How Ketosis Works: The Biochemistry Explained Simply

Ketosis is not a mysterious 'hack' — it is a normal, ancient metabolic state your body enters whenever carbohydrates are scarce. Understanding the biochemistry helps you troubleshoot problems and avoid the common myths that circulate online.

Fuel #1: Glucose and glycogen

On a typical Western diet, roughly half of your daily calories come from carbohydrate. Digestion breaks those carbohydrates into glucose, which enters the bloodstream and triggers the pancreas to release insulin. Insulin does two things: it escorts glucose into cells for immediate energy, and it signals the liver and muscles to store excess glucose as glycogen.

Your body carries roughly 400–500 grams of glycogen — enough for one to two days of normal activity. When glycogen stores are full and glucose keeps arriving, the excess is converted to fat and stored in adipose tissue.

Fuel #2: Fat and ketones

When carbohydrate intake drops below about 50 grams per day, blood glucose falls, insulin drops, and glycogen stores gradually deplete. Once glycogen is low, your body must switch fuels. Fatty acids stream out of adipose tissue and travel to the liver, where a portion of them are converted into three ketone bodies: beta-hydroxybutyrate (BHB), acetoacetate, and acetone.

BHB is the most important. It circulates through the blood, crosses the blood-brain barrier easily, and is used as fuel by the brain, heart, and skeletal muscle. Acetone is a byproduct exhaled through the lungs — this is what causes the mildly fruity 'keto breath' that some beginners notice.

The role of insulin

Insulin is the master switch. When insulin is high, fat storage is on and fat burning is off. When insulin is low (as on keto), fat burning is on and stored fat becomes accessible for fuel. This is why the primary lever of keto is carbohydrate restriction — it is the fastest, most reliable way to lower insulin.

The adaptation curve

Reaching ketosis and becoming fat-adapted are two different milestones:

• Day 1–3: Glycogen depletes. You will urinate frequently as each gram of glycogen releases about 3 grams of water. Weight drops quickly but this is water, not fat.
• Day 3–7: BHB rises. Some people feel sluggish (the 'keto flu') as their brain briefly runs low on fuel while ketone production ramps up.
• Week 2–4: Fat adaptation. Muscles preferentially burn fatty acids, sparing ketones for the brain. Energy stabilizes and hunger typically decreases.
• Week 4–8: Metabolic flexibility. Your body becomes efficient at switching between fuels, and performance during moderate exercise usually returns to normal or better.

Why brain fog fades

Your brain uses about 20% of your daily calories. In the fed state it prefers glucose, but during fasting or carbohydrate restriction it derives up to 70% of its energy from ketones. Ketones are actually a very stable brain fuel — they burn cleanly and do not cause the blood sugar swings some people experience on a carb-heavy diet.

How to measure ketosis

• Urine strips — cheap and easy for beginners but become inaccurate as your body reabsorbs ketones more efficiently.
• Breath meters — measure acetone in exhaled air. Reusable, moderately accurate, no consumables.
• Blood meters — gold standard. Measure BHB directly. Most useful if you are troubleshooting or optimizing.

A blood BHB reading of 0.5–3.0 mmol/L is nutritional ketosis. Higher is not necessarily better — chasing very high ketone numbers is not a goal in itself.

Ketosis is not ketoacidosis

Diabetic ketoacidosis (DKA) is a medical emergency that occurs almost exclusively in people with type 1 diabetes when ketones climb above 10 mmol/L alongside dangerously high blood glucose. Nutritional ketosis, driven by a well-formulated diet, is a self-regulating state — your body's own insulin response prevents ketones from spiraling upward.