For years, we’ve been presented with a straightforward narrative when it comes to shedding pounds.

Experts have long insisted that weight loss boils down to a simple equation: calories consumed minus calories burned equals body fat. The advice? Cut 500 calories each day, and you’ll drop a pound every week.

This approach seems neat, logical, and highly scientific.

However, in practical terms, it’s hardly helpful. The reality is that body weight isn’t governed by simple math; it’s determined by biology, like most aspects of human physiology.

And central to human biology is the role of hormones.

When we attempt to reduce our calorie intake by 500, our bodies might react by compensating in one of several ways:

1. Burning 500 calories of body fat. Result – weight loss. Yay!

2. Burning 500 fewer calories (lowered metabolism). Result – you feel cold, tired but no weight loss. Boo!

Body weight is not controlled by arithmetic. It’s controlled, like almost all human physiology, by biology

Which one does our body choose? That is the only question that matters. But that’s not decided by willpower. It’s decided by hormones.

I’m a practicing physician, with over 30 years experience in treating diabetes. I’ve seen what works, and what does not, firsthand. By focusing on diet, I’ve helped thousands of patients lose weight successfully and keep it off. 

And what I can confirm with absolute authority is that calorie restriction diets always follow the same unhappy pattern. Weight drops, but only at first. It quickly hits a plateau. Metabolism slows. Hunger rises. Weight goes back up. You cry. Lather, rinse, repeat.

So, what went wrong, and how can we fix it?

Dr Jason Fung is author of The Hunger Code: Resetting Your Body’s Fat Thermostat in the Age of Ultra-Processed Food, published by Greystone Books

Food contains more than just calories – it contains instructions

As soon as we eat food, we release hormones. 

Two foods with identical calories can generate completely different hormonal responses. This tells your body what to do with the energy (calories). Do you store it (as body fat) or do you burn it (for metabolism)?

A hormone like insulin tells our body to store more calories. 

Satiety hormones, like GLP-1, GIP, peptide YY and cholecystokinin tell your body that you’ve eaten enough and you should stop. This means that, for fat storage, there are good calories and there are bad calories.

And that’s the missing piece in calorie logic.

How does your body keep it all organized? Through a balancing mechanism like a thermostat. 

Satiety hormones, like GLP-1, GIP, peptide YY and cholecystokinin tell your body that you’ve eaten enough, and you should stop 

The ‘skinny’ secret 

A room thermostat maintains a constant temperature whether it is boiling hot or freezing cold outside. If the room cools, the heater turns on. If it overheats, the air conditioning kicks in. 

The ‘body fat thermostat’ does the same job. If your body fat thermostat is set too high and you lose weight, your body will respond by either making you hungrier or lowering the metabolic rate to make you burn fewer calories.

Your hormones set the thermostat, which then coordinates hunger, fullness and metabolic rate to gain or lose that weight. 

That’s the secret behind how ‘naturally’ skinny people effortlessly stay that way. Their body fat thermostat is cranked way down, which naturally turns their hunger down.

And that’s why different foods, even with the same calorie content, have dramatically different effects on eating behavior. 

Imagine that you ate donuts, full of sugar and refined carbohydrates for breakfast, perhaps accounting for 800 calories. Insulin spikes, telling your body to store the calories (as body fat) leaving few calories available for metabolism. 

At the same time, it doesn’t activate any satiety hormones. Result? Shortly after the donuts, you’ll be hungry again.

Now imagine that you ate an 800-calorie vegetable omelet instead. Insulin stays low, so few of those calories are shuttled into storage (body fat). Multiple satiety hormones are activated, which keeps us full at least until lunch. 

So if your plan depends on cutting calories by white-knuckling hunger forever, you’ve already lost. You can’t fight biology (the body fat thermostat) with slogans (a calorie is a calorie). 

You don’t get fat because you ate too many calories. You ate too many calories because you got fat (the body fat thermostat is increased). 

Why did you eat ‘too much’? Because you were hungry.

Different foods, even with the same calorie content, have dramatically different effects on eating behavior

You can reset the thermostat

To reduce hunger, we need to reduce the body fat thermostat, which means eating in a manner that best affects hunger and satiety hormones.

Avoiding ultra-processed foods (UPFs) is my #1 golden rule of weight loss. 

UPFs are digested and absorbed very quickly, allowing insulin to spike. 

These foods are deliberately engineered to provide maximum pleasure and minimal satiety.

They are also packaged, convenient and cheap, making it very easy to develop the bad habit of eating a package of cookies in front of the TV, something that is less easy for scrambled eggs.

Ultra-processed foods can often be identified by the presence of multiple ingredients that you can’t pronounce or have never heard of. 

If it contains xanthan gum, carrageenan calcium propionate, potassium sorbate, etc, it’s a UPF. 

The most common foods in this category are commercial white bread, pre-packaged meals, breakfast cereals, reconstituted meat products (sausages, fish sticks, chicken nuggets), candies, chocolates and snacks.

Avoiding ultra-processed foods (UPFs) is my #1 golden rule of weight loss 

Next, maintain a natural fasting period

Not eating (fasting) is the most efficient and effective way to lower insulin levels. It naturally allows the body to go from a fat-storage mode to a fat-burning mode, because insulin is the main switch. 

In the 1960s, people would naturally fast for 12-14 hours every day, without even considering it a fast. That’s where the term ‘breakfast’ comes from.

If you want to push it a little to lose weight, consider pushing that up to 16 hours daily or occasionally even up to 24 hours, which is a one-meal-a-day schedule.

Eat a low-insulin/high-GLP1 diet

There are other simple ways to reduce the levels of insulin released without changing the number of carbohydrates. This includes:

1. Eat earlier in the day and avoid late-night large meals

2. Eat natural rather than processed carbohydrates

3. Eat carbohydrates together with proteins and fats rather than ‘naked’ carbs

4. Eat vinegar or lemon juice together with carbs

5. Walk immediately after eating

Then prioritize foods that stimulate the ‘right’ kind of hormones instead.

For example, eating protein activates satiety signals like GLP1 and peptide YY, where a low-fat muffin does not. This means that eating a steak is more filling. 

Eating foods with fiber activates stretch receptors in the stomach wall, which is another satiety signal. So eating bulky foods like broccoli can fill you up, even though it has relatively few calories.

Weight loss is not about punishment. It’s about aligning your target weight with basic human physiology (hormones). 

Stop counting calories and start controlling biology that drives eating behavior. 

When you change the body fat thermostat, the math takes care of itself.

Dr Jason Fung is author of The Hunger Code: Resetting Your Body’s Fat Thermostat in the Age of Ultra-Processed Food, published by Greystone Books.