My family holidays in France are synonymous with one thing: bread. But this year, as everyone else sits down to their croissants and baguettes, my plate is bare. Before even my first sip of coffee, I have to take a long, slow inhalation through a handheld device resembling a sleek electronic cigarette, hold my breath for 10 seconds, then exhale through the mouthpiece. Instantly, an app on my phone delivers the verdict as a score from one to five. Today, it is a one, and I am elated. For the first time in weeks, I have woken up in “fat-burn mode”. I have taken my first steps to hacking my metabolism. Usually, I would be the first to roll my eyes at any gizmo associated with weight loss and healthy eating. You don’t need an app to tell you which foods are good or bad. Or do you? Evidence has been mounting that suggests the one-size-fits-all advice on nutrition is drastically failing us, and that we have been thinking about metabolism – the chemical reactions in your cells that change food into energy – all wrong. Against this backdrop, many new technologies are hitting the market, claiming to reveal what your metabolism is doing as you go about your day – and giving tips on how to improve it. Make the right tweaks and you could find it easier to manage your weight, ward off disease, sleep better, and more. I wanted to understand how much of a difference such changes could make and whether ignoring the new science might be leading me to metabolic meltdown. Dietary advice used to be straightforward: to maintain a healthy weight, calories in and out should roughly match up. If you ate too much, you could burn it off with exercise. And it was widely accepted that your metabolism slows with age, accounting for that middle-age spread. Finally, specific foods were thought to trigger the same response in everyone – a banana was as good for you as it was for me. Over the past decade, however, each of these tenets has been dismantled. The problem is that metabolism is extremely complex, and trying to measure what a person is doing at any point in time has typically required some pretty elaborate equipment, so studies involved only a small number of people. Let’s take the question of calories, used to express the energy content of food. General advice suggests that men should consume 2,500 calories and women 2,000 each day. “But it’s a total guess,” says epidemiologist Tim Spector, at King’s College London. He should know. One gold-standard way to see what someone’s metabolism is doing is to put them in a sealed room, pump fresh air in and continually analyse the amount of carbon dioxide and oxygen. That’s because oxygen is used by our cells as they work, producing carbon dioxide that is ultimately exhaled. When Spector spent 24 hours in such a metabolic chamber, he discovered that the amount of calories he burned while doing nothing was fewer than 1,600. “If I’m having a 2,500-calorie diet, it’s going to be very hard for me to balance it every day,” he says. Surely exercising should help him get rid of those extra 900 calories? Not so fast. In the past few years, Herman Pontzer, associate professor of evolutionary anthropology and global health at Duke University, in the United States, and his colleagues have conducted studies of the Hadza people in Tanzania, who live a very active hunter-gatherer lifestyle, and found that, despite this activity, they burn around the same calories as someone sitting at a desk all day. It turns out that you can’t outrun a bad diet. Pontzer has also laid to rest the idea of middle-age spread. It seems our metabolism stays pretty constant until around the age of 60. When it comes to food, things are even more surprising. Work by Spector and others has shown that if two people – even identical twins – eat the same food, their bodies’ reactions could vary wildly. One might have a huge spike of glucose after that banana, followed by a sugar crash and accompanying hunger pangs, whereas the other would see only a gentle rise and fall in blood glucose levels and feel satiated until lunch. It is clear that every individual has a different metabolism, and possibly it responds differently at varying times of day and at different stages of life. What’s more, traditional ideas about how to keep the metabolism in check are not going to work. It is no wonder, then, that people are turning to technologies that could help them better understand what is going on. The device I am using is made by a company called Lumen. Like Spector’s own personal nutrition firm, Zoe, the first thing Lumen does is throw away the concept of calorie counting.“Purely calorie-controlled diets don’t work,” says Spector. Instead, the companies focus on macronutrients, the core components of our diets – in particular protein, fat and carbohydrate, the last two of which are the only types of fuel our bodies can use. Protein is important for building muscle, but isn’t used as a direct energy source, except in extreme cases where all available carb and fat stores have been used. If carbohydrates, which are present in the blood as glucose, are available, the body will prefer to burn these over fats. “Everything in your body is designed to prevent you from burning fat,” says Spector. Metabolising fat is hard for the body – it requires more chemical reactions than metabolising carbs, so provides less energy in the same amount of time. This is the key concept behind the Lumen system, which was devised by twin sisters Merav and Michal Mor, both physiologists and endurance athletes.
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