Sugar: Why Your Body Treats It Differently to Everything Else You Eat

I used to eat a kilogram of ice cream in a sitting. A litre of Coca-Cola and a kilogram of chocolate to stay awake through late nights. I'm not exaggerating. This was just life, and I didn't think much about it because I was active, I wasn't obviously overweight, and nobody around me was raising the alarm.

What I didn't understand was what was happening inside my body every single time I did it.

This post is about sugar. Not in a moralistic, you-should-feel-bad-about-this way, but in a genuinely interesting, here-is-what-is-actually-happening way. Because once you understand the mechanism, the choices become a lot easier to make.

Sugar Is Not Just Food With Extra Calories

The standard way of thinking about food is that it provides energy, and too much energy stored as fat causes weight gain. By that logic, sugar is just a fast energy source, and the problem is simply eating too much of it.

This turns out to be an oversimplification that misses most of what's actually going on.

Food, in the broader sense, is supposed to give us energy, support our health, and do no harm. Most whole foods do this reasonably well. Sugar, and specifically fructose, the component of table sugar that makes it sweet, behaves differently to almost everything else we eat. It doesn't follow the normal energy pathways. It is processed almost entirely by the liver, in a way that more closely resembles how the liver handles alcohol than how it handles a piece of chicken or a bowl of oats.

This isn't a fringe claim. It's the basis of significant research by scientists including Dr Robert Lustig at the University of California and Dr Richard Johnson, whose book Nature Wants Us To Be Fat details over a decade of research into exactly this question. The picture that emerges from their work is both fascinating and sobering.

The Survival Switch

Dr Johnson's central finding is built around what he calls the survival switch, and it's one of the most useful frameworks I've come across for understanding weight and metabolic health.

In nature, many animals face periods of feast and famine. Bears fatten before winter. Migratory birds build fat reserves before long flights. Certain whales accumulate extraordinary fat stores. What triggers this process? Fructose.

When animals consume fructose, typically from ripe fruit at the end of summer, it activates a metabolic pathway that switches the body into fat storage mode. Energy that would normally be burned gets stored instead. Hunger increases. Movement decreases. The body is preparing for a period of scarcity that, in nature, reliably follows the abundance.

This is the survival switch. In animals it turns on and off seasonally. In humans eating a modern diet, it has been permanently switched on.

Every time you consume fructose, whether from table sugar, high-fructose corn syrup, fruit juice, or many processed foods, you are activating the same ancient survival mechanism. Your body responds as if winter is coming. It stores fat, drives hunger, and suppresses the signals that would normally tell you to stop eating.

This is not a character flaw or a lack of willpower. It is a biological response that evolved over millions of years to keep animals alive through scarcity. The problem is that the scarcity never arrives anymore, so the switch never turns off.

The Insulin Connection

When you eat carbohydrates, your body breaks them down into glucose, which enters the bloodstream. Your pancreas releases insulin to move that glucose into your cells where it can be used for energy. This is normal and healthy when it happens occasionally and in response to moderate glucose levels.

The problem arises when glucose spikes rapidly and repeatedly. This happens with sugar, obviously, but also with refined carbohydrates like white bread, white rice, pasta, and potatoes, which your digestive system converts to glucose almost as quickly as pure sugar. Your body responds to these spikes by releasing large amounts of insulin to bring blood sugar back down as fast as possible.

Over time, with repeated spikes, cells start to become less responsive to insulin's signal. You need more insulin to achieve the same effect. This is insulin resistance, and it sits at the root of type 2 diabetes, metabolic syndrome, and a cluster of conditions that have become remarkably common in societies eating a Western diet.

The fructose component of sugar makes this worse in a specific way. Unlike glucose, fructose does not trigger insulin directly. This sounds like good news but it isn't, because fructose drives insulin resistance in the liver through a different mechanism, and it does so without the satiety signals that glucose produces. You can consume a large amount of fructose without your body registering that it has received significant energy. The hunger switch stays on.

Where Fructose Hides

This is where most people are genuinely surprised, because fructose is not just in the obvious places.

Table sugar is sucrose, which is half glucose and half fructose. High-fructose corn syrup, found in an extraordinary range of processed foods, is as the name suggests heavily fructose. Fruit juice concentrates fructose from multiple pieces of fruit into a single glass, strips away the fibre that would slow its absorption, and delivers it to your liver in a rush. A glass of orange juice can contain the fructose of four or five oranges with none of the protective effect of eating those oranges whole.

Smoothies are better than juice but not as good as most people assume. Blending fruit breaks down its fibre structure, which increases how quickly the sugars are absorbed. A smoothie made from several pieces of fruit is still a significant fructose load, delivered faster than your body was designed to handle.

Then there are the foods most people consider healthy. Bread, pasta, rice, and potatoes are not high in fructose directly, but they spike blood glucose rapidly and trigger the insulin response described above. The body's reaction to a bowl of white rice is not dramatically different from its reaction to a bowl of sugar when you measure what happens in the bloodstream. This was one of the more confronting things I learned when I started reading seriously about this, because these foods had been staples my entire life and nobody had suggested there was anything wrong with them.

Whole Fruit Is Different

Before anyone starts feeling anxious about fruit, it's worth being clear about this distinction because it matters.

Whole fruit contains fructose, yes. But it also contains fibre, water, and a range of nutrients that change how that fructose behaves in your body. The fibre slows digestion, blunts the absorption of sugar, feeds beneficial gut bacteria, and produces satiety signals that tell you when you've had enough. Eating two apples is a very different metabolic event to drinking the juice of two apples, even though the fructose content might be similar.

The general consensus among researchers in this area is that whole fruit, consumed in reasonable amounts, is not a significant problem for most people. The fibre is the key. It's not the fruit that's the issue. It's the removal of everything that comes with the fruit.

What We Experienced

When my wife and I started a ketogenic diet, removing sugar and refined carbohydrates, something happened that we hadn't expected and hadn't been told to expect.

My IBS symptoms largely disappeared. The bloating, the discomfort, the unpredictability, most of it gone. My gut settled in a way it hadn't in years. We both lost weight without any change in how much we were eating in terms of volume. The sugar cravings, which had felt like a permanent feature of life, faded within a few weeks.

I also noticed that without sugar I had less of that mid-afternoon energy crash that I'd spent years fighting with more food. The energy was more even throughout the day, which turned out to be exactly what you'd expect when you stop riding the blood sugar rollercoaster.

I won't pretend we've been perfect. I've eaten entire tubs of ice cream since starting this journey and I'll probably do it again. But the difference between how I feel eating this way and how I felt before is clear enough that going back entirely is not something I want to do. The body keeps honest records even when you'd rather not look at them.

The Practical Question

Knowing all of this, what do you actually do?

The goal doesn't have to be eliminating sugar entirely, though some people find that easier than moderation. A more practical starting point is simply becoming aware of where fructose is hiding in your diet and making deliberate choices about it.

Read labels. Sugar appears under dozens of names: sucrose, high-fructose corn syrup, dextrose, maltose, agave nectar, fruit juice concentrate, and many others. If something sweet and processed is in the ingredient list, fructose is almost certainly present.

Swap juice for whole fruit. Same nutrients, completely different metabolic effect.

Treat refined carbohydrates like bread, white rice, pasta, and potatoes as sugar by another name, because in terms of what they do to your blood glucose, they largely are.

And give yourself time. Sugar affects the brain's reward system in ways that make it genuinely difficult to reduce. The first week or two without it can feel disproportionately hard. That difficulty is itself worth understanding, because it tells you something about what it was doing to you all along.

Next post: the gut, and why what's living in your digestive system might be shaping your mood, your energy, and your immune system in ways you haven't considered.