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. Dogs can smell our stress, experts find. How can that help anxiety sufferers? 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. Purely calorie-controlled diets don’t work Tim Spector, epidemiologist, King’s College London This means that advice had to be generalised. But start to look at the individual and a different picture emerges. 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. How intermittent fasting and exercise helped Hong Kong banker lose 32kg 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. Anyone competing in endurance events is acutely aware of the body’s preference for carbs over fats. Once you run out of carbs, you tend to “hit the wall”, where muscles feel heavy and everything tells you to rest. The twins wanted to devise a way to hack their metabolism to improve their ability to burn fat, so they could fuel themselves better for their Ironman races. They soon realised this might also be useful for anyone who wanted to improve their metabolism. Lumen claims to be able to monitor whether your body is burning carbs or fat in real time and to train it to switch between the two, burning fat when you want it to and using carbs only when you need that disposable energy. It works by monitoring the amount of carbon dioxide and oxygen in your breath, replicating a test of metabolism that has, until now, been available only in a laboratory setting. The accuracy of the device was verified against the lab version in a small, peer-reviewed study. Aside from losing weight, there are other reasons why you might want to know whether your body is burning carbs or fats, and get better at swapping between the two. Being able to switch between burning fats and carbs is a measure of metabolic flexibility. And it turns out that being metabolically flexible could be hugely beneficial for your health. The term “metabolic flexibility” was first used in the 1980s to describe an unusual ability of parasitic worms, which could switch between using different energy sources depending on their environment. Scientists started to wonder how important metabolic flexibility was to our own health. They found that people who were lean and healthy were good at using up glucose in the blood, and that people who were overweight had a lower capacity to burn carbohydrates. How to beat burnout as a stressed Hong Kong ranks bottom in well-being survey They were also interested in whether people were able to switch their fuel source depending on whether they needed a burst of energy for exercise. The answer emerged: people who are metabolically flexible rely on fats after not eating for several hours, then shift to carbohydrates after they have eaten. They find it easier to burn through the carbs available and switch back to fats when necessary. Importantly, the ability to shift between the two was a marker of overall metabolic health. The question then became whether we can help people to improve their metabolic flexibility, says integrative physiologist Audrey Bergouignan, at the University of Colorado, Denver. And this is where Lumen aims to help. The idea is that by switching more often between burning fats and carbs, you can train your body to do it more efficiently. This tallies with my experience. For the first couple of weeks, I found it impossible to avoid getting a reading of at least four on the five-point scale, where a one or two indicates fat burn, three suggests you are burning both carbs and fat, and four or five is a total carb fest. Waking up in a state of fat burn is one of the most important ways of testing metabolic flexibility. “Overnight, your body has time to burn through all of your carb stores. You should be like an empty tank,” says Kyla Blumenfeld, PR manager at Lumen. Then, when you start eating again, you fuel your day without an overload of carbs. This is important because if you have carbs in your bloodstream constantly, your body doesn’t have a chance to get in the mode of burning fats. Thankfully, you aren’t trying to cut carbs from your diet completely. [Fasting for too long] causes stress on the body that, in the long term, piles up Tim Spector, epidemiologist, King’s College London In fact, to encourage metabolic flexibility, Lumen will suggest a low-carb quota if you wake up burning carbs. Once you are better at burning fat, it will recommend high-carb days to keep your metabolism on its toes, making that important switch. Research indicates that being metabolically flexible is key to beating metabolic syndrome, which is a cluster of symptoms such as high blood pressure, high cholesterol and obesity that puts you at greater risk of heart disease, type 2 diabetes and stroke. Indeed, studies show that people with metabolic syndrome are much worse at switching between carbs and fats – they are metabolically inflexible. Of course, it could be that they are metabolically inflexible because they already have health issues. To get to the bottom of this, Bergouignan and her team went to the extreme. They knew that habitual physical activity is a good predictor of metabolic flexibility, so they took a group of women without any significant health problems and confined them to bed rest for two months. The big risks of leaving high blood pressure untreated – doctors explain The participants were fed a balanced diet, and yet not only did they become metabolically inflexible, they also became worse at controlling their blood glucose levels – predisposing them to metabolic syndrome. In addition, long-term research has found that metabolic flexibility is a predictor of how much body fat and weight gain a person will go on to have five years later. The takeaway from all this is that metabolic flexibility is to be taken seriously. Luckily, we can improve it, with exercise being one option. Bergouignan’s bed-rest studies may have been extreme, but her team has discovered that the same things happen when people take fewer steps. One of the biggest predictors for me of whether I wake up burning fats is whether I have been physically active the day before. The key is to exercise regularly. In research due to be published soon, Bergouignan’s team found that just three days of physical inactivity can make you metabolically inflexible. Responsible fasting is another way to improve metabolic flexibility by giving your body enough time to burn through all of the blood glucose from carbs and switch to using fats. Your body doesn’t need large amounts of energy at night, so Lumen advises that you stop eating a couple of hours before you go to bed. By pooling data from thousands of individuals using its devices, Lumen has also discovered surprising patterns that could help people improve their metabolic health. For instance, it found that 40 per cent of users fast for too long, putting their body in a state of stress. Stress, as well as poor sleep, promotes the production of cortisol, a hormone that can interfere with metabolism and result in surges in blood sugar. Aside from the risk of diabetes, such surges are bad for us because regular periods of high blood glucose have been linked to lethargy, ageing of the skin and an increased risk of cancer and dementia, as well as causing damage to blood vessels, which could increase heart disease risk. “It causes stress on the body that, in the long term, piles up,” says Spector. There are now devices that help you monitor these surges in real time and understand which foods cause them. Blood glucose monitoring was previously the preserve of people with diabetes and, until 2014, would have involved a pinprick of blood. Continuous monitors are small, wearable devices that constantly read glucose levels in fluid between cells in the skin, which is a good proxy for blood glucose. These personalised devices could make a big difference to health, even for those who think they are in good shape. One landmark study in 2018 used continuous glucose monitors on a group of 57 people and found that even those considered healthy experienced extreme spikes in glucose levels. In some people, just eating a bowl of cornflakes with milk triggered blood glucose spikes associated with prediabetes. Thankfully, there are simple ways we can all keep our blood sugar levels, well, level. One is the order in which you eat your food, which studies show can significantly affect your glucose response. To minimise a post-meal surge in glucose, you should start with fibre, such as veg, follow this with protein and end with any carbs and sugars. The fibre lines your intestine and the protein slows down digestion. Taken together, this will mean that the carbohydrates are digested more slowly, causing less of a steep rise in blood sugar levels. How ginger shots improve your well-being and boost your immune system For the same reason, if you have a sweet tooth, it is wise to get that sugar hit at the end of a meal rather than a few hours later as a snack on an empty stomach. Another trick is to eat whole foods and fibre that also contains starch to slow down digestion. Finally, avoid too much stress, which research shows prompts your body to release lots of glucose to give you the energy to fight or flee – not necessary when you are sitting down for dinner. As we start to understand individual variability in our metabolisms, it is likely that we will see more devices come onto the market. How they will affect our lives and health remains to be seen, but my experience with Lumen has at least left me making small shifts in my habits that could, in theory, drastically improve my metabolic health. I try to eat earlier in the evening and try to consume carbs only when I need them to fuel exercise. And it has certainly given me a renewed appreciation of the importance of sleep. Of course, no technology will make it any easier to keep up those changes – when it comes to resisting that morning croissant, the hard work is still on me. Catherine de Lange is New Scientist’s Magazine Editor.