Ketosis
When the liver runs on fat and makes fuel for the brain
Plain English
Ketosis is a metabolic state in which the liver converts fat into molecules called ketone bodies, which the brain and muscles use as fuel in place of glucose. It occurs when carbohydrate intake is very low or absent, forcing the body to rely on fat stores. Ketosis is a normal and ancient metabolic adaptation; humans have entered it during periods of fasting, food scarcity, and extended exercise throughout evolutionary history.
The Mechanism
When carbohydrate intake drops below roughly 20-50 grams per day (or during extended fasting), blood glucose and insulin levels fall. Low insulin removes the suppression on fat release, allowing fat cells to release fatty acids into the bloodstream at high rates. The liver takes up these fatty acids and converts them through a process called beta-oxidation, producing acetyl-CoA. When acetyl-CoA accumulates faster than it can enter the standard energy cycle, the liver diverts it into ketone production, generating three primary ketone bodies: beta-hydroxybutyrate (BHB), acetoacetate, and acetone.
BHB is the most abundant ketone in the blood and crosses into the brain freely, where it provides a highly efficient fuel source. The brain normally depends almost entirely on glucose, but during sustained ketosis, it can meet 60-70% of its energy needs from BHB (Cahill, 1970). This shift is why fasting and ketogenic diets are associated with mental clarity in some individuals: the brain is running on a stable, non-fluctuating fuel rather than a glucose supply that rises and falls with meals.
Nutritional ketosis is typically defined as blood BHB between 0.5 and 3.0 mmol/L. This is distinct from diabetic ketoacidosis, a dangerous condition where BHB exceeds 10 mmol/L in the context of absent insulin. In healthy individuals with functioning insulin regulation, ketosis is self-limiting: rising BHB stimulates just enough insulin release to prevent runaway ketone production.
Why It Matters
Ketosis is a fuel state, not a cure; it works when it matches your goals.
Ketosis is not inherently better or worse than running on carbohydrates; the relevant question is whether it is appropriate for your goals. It reduces blood sugar variability, is used therapeutically for epilepsy (with strong clinical evidence since the 1920s), and is a useful tool for fat loss when it creates a meaningful calorie deficit. For high-intensity athletes, it is a poor primary fuel state because ketones cannot be oxidized fast enough to power maximum-effort work.
Common Misconception
Many people confuse ketosis with fat loss, but the two are not the same. Being in ketosis means your liver is producing ketones; it does not guarantee you are losing body fat. If you are consuming enough dietary fat to fuel your energy needs, you will stay in ketosis without drawing on stored fat at all. Fat loss requires a caloric deficit regardless of which fuel state you are in.
Signs It Is Disrupted
- Strong carbohydrate or sugar cravings, particularly in the afternoon or evening
- Energy slumps tied to meal timing, suggesting glucose dependence rather than fat flexibility
- Difficulty fasting for more than 6-8 hours without cognitive fog or irritability
- Brain fog or mood instability that improves immediately after eating carbohydrates
How to Improve It
3 Things to Remember
Ketosis is a normal metabolic state triggered by very low carbohydrate intake or fasting, where the liver produces ketone bodies as fuel for the brain and muscles.
Being in ketosis does not guarantee fat loss; a caloric deficit is required for fat loss regardless of fuel state.
Nutritional ketosis (blood BHB 0.5-3.0 mmol/L) is self-limiting in healthy individuals and is entirely distinct from diabetic ketoacidosis, which requires absent insulin to develop.
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