Anyone who’s fought a lifelong battle of the bulge – cutting calories, working out – might wonder why they still can’t lose weight. That’s because, according to Dr Jason Fung, a practising kidney specialist, based in Toronto, Canada, the physiology of obesity is far more complex than “eat less, move more.”
In his 2016 multi-million copy bestseller, The Obesity Code, he argued that obesity is a hormonal imbalance, not a calorific one, as having chronically high insulin levels, for instance, drives you to store fat. He’s now written another excellent, revelatory science-based book, The Hunger Code: Resetting your body’s fat thermostat in the age of ultra-processed food, in which he reveals the secrets of long-term weight loss and better health. They may surprise you.
It’s not the calories you eat, it’s the calories you store
Dr Fung doesn’t claim that calories don’t matter at all. He argues that weight loss – or gain – is not as straightforward as the basic maths of calories in, calories out. This is “too simplistic,” he says.
What actually matters, he believes, is what your body does with those calories – how much of what you eat you burn, or store as fat. “Every single study of weight loss shows that when you just count your calories and eat less, your basal metabolic rate – the number of calories that you burn – goes down. So you could eat less, but you’ll burn less too, so still not lose that body weight,” he says.
Fung cites three large studies, including The Women’s Health Initiative, in which participants reduced their intake by 371 calories, and increased their physical activity by 10 per cent. “Exercise, while terrific for health, doesn’t increase calorie burn, as the body compensates by slowing metabolism afterwards,” he notes. Meanwhile, after seven years of dieting, he reports, “they did not weigh any less than the women in the study who didn’t change their diet at all.”
Dr Fung says that it’s not just the number of calories that matters, but what your body does with them – Bernard Okulaja Photography
To reiterate, the key is what your body does with your calories. “For every calorie you eat, your body could either store it or burn it. So it’s not just the number of calories that’s important. If your body stores it, you’re going to get fat. If it burns it, you’re not going to get fat.”
Which of these it does depends on what you eat. Think in terms of “good calories” and “bad calories.” Treating all calories as equal is a recipe for weight gain. Most of us know that some foods are more fattening than others, says Fung. Two hundred calories of biscuits is more fattening than 200 calories of carrot. But why? Because as well as calories, he says, all food contains “information” for the body. Critical to weight loss (or gain) is what hormones are stimulated by the things you eat.
Hormones make you hungry
The most important factor determining our body weight is hormones. The hormone GLP-1 tells us to feel full. Ghrelin tells us to be hungry. Leptin tells us to lose body fat. And the hormones insulin and cortisol tell us to store body fat. So if what you eat largely stimulates insulin, you’ll gain fat.
Fung explains, “If you eat an 800-calories, three-egg vegetable omelette for breakfast, you’re full. It lasts you till lunch, even dinner. If you drink an 800-calorie Starbucks frappuccino, you’re hungry five minutes later.” The calorific content is the same, but the two portions are completely different. “Why? Because they stimulate different hormones. Every food you eat contains calories, but also instructions and information on what the body should do with those calories.”
When you eat that vegetable omelette, it doesn’t stimulate much insulin, he says. “So most of the calories are available as energy for the body.” GLP-1 is therefore released, telling us we’re full. “But after the frappuccino, insulin spikes like crazy, all of those calories immediately are stored as body fat, so you have no energy for the body.” Your blood sugar drops and – understandably – your body sends out hunger signals by releasing a ghrelin surge. “Five minutes later, you’re thinking ‘I need something to eat,’” he says. “It’s the hormones that ultimately determine whether you gain weight or not, not the calories.”
The timing of food matters too. Even if you eat exactly the same meal in the morning and at night, “they produce very different effects on the insulin levels,” says Fung, which makes perfect sense. “Insulin’s job is to tell you to store fat, so if you eat very late at night, you’ll have about 25 per cent higher insulin levels than in the morning. It’s because you’re going to go to bed, so your body is thinking, ‘What am I going to do with all these calories? I should put them into storage’, whereas in the morning it knows you have just got up and might need the energy for the day ahead.”
Eggs and vegetables are amongst the foods to eat on a low-insulin diet
Meal order also contributes to how much insulin is released. Eat fish and vegetables before pasta, eggs before toast – as fibre, fat, and protein don’t stimulate much insulin, and eating them first slows your digestion and absorption of carbs, meaning less insulin is stimulated.
Avoid eating only carbs, which Fung calls naked carbohydrates. “If you eat white bread and jam, for example, it’s all pure carbohydrate, which is all glucose.” This means it’s digested and absorbed at speed, spiking insulin. He cites experiments where participants ate two meals, “bread and orange juice, then 10 minutes later, chicken and vegetables – or you flip it around, chicken and vegetables then 10 minutes later, bread and orange juice. The insulin level is 50 per cent lower when you eat the carbs last.”
Foods to eat on a low-insulin diet include meat and poultry, fish, eggs, cheeses, full fat yogurt, nuts and seeds, vegetables, pulses, legumes, fermented foods, fruit (low-fructose like berries), healthy fats, and spices.
How to reset your fat thermostat for good
As Dr Fung says in The Hunger Code, our body keeps all its critical systems (including hydration, temperature, blood oxygen levels, and – yes – body fat) automatically in balance. These are its “homeostatic mechanisms.” How much body fat we carry is set at an optimal point called “the body set weight” by our body’s “fat thermostat”. This is because, “like wild animals who maintain relatively constant body fatness despite varying conditions” to optimise survival, we’re not meant to be very thin or fat.
Fung explains, “The amount of body fat we carry is very tightly controlled.” If it’s too high or low, our body adjusts, boosting or lowering our hunger levels to reset it, then slowing or speeding up our metabolism. Or it should do. Obesity is “a disorder of the body set weight being too high”.
So how does our fat thermostat get out of whack? “There are certain hormones that push it up” – insulin, cortisol – “and there are certain hormones that push it down,” says Fung. “If you stimulate GLP-1, like Ozempic and Mounjaro weight loss jabs do, your bodyweight goes down.”
Thick smoothies packed with seeds are more satiating than thin liquids (like juices or fizzy drinks) – Moment RF
GLP-1 hormones act indirectly on fat levels by increasing or decreasing hunger or satiety. A diet of mainly processed foods and refined carbs spikes insulin and drives hunger, overeating and weight gain. It can lead to insulin resistance – where increasing amounts of insulin are needed to clear blood glucose after eating – leading to chronically high insulin levels. This “keeps pushing that thermostat up,” says Fung. Then it’s hard to sustain weight loss as your body defends that high set point.
Other hormones also affect our weight and propensity to store fat. Testosterone, higher levels of which drive the body to make muscle and burn fat, explains why teen boys can eat 5,000 calories a day (“they clean out your fridge!” exclaims Fung, who has two sons aged 19 and 22) and not gain fat. Why? “Because the hormones are directing those calories to burning.”
Oestrogen, meanwhile, is an appetite suppressant, so perimenopause – when it fluctuates and drops – is the “highest risk period of weight gain for a woman,” says Fung. “When you have less oestrogen, you tend to eat more. It’s pushing that thermostat up.” If you try to outwit it by cutting calories, your body “ramps down your metabolic rate.”
So what can you do? What you eat (or don’t) is key. “Fasting is actually a very good way to get that [thermostat] down. You turn off the food, you try to get the insulin down,” he says. This forces the body to burn fat for fuel.
Fung advises a low carb, high fat diet – a “low insulin diet” – plus intermittent fasting to lose weight and keep it off. He admits, “I actually have a tendency to gain weight” – but following his own advice recently enabled him to fit into some 25-year-old green trousers. “They’re awful, but I put them on anyway. My wife was not impressed!”
