How it works
The switch into ketosis begins with a hormonal shift. When carbohydrate intake falls below roughly 50 g per day, or when a fast extends beyond 12-18 hours, insulin concentrations drop and glucagon rises. This signal activates hepatic lipolysis, freeing fatty acids from adipose stores and flooding them into the liver. There, beta-oxidation cleaves the fatty acids into acetyl-CoA units that, in excess of the TCA cycle's capacity, are channelled through HMGCS2 into two primary ketone bodies: acetoacetate and beta-hydroxybutyrate. 12
The liver synthesises ketone bodies entirely for export. Hepatocytes lack succinyl-CoA transferase, the enzyme required to oxidise ketones, so they cannot consume what they produce. Peripheral tissues (brain, heart, and skeletal muscle) absorb ketones via monocarboxylate transporters and reconvert them to acetyl-CoA for ATP generation through the TCA cycle. 1 Nutritional ketosis is operationally confirmed at a fasting blood beta-hydroxybutyrate of 0.5 mmol/L or above; the 1-3 mmol/L range is typical in sustained dietary ketosis, and levels above 5 mmol/L are rarely reached without pathology. 4
Beta-hydroxybutyrate also functions as a signalling molecule beyond its fuel role. It inhibits class I histone deacetylases, activates the NLRP3 inflammasome inhibitory pathway, and modulates gene expression in ways that couple metabolic state to cellular stress responses. 3 This signalling dimension explains why nutritional ketosis has attracted sustained interest across metabolic medicine, neurology, and longevity science.
Example
After three to four days of restricting carbohydrate intake to under 50 g daily, a fasting morning blood draw measures beta-hydroxybutyrate at 1.2 mmol/L, confirming nutritional ketosis. 4 Cognitive performance across the morning remains stable; the brain is drawing on ketone bodies rather than glucose. Aerobic zone training proceeds without issue, but a subsequent sprint session demands higher perceived effort than usual.
The blood marker confirms the metabolic shift; the sprint session reveals precisely where its fuel advantage ends.
Why it matters
The primary physiological significance of ketosis is its glucose-sparing effect on the brain. During prolonged fasting, ketone bodies can cover up to 60% of brain energy requirements 13, sustaining cognitive function at a time when glucose supply falls well below normal. This adaptive capacity is the primary reason humans survive multi-week starvation; the brain shifts its fuel dependence as glucose becomes scarce rather than failing when supply drops.
For high-performance contexts, the intensity ceiling is the critical constraint. At exercise intensities above 70-80% VO2max, athletes restricting carbohydrate show impaired performance relative to carbohydrate-fed counterparts; fat oxidation cannot sustain the ATP resynthesis rates required for high-intensity work, increasing both oxygen cost and perceived exertion. 4 Ketosis is not a universal performance enhancer. It is a metabolic mode with specific advantages at lower intensities and clear limitations at maximal outputs.
What is the difference between nutritional ketosis and diabetic ketoacidosis?+
Nutritional ketosis is a regulated, insulin-present state in which blood beta-hydroxybutyrate sits between 0.5 and 5 mmol/L. Diabetic ketoacidosis occurs when severe insulin deficiency allows unregulated ketone production, driving blood ketones to 15-25 mmol/L and dangerously acidifying the blood. The two conditions are physiologically and clinically distinct. {{cite:10.1002/(sici)1520-7560(199911/12)15:6<412::aid-dmrr72>3.0.co;2-8}}{{cite:10.1080/15502783.2024.2368167}}
What blood ketone level confirms you are in ketosis?+
A fasting blood beta-hydroxybutyrate measurement of 0.5 mmol/L or above confirms nutritional ketosis. The 1-3 mmol/L range is typical during sustained dietary ketosis. Breath and urine ketone tests are less accurate than blood measurement and may misclassify status. {{cite:10.1080/15502783.2024.2368167}}
How long does it take to enter ketosis on a low-carbohydrate diet?+
Most people enter ketosis within two to four days of restricting carbohydrate to under 50 g per day, provided hepatic glycogen stores are depleted. The transition timeline varies with metabolic rate, habitual carbohydrate intake, and activity level; exercise accelerates glycogen depletion and can shorten entry time. {{cite:10.1146/annurev.nutr.26.061505.111258}}{{cite:10.1080/15502783.2024.2368167}}
Does ketosis improve athletic performance or cognitive function?+
Evidence on cognitive benefit is promising but limited; ketone bodies can supply a substantial portion of brain energy and may sustain focus during prolonged fasting. For athletic performance, the effect is intensity-dependent: ketogenic adaptation supports endurance at lower intensities but impairs high-intensity output above 70-80% VO2max, where carbohydrate remains the superior fuel. {{cite:10.1080/15502783.2024.2368167}}{{cite:10.1146/annurev-nutr-071816-064916}}
