Ketosis

Ketosis is a metabolic state in which the body shifts to using fat as its primary energy source instead of carbohydrates. Normally, the body primarily uses glucose (blood sugar) as fuel, but when carbohydrate intake drops very low - typically below 20-50 grams per day - glycogen stores are depleted within 24-72 hours. The liver then begins producing ketone bodies from fatty acids: beta-hydroxybutyrate (BHB), acetoacetate, and acetone. These ketone bodies replace glucose as fuel for the brain, heart, and muscles.

Achieving ketosis typically takes 2-7 days, depending on the individual, previous diet, and activity level. In the initial phase, many experience the so-called 'keto flu': fatigue, headaches, irritability, nausea, and brain fog. This is due to the body adapting to a new fuel source and significant fluid loss, as glycogen binds approximately 3 grams of water for each gram of glycogen. Adequate electrolyte intake (sodium, potassium, magnesium) helps alleviate symptoms. Keto flu symptoms generally ease within 5-10 days, once the body has adapted to efficiently using ketone bodies - this state is called keto-adaptation.

The ketogenic diet is typically based on the following macro distribution: fat 70-80% of energy, protein 15-20%, and carbohydrates only 5-10%. In practice, this means plenty of fat (avocado, nuts, olive oil, butter, fatty fish), moderate protein (meat, fish, eggs, cheese), and very few carbohydrates - practically no bread, pasta, rice, potatoes, fruits (berries can be eaten in small portions), or sugar. Among vegetables, low-carbohydrate options like spinach, broccoli, cauliflower, and salads are preferred.

Benefits of ketosis for weight loss: fat and protein keep you full effectively, and many people naturally eat less without conscious calorie counting. Ketone bodies themselves reduce hunger by affecting the ghrelin hormone. Some report better concentration and more stable energy when blood sugar fluctuations decrease. In studies, the ketogenic diet has proven particularly beneficial for insulin resistance and type 2 diabetes treatment because it minimizes insulin requirements.

However, it's important to understand the limitations: the rapid weight loss in the first weeks (often 2-4 kg) is largely due to fluid loss, not fat loss. In long-term studies, keto is not a more effective fat burner than moderate energy restriction at the same calorie level. Weight loss results always depend on total energy intake, not just carbohydrate restriction alone.

Potential risks and downsides of ketosis: digestive function may be disrupted due to lack of fiber, breath and sweat may smell of acetone, cholesterol may rise in some individuals (especially LDL), and kidney stone risk may increase. Long-term ketogenic diets have been studied less, so there is no strong evidence for the safety of periods lasting over a year. Exercise performance may suffer, especially in anaerobic and intense exercise, because muscles primarily use glycogen when producing high power output.

Who ketosis is not suitable for: type 1 diabetics (risk of ketoacidosis), pregnant and breastfeeding women, those with severe kidney disease, individuals with fat metabolism disorders, and those with an eating disorder history, for whom strict dietary rules may trigger a cycle. Those on medication (especially diabetes and blood pressure medications) should always consult a doctor before starting a keto diet, as medication dose adjustments may be necessary.

Practical tip: if you want to try a ketogenic diet, start with a 2-4 week trial period. Take care of electrolytes (salt, potassium, magnesium), eat plenty of green vegetables for fiber, and monitor how you feel. Ketosis can be verified with a blood, urine, or breath meter - blood measurement (BHB 0.5-3.0 mmol/l) is the most reliable. Ketosis is closely related to carbohydrate restriction, using fat as energy, and calorie deficit concepts.

Last updated: March 30, 2026