How Ketosis Actually Works Inside Your Body

The ketogenic diet shifts your metabolism into a state called ketosis, where fat becomes the primary fuel source instead of glucose. This metabolic adaptation occurs when carbohydrate intake drops below 50 grams per day, though individual thresholds vary. The process hinges on three biological mechanisms: glycogen depletion, hepatic ketogenesis, and cellular adaptation to ketone bodies.

The glycogen depletion phase

Your liver stores roughly 100-120 grams of glycogen, the body’s carbohydrate reserve. When dietary carbohydrates are scarce, these stores deplete within 24-48 hours. Glycogen molecules break down into glucose molecules through glycogenolysis, maintaining blood sugar levels during the initial transition. Once depleted, the liver initiates ketogenesis – the production of ketone bodies from fatty acids. This phase often coincides with temporary fatigue or brain fog, sometimes called the “keto flu”. symptoms of keto adaptation typically resolve within a week as the body shifts fuel systems.

Hepatic ketogenesis explained

The liver converts fatty acids into three ketone bodies: acetoacetate (AcAc), beta-hydroxybutyrate (BHB), and acetone. BHB becomes the primary circulating ketone, reaching concentrations of 0.5-3.0 mM in nutritional ketosis compared to 0.1 mM on a standard diet. Mitochondrial enzymes break down fatty acids through β-oxidation, producing acetyl-CoA which then enters the HMG-CoA pathway – the same pathway targeted by statin drugs. Unlike glucose metabolism, this process generates no reactive oxygen species, which may explain some reported cognitive benefits.

Cellular energy production in ketosis

Ketones cross the blood-brain barrier and replace glucose as the brain’s primary fuel source, providing up to 70% of its energy needs. Muscle tissue also oxidises ketones efficiently, particularly during endurance exercise. Research shows ketones yield more ATP per oxygen molecule than glucose, making them a more efficient fuel under certain conditions. A 2013 meta-analysis found this metabolic efficiency correlates with reduced hunger signals, possibly due to ketones’ effects on ghrelin and leptin sensitivity.

What this means in practice

Achieving ketosis requires consistently limiting net carbohydrates to 20-50 grams daily – about one slice of bread or half a banana. UK supermarkets like Tesco now stock keto-friendly staples like almond flour (£3.20 for 200g) and erythritol sweetener (£4.50 per 250g). Seasonal berries in summer provide lower-carb fruit options. The NHS recognises very-low-carb diets as a potential management strategy for type 2 diabetes, though medical supervision is advised for those on insulin. keto electrolyte supplements can help mitigate transition symptoms.

Frequently asked questions

How long does it take to enter ketosis?

Most people achieve measurable ketosis within 2-4 days of restricting carbohydrates below 50 grams daily. Using urine strips or blood meters, ketone levels typically reach 0.5 mM or higher during this period.

Can you build muscle in ketosis?

Yes, though the process differs from traditional bulking. Adequate protein (1.6-2.2g per kg of body weight) and resistance training remain essential. Some athletes report better endurance but slightly reduced explosive power.

Is ketosis dangerous for the liver?

Nutritional ketosis differs from diabetic ketoacidosis. For those without pre-existing liver conditions, studies show ketogenic diets may improve markers of fatty liver disease by reducing hepatic fat accumulation.

The bottom line

Ketosis represents a fundamental shift in human metabolism, activating evolutionary pathways for fat-based energy production. While the biochemistry appears complex, the practical application involves carbohydrate restriction, electrolyte management, and patience during adaptation. If you’d rather not do the macro maths yourself, the Keto Dieting app does it for you on Google Play and the App Store.

Educational only — not medical advice. This article is for general information. Speak to your GP before changing your diet, especially if you have type 1 or type 2 diabetes, kidney or liver disease, are pregnant or breastfeeding, or take medication for blood pressure, cholesterol, or blood glucose.

References

Bueno NB, de Melo IS, de Oliveira SL, da Rocha Ataide T (2013). Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. British Journal of Nutrition. https://doi.org/10.1017/S0007114513000548
Sumithran P, Prendergast LA, Delbridge E, et al. (2013). Ketosis and appetite-mediating nutrients and hormones after weight loss. European Journal of Clinical Nutrition. https://doi.org/10.1038/ejcn.2013.90