The science behind why you can’t lose weight  

If weight loss were simply an accounting problem, you could solve it with a calculator: eat less, move more, done. Yet millions of people have followed that advice for years and failed. Miserably. The issue is not effort or lack of willpower. Weight loss is not maths, it’s about physiology and psychology.  

You would not treat someone with alcoholism by telling them to “just drink less”. You would ask what is driving the behaviour and then deal with the root cause. Is it depression? Is it addiction? The same question matters for food. We need to ask not whether calories in exceeds calories out, but why?  

Calories in and calories out is bookkeeping. It is downstream thinking, meaning that it describes what happened after the fact. It does not explain what pushed you to keep eating, to crave, to snack, to finish the bag of chips. Real solutions live in upstream thinking, by correcting the root cause of weight gain, which is hunger. 

We eat because we are hungry and we stop eating because we are full. That’s it. If overeating is the issue, then the root cause is really ‘over-hunger’. So, the real question becomes: what makes us hungry in the first place?  

1. Physical (homeostatic) hunger: hormones and neurotransmitters, not willpower 

Physical hunger, the stomach-growling kind, and its opposite, satiety are controlled by hormones, including ghrelin, peptide YY, cholecystokinin, GLP-1 and GIP and appetite centres in the brain that blunt appetite.  

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Imagine that for breakfast you ate a three-egg vegetable omelette with cheese, which contains perhaps 500 calories. The omelette will keep you full at least until lunch. Now imagine instead that you drank a 500-calorie Frappuccino. You’d probably be hungry 10 minutes later. The omelette and the Frappuccino provoke dramatically different hormones and have dramatically different effects on eating behaviour.

This means that some foods are more fattening than other foods. Which seems self-evident, despite the constant exhortations that ‘a calorie is a calorie’. Cookies are more fattening than broccoli. Brownies are more fattening than eggs. That’s life. The importance of controlling hunger is the key lesson of weight-loss drugs like Ozempic, which act on the GLP-1 hormones. This drug neither contains nor blocks calories. Instead, the hormones suppress hunger.  

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2. Emotional (hedonic) hunger: pleasure that overrides fullness 

Hedonic hunger is eating for pleasure or comfort. The emotional pay-off is the reason that dessert still looks ‘worth it’ even after a full meal. It is also the reason that comfort foods soothe our stress and boredom and bring us joy during celebrations. Ultra-processed foods are engineered for maximum reward and minimum satiety, which trigger the brain’s reward system so hard that we can’t possibly resist ‘one more bite’. 

3. Social and environmental (conditioned) hunger: you were trained to eat all the time 

Conditioned hunger is triggered by routine and surroundings. Breakfast time. Snack time. A movie. A sports game. Conditioning is a learned behaviour. Your brain links cues such as specific times and events to rewards and behaviours, like eating. When almost every moment can trigger eating, ‘food noise’ can be the loudest voice in the room.  

All of us need to stop treating hunger like a moral failure. Instead, focus on avoiding ultra-processed foods; on eating whole, natural foods that create satiety and reduce hunger. Not eating all the time allows our body to use the food energy (calories) we’ve stored away. Choose activities and friend groups that don’t involve food. Discard as many food cues as you can.

Instead of defaulting to ‘eating all the time’, develop the mindset and habits that support weight loss. When you feel hungry, ask yourself the key question: What kind of hunger is this? Physical, emotional or social? Each type of hunger needs a different toolkit. In other words, don’t bring a snorkel to a bike race. 

The premise behind The Hunger Code is simple: hunger is not a choice. You can decide not to eat, but you can’t decide to not be hungry. When you understand what drives your appetite, you can start working with physiology instead of against it. And once you control hunger, eating less stops feeling like punishment and starts becoming automatic. Weight loss is not maths and hunger is not a choice. Achieving a healthy weight is about understanding the different types of hunger. 

The Hunger Code by Dr Jason Fung is out now (Greystone Books, £21.99).

You can buy it from the Big Issue shop on bookshop.org, which helps to support Big Issue and independent bookshops.

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