As a general rule it is true that if you eat vastly fewer calories than you burn, you’ll get slimmer (and if you consume far more, you’ll get fatter). But the myriad faddy diets flogged to us each year belie the simplicity of the formula that Camacho was given. The calorie as a scientific measurement is not in dispute. But calculating the exact calorific content of food is far harder than the confidently precise numbers displayed on food packets suggest. Two items of food with identical calorific values may be digested in very different ways. Each body processes calories differently. Even for a single individual, the time of day that you eat matters. The more we probe, the more we realise that tallying calories will do little to help us control our weight or even maintain a healthy diet: the beguiling simplicity of counting calories in and calories out is dangerously flawed.
Susan Roberts, a nutritionist at Tufts University in Boston, has found that labels on American packaged foods miss their true calorie counts by an average of 18%. American government regulations allow such labels to understate calories by up to 20% (to ensure that consumers are not short-changed in terms of how much nutrition they receive). The information on some processed frozen foods misstates their calorific content by as much as 70%.
That isn’t the only problem. Calorie counts are based on how much heat a foodstuff gives off when it burns in an oven. But the human body is far more complex than an oven. When food is burned in a laboratory it surrenders its calories within seconds. By contrast, the real-life journey from dinner plate to toilet bowl takes on average about a day, but can range from eight to 80 hours depending on the person. A calorie of carbohydrate and a calorie of protein both have the same amount of stored energy, so they perform identically in an oven. But put those calories into real bodies and they behave quite differently. And we are still learning new insights: American researchers discovered last year that, for more than a century, we’ve been exaggerating by about 20% the number of calories we absorb from almonds.
The process of storing fat – the “weight” many people seek to lose – is influenced by dozens of other factors. Apart from calories, our genes, the trillions of bacteria that live in our gut, food preparation and sleep affect how we process food. Academic discussions of food and nutrition are littered with references to huge bodies of research that still need to be conducted. “No other field of science or medicine sees such a lack of rigorous studies,” says Tim Spector, a professor of genetic epidemiology at Kings College in London. “We can create synthetic DNA and clone animals but we still know incredibly little about the stuff that keeps us alive.”
Research published this year showed that a certain set of genes is found more often in overweight people than in skinny ones, suggesting that some people have to work harder than others to stay thin (a fact that many of us already felt intuitively to be true). Differences in gut microbiomes can alter how people process food. A study of 800 Israelis in 2015 found that the rise in their blood-sugar levels varied by a factor of four in response to identical food.
Some people’s intestines are 50% longer than others: those with shorter ones absorb fewer calories, which means that they excrete more of the energy in food, putting on less weight.
The message from many public authorities and food producers, especially fast-food companies that sponsor sports events, is that even the unhealthiest foods will not make you fat if you do your part by taking plenty of exercise. Exercise does, of course, have clear health benefits. But unless you’re a professional athlete, it plays a smaller part in weight control than most people believe. As much as 75% of the average person’s daily energy expenditure comes not through exercise but from ordinary daily activities and from keeping your body functioning by digesting food, powering organs and maintaining a regular body temperature. Even drinking iced water – which delivers no energy – forces the body to burn calories to maintain its preferred temperature, making it the only known case of consuming something with “negative” calories. A popular expression in English tells us not to “compare apples and oranges” and assume them to be the same: yet calories put pizzas and oranges, or apples and ice cream, on the same scale, and deems them equal.
The scientific and health establishment knows that the current system is flawed. A senior adviser to the UN’s Food and Agriculture Organisation warned in 2002 that the Atwater “factors” of 4-4-9 at the heart of the calorie-counting system were “a gross oversimplification” and so inaccurate that they could mislead consumers into choosing unhealthy products because they understate the calories in some carbohydrates. The organisation said it would give “further consideration” to overhauling the system but 17 years later there is little momentum for change. It even rejected the idea of harmonising the many methods that are used in different countries – a label in Australia can give a different count from one in America for the same product.
The calorie system, says Camacho, lets food producers off the hook: “They can say, ‘We’re not responsible for the unhealthy products we sell, we just have to list the calories and leave it to you to manage your own weight’.” Camacho and other calorie dissidents argue that sugar and highly processed carbohydrates play havoc with people’s hormonal systems. Higher insulin levels mean more energy is converted into fat tissues leaving less available to fuel the rest of the body. That in turn drives hunger and overeating. In other words the constant hunger and fatigue suffered by Camacho and other dieters may be symptoms of being overweight, rather than the cause of the problem. Yet much of the food industry defends the status quo too. To change how we assess the energy and health values of food would undermine the business model of many companies.