The Truth About Sugar: Spikes, Crashes and What Actually Harms You
ZOE scientists Tim Spector and Sarah Berry explain why sugar isn't the villain it's made out to be: the real problem is how quickly modern foods flood our blood with glucose, and the energy-sapping crash that follows. They reveal why free sugars matter far more than the sugar naturally locked inside whole fruit, and share simple ways to blunt the spike, from a 10-minute walk to pairing your cake with full-fat yogurt.
Overview
In this ZOE Science & Nutrition conversation, host Jonathan Wolf sits down with Professor Tim Spector and Professor Sarah Berry to untangle one of nutrition's most confusing topics: sugar. They start by defusing the panic: we need glucose in our blood to survive, but we don't actually need to eat sugar, because the body can make its own. The trouble, they explain, is that modern ultra-processed foods release glucose into the bloodstream so fast that they trigger a sharp spike, an overshoot of insulin, and then a crash that leaves us tired, hungry and eating around 320 extra calories a day.
Repeated high spikes drive inflammation, which travels from the gut to the brain and is linked to everything from low mood to a raised risk of dementia and, indirectly, some cancers. Crucially, the pair distinguish 'free' sugars, meaning added sugar, honey, syrups and fruit juice, from the 'intrinsic' sugar locked inside whole fruit, where fiber and polyphenols change the whole picture. That's why an orange behaves very differently from orange juice, and why fruit is not the enemy.
They flag the sneaky sources most people miss: breakfast cereals, low-fat and fruit yogurts, energy drinks and even ketchup. Their advice is refreshingly gentle: don't go cold turkey and don't swap to sweeteners, but wean down gradually and read labels for free sugar. And you can still have the cake: move afterwards and pair it with healthy fats and protein to soften the spike and prevent the dip.
Key quotes
5We shouldn't fear sugar itself. The problem is we're now consuming foods that release glucose often very quickly into our bloodstream.
We need sugar in our blood to survive, but we actually don't need to consume sugar to survive.
Those who have a dip after their breakfast consume on average 320 calories more that day.
We cannot think of sugar in fruit in the same way as the sugar you added to your tea, because of all the other wonderful components in that fruit.
Have that cake, Jonathan. Life is short.
Key ideas
8Sugar is just a type of carbohydrate
Sugar is a carbohydrate made of saccharides: single-molecule sugars like glucose, fructose and galactose, or two joined together like sucrose (table sugar) and lactose. Even starchy foods like rice, pasta and potatoes are long chains of these same sugar molecules.
Your body can make its own glucose
We need glucose circulating in our blood, but we don't need to eat sugar to get it. The body can build glucose from protein and, to a lesser extent, fat, and stores spare sugar as glycogen for when we need it.
How sugar is absorbed
After you eat, glucose passes through the gut into the blood, peaking around 15 to 30 minutes and returning to baseline by about two hours. Liquids and processed foods are absorbed far faster than whole foods.
High blood glucose is toxic, so insulin steps in
At high levels glucose damages blood vessels, so the body releases insulin within minutes to clear it from the bloodstream. When sugar arrives too fast and too often, this system gets overworked and can eventually break down.
The crash matters more than the spike
A fast sugar hit can trigger an insulin overshoot and a dip in blood sugar two to four hours later. ZOE research shows this dip leaves people tired and hungry, and leads them to eat around 320 extra calories a day.
Sugar, inflammation and the brain
Repeated high sugar spikes stimulate the immune system to create inflammation, which travels from the gut to the brain via the vagus nerve. This is linked to low mood, cognitive decline and a higher long-term risk of conditions like Alzheimer's.
Free sugar vs intrinsic sugar
What matters most is whether sugar is 'free', meaning added by us or manufacturers, plus honey, syrups and fruit juice, or 'intrinsic', locked inside the structure of a whole food. Free sugars are the ones to watch.
Whole fruit is not the enemy
The sugar in whole fruit comes wrapped in fiber and polyphenols that slow absorption and blunt inflammation, so an apple behaves very differently from apple juice. Whole fruit beats puree, which beats juice, which beats plain sugar.
Practical takeaways
6- 1
Read labels for free sugar 49:00
Where you can, look for the free-sugar figure and keep it low; in the UK check the ingredient list, since items are listed in order of quantity. Aim for well under 10% of calories from free sugar, ideally under 5%.
- 2
Rethink your drinks 51:00
A can of cola can hold around eight teaspoons of sugar, and 'healthy' iced teas and energy drinks often have more. Unless you're doing hours of hard exercise, skip them, and treat orange juice as a rare treat, not a daily habit.
- 3
Choose whole fruit, keep the pulp 56:00
Eat the whole apple rather than the juice, and if you make a smoothie, leave the pulp in for the fiber. Dried fruit is fine in small amounts despite being concentrated.
- 4
Wean down, don't swap to sweeteners 57:30
Cutting sugar cold turkey is hard and rarely lasts; reduce gradually and let your sweet-taste receptors adjust. Switching to sweeteners keeps your sweet tooth alive, so aim to cut both.
- 5
Move after you eat 58:30
A 10-minute walk within 20 to 30 minutes of a meal lets your muscles pull glucose from the blood and lowers the spike. Even fidgeting or clenching your leg muscles at your desk helps.
- 6
Have your cake, but pair it 59:00
You don't need to give up treats. Pair a sugary food with healthy fat or protein, such as full-fat yogurt, to reduce the spike and prevent the energy-draining dip.
Topics & chapters
14The sugar myth
The hosts introduce Professors Tim Spector and Sarah Berry and challenge the biggest myth: that all sugar is bad for us.
What sugar actually is
Sarah Berry explains that sugar is a carbohydrate made of simple and paired sugar molecules, and that even starchy foods break down into the same sugars.
We need blood sugar, not dietary sugar
Why our cells and brain need glucose, yet the body can manufacture its own and store it as glycogen, so we don't have to eat sugar to survive.
What happens when you eat sugar
The journey from mouth to bloodstream, why liquids hit fastest, and how blood glucose peaks and then returns to baseline within a couple of hours.
Insulin and why high glucose is toxic
High blood sugar damages vessels, so insulin rushes to clear it, and why overworking that system leads toward type 2 diabetes.
Evolution vs the modern sugar environment
Tim Spector describes the Hadza gorging on honey for two days, then going weeks without, contrasted with today's constant drip of hidden sugar.
The crash and the 320-calorie effect
How an insulin overshoot causes a blood-sugar dip that makes us tired, hungry and prone to eating hundreds of extra calories.
Sugar, inflammation and the brain
Why repeated spikes fuel inflammation linked to mood, cognition and a raised risk of dementia.
Thin outside, fat inside
Being a healthy weight isn't a free pass: hidden visceral fat and its risks, especially for South Asian populations.
Sugar, cancer and acne
Why sugar's link to cancer is mostly indirect through excess calories and obesity, and why acne is largely down to genetics.
Free vs intrinsic sugar
The single most useful distinction: sugar added to foods versus sugar locked inside the structure of whole foods.
Drinks: soda, energy drinks and juice
Why liquid sugar hits hardest, how much hides in everyday drinks, and why orange juice isn't the health halo it seems.
Why whole fruit isn't the enemy
Fiber and polyphenols in whole fruit change how sugar behaves, and the whole-fruit-to-juice-to-sugar hierarchy.
Cutting down and eating cake wisely
Wean off gradually rather than going cold turkey, avoid sweeteners, and use walking and food pairing to enjoy treats with fewer downsides.