How refined carbs affect your blood sugar, energy and cravings

You've probably felt it: a pastry or a big bowl of sugary cereal at breakfast, and by mid-morning you're tired, hungry and reaching for a snack. This is the second article in our carbs series, and it looks at what refined carbs actually do in your body, and what the research says about energy and cravings.

If you missed it, start with part 1: Are carbs bad for you? Good carbs vs refined carbs explained.

What happens when you eat refined carbs

All digestible carbs end up as glucose in your blood. The difference is speed.

• Refined carbs (white bread, sugary cereal, pastries, sweets, cold drinks) have had most of their fibre removed. Your body breaks them down fast, so glucose floods into your blood quickly.
• Whole carbs (oats, beans, whole grains, vegetables, whole fruit) come wrapped in fibre. They're digested more slowly, so glucose arrives as a trickle rather than a flood.

When blood sugar rises fast, your pancreas releases a lot of insulin to move that glucose into your cells. In some people, a few hours later blood sugar can dip below where it started. That dip is what many of us feel as the mid-morning slump.

Refined carbs and cravings

A small but clever 2013 study tested this directly. Overweight men drank 2 milkshakes on different days. The shakes had the same calories, taste and sweetness; the only difference was the type of carb. One was fast-digesting, the other slow.

After the fast-digesting shake, blood sugar rose about 2.4 times as much. Four hours later it had dropped lower than after the slow shake, the men felt hungrier, and brain scans showed more activity in a region linked to reward and craving. Four hours is roughly when most of us start thinking about the next meal.

The caveat: this was 12 men, in 1 study. It fits what many people notice, but it doesn't prove refined carbs are "addictive".

The "sugar rush" is a myth

Many of us reach for something sweet to pick us up. A 2019 review pooled 31 studies with almost 1,300 adults to test whether it works. Sugar and other carbs did not improve mood at any point. Within the first hour, people who'd had them actually felt more tired and less alert than those who hadn't.

So the afternoon chocolate bar isn't giving you the energy boost it seems to promise. A short walk, a glass of water or a protein-based snack is a better bet.

Why processed food makes it easy to overeat

In a 2019 trial at the US National Institutes of Health, 20 adults lived in a research unit for a month. For 2 weeks they ate mostly ultra-processed food, and for 2 weeks mostly unprocessed food. The meals offered the same calories, sugar, fat and fibre, and people could eat as much as they wanted.

On the processed diet they ate about 500 calories more a day, ate faster, and gained about 0.9 kg. On the unprocessed diet they lost about the same amount. The researchers don't yet know exactly why; refined carbs are only part of what makes processed food different. But it's the clearest evidence so far that how processed your food is affects how much you eat.

Over the long term

A blood sugar spike now and then is normal. The concern is a diet built mostly on refined carbs, year after year. As we covered in part 1, large studies link high intakes of refined grains with higher risk of heart disease and early death, and high fibre intakes with lower risk of type 2 diabetes. If you already have type 2 diabetes or prediabetes, how you handle carbs matters more; our guide Supplements and type 2 diabetes: what lowers blood sugar covers what works.

Everyone responds differently

A 2015 study tracked the blood sugar of 800 people across almost 47,000 meals. People's responses to the exact same food varied hugely: for some, bread spiked blood sugar more than ice cream, and for others the reverse. Your sleep, activity, body size and gut bacteria all play a part.

That's why the steps below are general principles, not rigid rules. Notice how different meals leave you feeling 2 to 3 hours later; it's useful information.

5 ways to keep your blood sugar steadier

1. Eat your vegetables and protein first, carbs last. In a small 2015 study of people with type 2 diabetes, eating the same meal in this order lowered the blood sugar rise by about 29% at 30 minutes and 37% at 1 hour, compared with eating the bread and juice first. Same food, different order.
2. Never eat carbs alone. Pair toast with eggs, fruit with nuts or yoghurt, pap with beans or meat and vegetables.
3. Choose the slower version. Oats instead of sugary cereal, whole-grain or seeded bread instead of white, whole fruit instead of juice.
4. Swap the sugary pick-me-up. Try water, a short walk or a protein snack when the slump hits.
5. Move after meals. Even a 10 to 15 minute walk helps your muscles use glucose.

Frequently asked questions

Is a blood sugar spike after a meal bad?
Some rise after eating is normal and healthy. What matters is a pattern of large, frequent spikes, especially if you already have prediabetes or diabetes.

Do I need a glucose monitor?
Not if you don't have diabetes. Noticing how hungry and energetic you feel a few hours after different meals tells you most of what you need. If you're worried about your blood sugar, ask your doctor for a test.

Are all white foods bad?
No. A white potato with its skin, eaten with protein and vegetables, behaves very differently from a white-bread sandwich on its own. The meal as a whole matters more than any single food.

More in this series

• What's really in supermarket bread and pasta?
• How many carbs do you need, and does it change with age?
• Simple carb swaps that work in a South African kitchen

The sources

• Lennerz BS et al. (2013). Effects of dietary glycemic index on brain regions related to reward and craving in men. American Journal of Clinical Nutrition 98:641–647. View →
• Mantantzis K et al. (2019). Sugar rush or sugar crash? A meta-analysis of carbohydrate effects on mood. Neuroscience & Biobehavioral Reviews 101:45–67. View →
• Hall KD et al. (2019). Ultra-processed diets cause excess calorie intake and weight gain: an inpatient randomized controlled trial. Cell Metabolism 30:67–77. View →
• Zeevi D et al. (2015). Personalized nutrition by prediction of glycemic responses. Cell 163:1079–1094. View →
• Shukla AP et al. (2015). Food order has a significant impact on postprandial glucose and insulin levels. Diabetes Care 38:e98–e99. View →

Find out more

• Are carbs bad for you? Good carbs vs refined carbs explained
• How much sugar is too much? Daily limits, hidden sugars and how to cut back
• Supplements and type 2 diabetes: what lowers blood sugar, what doesn't
• How sleep improves your health

This article is general information, not medical advice. If you have diabetes or prediabetes, or take medicine that affects blood sugar, speak to your doctor before changing how you eat.

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