Ketosis

What Is Ketosis?

Ketosis is a metabolic state in which the body shifts its primary fuel source from glucose (derived from carbohydrates) to ketone bodies (derived from fat). It occurs when carbohydrate intake is sufficiently restricted — typically below 20–50 grams of net carbohydrates per day — causing liver glycogen stores to deplete and the liver to begin converting fatty acids into three ketone bodies: beta-hydroxybutyrate (BHB), acetoacetate, and acetone.

Nutritional ketosis is distinct from diabetic ketoacidosis (DKA), a dangerous condition seen in uncontrolled type 1 diabetes. In nutritional ketosis, blood ketone levels typically range from 0.5 to 3.0 mmol/L, whereas DKA involves levels above 10 mmol/L alongside hyperglycemia.

How Ketosis Works

When dietary carbohydrate is low, insulin levels fall and glucagon rises. This hormonal shift signals adipose tissue to release stored fatty acids into the bloodstream. The liver takes up these fatty acids and, through beta-oxidation followed by ketogenesis, produces ketone bodies. These ketones are then exported to peripheral tissues — including the brain, heart, and skeletal muscle — which use them as a highly efficient energy substrate.

• Brain adaptation: The brain, which normally relies on glucose, can derive up to 70% of its energy from ketones after several weeks of adaptation.
• Respiratory quotient: Fat oxidation produces a lower respiratory quotient (approximately 0.70) than carbohydrate oxidation (1.00), reflecting more efficient oxygen use per calorie.
• Protein sparing: Ketones reduce the need for gluconeogenesis from amino acids, partially protecting lean muscle mass during energy restriction.

Practical Considerations

Achieving ketosis requires consistent adherence to a very low-carbohydrate diet. Most individuals enter ketosis within 2–4 days of restricting carbohydrates below 50 grams per day, though the timeline varies with activity level, individual metabolism, and prior glycogen stores. Blood ketone meters provide the most accurate measurement; urine strips are less reliable after the first few weeks of adaptation.

During the initial transition — sometimes called the keto-adaptation phase — individuals may experience fatigue, headache, and irritability. Adequate electrolyte intake (sodium 2,000–4,000 mg, potassium 3,500–4,700 mg, magnesium 300–500 mg daily) helps mitigate these symptoms. Long-term sustainability, lipid responses, and individual tolerance vary considerably and should be evaluated with a healthcare provider.