Yet a number of researchers have come to believe, as Wells himself wrote earlier this year in the European Journal of Clinical Nutrition, that ‘all calories are not equal’. The problem with diets that are heavy in meat, fat or sugar is not solely that they pack a lot of calories into food; it is that they alter the biochemistry of fat storage and fat expenditure, tilting the body’s system in favour of fat storage. Wells notes, for example, that sugar, trans-fats and alcohol have all been linked to changes in ‘insulin signalling’, which affects how the body processes carbohydrates. This might sound like a merely technical distinction. In fact, it’s a paradigm shift: if the problem isn’t the number of calories but rather biochemical influences on the body’s fat-making and fat-storage processes, then sheer quantity of food or drink are not the all-controlling determinants of weight gain. If candy’s chemistry tilts you toward fat, then the fact that you eat it at all may be as important as the amount of it you consume.
More importantly, ‘things that alter the body’s fat metabolism’ is a much wider category than food. Sleeplessness and stress, for instance, have been linked to disturbances in the effects of leptin, the hormone that tells the brain that the body has had enough to eat. What other factors might be at work? Viruses, bacteria and industrial chemicals have all entered the sights of obesity research. So have such aspects of modern life as electric light, heat and air conditioning. All of these have been proposed, with some evidence, as direct causes of weight gain: the line of reasoning is not that stress causes you to eat more, but rather that it causes you to gain weight by directly altering the activities of your cells. If some or all of these factors are indeed contributing to the worldwide fattening trend, then the thermodynamic model is wrong.