WHEN you are navigating the complex world of nutrition, the calorie feels mercifully simple. A straightforward measure of the energy in our food, calories sit at the heart of conventional weight control advice. Consume too many, or fail to burn enough, and you will put on weight. Want to slim down? Eat less and move more. As the saying goes, 鈥渁 calorie is a calorie is a calorie鈥�. Dig a little deeper, however, and you find it isn鈥檛 that simple.
This idea of the body as a furnace and food as its fuel was popularised by Wilbur Atwater in the 19th century. He deduced that there are three main components, or macronutrients, with which we can calculate the calorie content of any food: protein and carbohydrates contain 4 calories per gram, while fats contain 9. Combine these and the result informs the calorie content on food labels everywhere.
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But think about how food is consumed and you hit your first problem. Take an orange, says , a geneticist at the University of Cambridge and author of Why Calories Don鈥檛 Count. An orange is mainly carbohydrate, or sugar. 鈥淲hen you drink orange juice, your body will absorb the sugar [quickly] because there is no digestion required,鈥� says Yeo. Eat orange segments, however, and it takes energy for the digestive system to work on the fibre and extract the sugar. 鈥淧hysiologically, your body does completely different things with it, with exactly the same calorie hit,鈥� says Yeo.
Orange juice causes a more rapid spike in blood sugar levels than the orange segments, for example, and regular surges in blood sugar levels can increase the risk of many conditions, from diabetes to dementia. The orange segments also take longer to reach the gut, stimulating hormones that help us feel full.
Next, throw in the effects of cooking. The body uses energy to digest food. Cooking is essentially an extension of digestion: it breaks down ingredients, making it easier for the body to extract the energy. For processed and cooked foods, the calorie count you see on labels is most likely to be an underestimate. A stick of raw celery has about 6 calories in it, for example. 鈥淚f you cook that stick of celery, that 6 calories becomes 30,鈥� says Yeo.
Atwater鈥檚 calculations also fail to account for the cost of digestion. Protein may contain 4 calories per gram, but because of the work needed to digest it, we will only ever extract 70 per cent of those, says Yeo. This is why labels on foods containing protein overestimate calories.
The other thing to consider is that people can respond very differently to the same food, resulting in . And finally, calories vary in their nutritional content. A lollipop contains roughly the same number of calories as a small apple, for instance, but consuming lots of nutritionally 鈥渆mpty鈥� calories will soon take a toll on health.
Why exercise won鈥檛 make you thin
So much for the 鈥渃alories in鈥� side of the equation. But 鈥渃alories out鈥� is equally bamboozling. We think of exercise as a 鈥渂ig lever鈥� to get rid of excess calories, says at Duke University in North Carolina. But work by Pontzer and others looking at various populations, from hunter-gatherers to ultra-runners and sedentary desk workers, has found that pretty much everyone burns around the same number of calories each day no matter how much exercise they do.
Pontzer鈥檚 suspicion is that the body adapts to increased exercise in ways that we don鈥檛 fully understand. Exercise is great for overall health 鈥� but it doesn鈥檛 help much with weight loss, which suggests the solution to obesity isn鈥檛 quite as simple as eating less and moving more.
The upshot is that the body isn鈥檛 just a furnace. It has a sensitive thermostat, constantly adjusting to the environment. So while calories are temptingly simple at first glance, we have to reckon with the complexity of the human body.
