What happens in your body when you eat sugars: glucose and insulin explained simply

You know that drowsiness after a big plate of pasta or a slice of cake? It's no coincidence: it's your metabolism at work. Understanding what happens in your body when you eat sugars is the first step to making sense of the ketogenic diet. Let's look at it without beating around the bush.

From the plate to the blood: sugars and starches

When you eat something sweet (sugar) or rich in starches (bread, pasta, rice, potatoes), the body breaks these carbohydrates down into glucose, a simple sugar. Starch, after all, is nothing more than lots of glucose molecules stuck together: that's why even a "non-sweet" food like bread raises blood sugar.

The glucose passes into the blood. Blood sugar is precisely the amount of glucose in the blood at a given moment. After a meal rich in refined carbs, it rises quickly.

Insulin: the "storekeeper" hormone

When blood sugar rises, the pancreas releases a hormone: insulin. Picture it as a storekeeper knocking on the cells and saying: "Glucose has arrived, let it in."

Insulin does three things:

1. Opens the doors of the cells (muscles, organs) so they use glucose for energy.
2. Has excess glucose stored in the liver and muscles as glycogen (the short-term "pantry").
3. When the pantry is full, it converts the excess into fat.

Key point: when insulin is high, the body stores (fat included) and stops burning it.

The role of the liver

The liver is the silent director: it stores glycogen when there's plenty and releases it when you're fasting, to keep blood sugar stable. It's also the organ that, when carbs are scarce, produces ketones.

When the system breaks down: insulin resistance

If blood sugar keeps spiking — lots of sugars and refined carbs, every day — the pancreas has to pump out more and more insulin. Over time the cells "stop responding well" to its signal: this is insulin resistance, now considered the root of many metabolic problems. It's exactly the topic that Dr. Benjamin Bikman explains in Why We Get Sick, and that Dr. Robert Lustig links to sugar in Metabolical.

And the ketogenic diet?

The logic of keto starts right here: if you eat very few carbs, blood sugar stays low and stable, so insulin stays low. With little insulin around, the body no longer has a reason to store and starts drawing on fat. When carbs are scarce for a long time, the liver produces ketones, an alternative fuel derived from fats: this is the state of ketosis.

In plain terms: fewer sugar spikes → less insulin → an easier time using fat for energy.

In practice: measure to understand

The most direct way to see all this in your own body is to measure how you react to different foods with a glucose meter: discover, with the data in hand, which foods make your blood sugar spike and which don't.

And if you want to calculate where to start with macronutrients, use our keto macro calculator.

Further reading

• Dr. Robert Lustig — Metabolical: sugar and processed food.
• Dr. Benjamin Bikman — Why We Get Sick: insulin resistance well explained.
• Dr. Jason Fung — The Obesity Code: insulin and weight loss.

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This article is for purely informative and educational purposes and does not constitute personalized medical or nutritional advice. Some links are affiliate links: by buying through them we may receive a small commission, at no extra cost to you.

🛒 Useful products

• Blood glucose meter (measures blood sugar)

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