wouldn't have the massive problem we are facing all over the world."
The small surplus proposition is based on the assumption that calories are simply converted into fat stores at no energy cost and with no resulting changes in an individual's energy requirements. But Bouchard said that not all surplus calories are stored with about a third are used up in the process of converting excess energy to fat stores.
The imbalance in the early stages of weight gain may be very small compared to the much larger surplus involved in weight gain at the later stages of obesity.
Bouchard claims that overweight and obese people actually have a normal metabolism, but that their basic resting energy requirements increase as they gain weight. In addition, obese people need to burn more energy to move around.
"This situation would be typical of someone who has become moderately overweight. In the case of a greater weight gain, as in frank obesity, the energy gap is much more substantial."
Someone at an end point with an excess weight gain of 30 kg would have an imbalance of about 600 to 700 calories per day. An excess weight of 40 kg and more - not untypical in the one third of the adult population who are obese in the USA, - can easily translate in an energy gap of 1,000 calories per day to sustain the new body mass in comparison the prior normal weight level.
"This indicates that much larger numbers of calories are consumed compared to the situation when the same individual was normal weight," he added.
One critical lesson, warned Bouchard, was that narrowing the energy gap was likely to be much more successful when both energy intake and energy expenditure are addressed - cutting down food intake, increasing physical activity, and decreasing time spent in sedentary inactivity. This, said Bouchard, can be achieved much more easily at the earlier stage of weight gain, before serious levels of overweight are attained when the energy gap or surplus is still relatively small.
"It is important that we have a much clearer understanding of this issue, which is not as complex as it sounds, and reinforces the message that prevention is much better than cure. So we need to focus on those who are shifting from normal weight into the overweight category to attack that smaller weight gain before it becomes much harder to overcome."