Keto Diet Explained — How It Works, Benefits and Risks
The ketogenic diet is the most argued-about diet in fitness: a very-low-carbohydrate, high-fat diet that forces the body to burn fat and produce ketones for fuel. Its supporters credit it with rapid fat loss, steady energy and reduced hunger; its critics point to the side effects, the difficulty of sustaining it, and the loss of high-intensity performance. Both sides have a point, and the evidence is more nuanced than either camp admits.
The headline definition is that a ketogenic diet keeps carbohydrate under about 50 g a day (usually 20–50 g), with fat providing roughly 70–80% of calories and protein moderate at about 1.2–1.6 g/kg. At that carbohydrate level, liver glycogen empties within a few days and the liver starts producing ketones — acetoacetate and beta-hydroxybutyrate — from fat, which the brain and muscles can use as an alternative fuel. Blood ketones above 0.5 mmol/L define nutritional ketosis.
Use the calculator
Start with your own numbers: try our keto macro calculator to work out your figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: Fats Explained, Carbs Explained, What Are Macros?. You can also browse the full guides section for the rest of the series.
The key points
Ketosis is a metabolic state, not a magic one. When carbohydrate is very low, insulin falls and the liver converts fatty acids into ketones, which the brain uses in place of glucose. This is a real, measurable change — but it does not bypass energy balance. Keto diets cause fat loss because they reduce appetite and total calories, not because ketones burn fat by themselves. Trials that match calories find similar fat loss from keto and from other diets.
Appetite suppression is keto's main advantage. High-fat, moderate-protein meals are very filling, and ketones themselves appear to reduce hunger. Many people on keto spontaneously eat 300–500 kcal less a day without counting, which is why it works well for people who struggle with hunger on other diets. For people who find carbohydrate-rich foods hard to stop eating, removing them can be liberating.
High-intensity performance suffers. Glycogen is the fuel for efforts above roughly 70% of maximum heart rate, and keto depletes it. Studies consistently find that sprint, interval and heavy lifting performance fall on keto, at least for the first few weeks and often permanently for the highest intensities. Endurance at moderate intensity can be maintained or even improved after full adaptation (three to four weeks), but the top end of performance is usually lower.
The first week is the hardest. 'Keto flu' — headache, fatigue, irritability, constipation and cramps — affects many people in the first week, and it is mostly caused by water and salt loss as glycogen empties (each gram of glycogen holds about 3 g of water). Drinking water and eating salt helps, and symptoms usually pass within a week. The diet also requires careful planning: carbohydrate hides in sauces, vegetables and drinks, and a single high-carb meal can end ketosis for a day.
At a glance
| Food | Carbs per 100 g | Keto-friendly? |
|---|---|---|
| Olive oil, butter | 0 g | Yes — main fat sources |
| Eggs | 1 g | Yes |
| Salmon | 0 g | Yes |
| Chicken breast | 0 g | Yes |
| Avocado | 9 g (7 g fibre) | Yes — net carbs 2 g |
| Leafy greens | 2–3 g | Yes |
| Broccoli | 7 g (3 g fibre) | Yes in moderation |
| Bread, rice, pasta | 40–75 g | No |
| Fruit (most) | 10–20 g | No, except berries |
| Sugar, honey | 100 g | No |
The formula behind it
The ketogenic diet is usually defined as carbohydrate 20–50 g a day, fat 70–80% of calories, protein 1.2–1.6 g/kg. At 2,000 kcal a day with 30 g of carbohydrate (120 kcal) and protein at 1.4 g/kg for an 80 kg person (112 g, 448 kcal), fat is the remainder: 2,000 − 120 − 448 = 1,432 kcal, or 159 g of fat. Nutritional ketosis is defined as blood beta-hydroxybutyrate above 0.5 mmol/L, usually reached within two to four days under 50 g of carbohydrate. The safety floors still apply: a keto diet must keep total calories above 1,200 kcal a day for women and 1,500 kcal a day for men unless medically supervised.
Worked example — an 80 kg man, maintenance 2,400 kcal, cutting at 1,900 kcal on keto (above the 1,500 kcal floor). Carbohydrate: 30 g (120 kcal). Protein: 1.4 g/kg = 112 g (448 kcal). Fat: (1,900 − 120 − 448) ÷ 9 = 148 g. A day might be: eggs and avocado for breakfast, salmon and leafy greens for lunch, chicken thighs and broccoli cooked in olive oil for dinner, with nuts as a snack. The maths works, but the meals are repetitive and eating out is difficult — which is why adherence, not the formula, decides whether keto succeeds.
Your plan, step by step
- Decide whether keto fits your training: it suits steady fat loss and moderate endurance, not sprinting, heavy lifting or team sports.
- Set carbohydrate at 20–50 g a day and count it strictly for the first two weeks — hidden carbs in sauces and vegetables add up.
- Set protein at 1.2–1.6 g/kg (not higher — excess protein can reduce ketone production, though the effect is modest).
- Eat salt and drink water deliberately in the first week to reduce keto flu symptoms.
- Give it three to four weeks before judging it: adaptation takes time, and the first week is the worst week, not a representative one.
How to use this guide
Use this guide to make an informed decision, not to be talked into keto. If you enjoy high-fat foods, struggle with hunger on other diets, and do not care about top-end sprint performance, keto can work well. If you love bread and pasta, train at high intensity, or find strict rules stressful, another diet with the same calorie deficit will work just as well. Judge keto by the same measures as any diet: weight change at 0.5–1% of body weight a week, energy, sleep and training quality over a month.
Common mistakes
Eating too much protein and too little fat. Keto is a high-fat diet, not a high-protein one. Protein above about 1.6 g/kg makes ketosis harder to maintain and adds calories that could have come from fat — set protein first, then fill the rest with fat.
Forgetting salt and water in week one. Glycogen loss dumps water and salt, which causes most keto flu symptoms. Drink water and eat salty food deliberately for the first week, and the transition is usually much easier.
Treating keto as a licence to eat unlimited fat. Keto reduces appetite, but it does not remove calories. A 500 kcal surplus on keto is still a surplus, and fat is 9 kcal per gram — the easiest macronutrient to overeat. The deficit still matters.
What this guide does NOT tell you
Keto diets are population-level guidance for healthy adults; they are not suitable for everyone. People with diabetes (especially type 1), kidney disease, liver disease, pancreatitis, or a history of eating disorders should not start keto without medical supervision, because the risks are real — including ketoacidosis in type 1 diabetes. Pregnant people should not follow a ketogenic diet. Long-term effects on cholesterol and heart health are still debated, and LDL cholesterol rises in some people on keto. This guide is general information, not medical advice.
Frequently asked questions
Does keto work for weight loss?
Yes, mainly because it reduces appetite and total calories — high-fat, moderate-protein meals are very filling, and many people spontaneously eat 300–500 kcal less. Trials that match calories find similar fat loss from keto and other diets, so the advantage is adherence, not metabolism. The diet works if you can sustain the calorie deficit; the ketones themselves do not burn extra fat.
How many carbs can I eat on keto?
Usually 20–50 g of total carbohydrate a day, with most people aiming for 20–30 g to be safely in ketosis. That is roughly the carbohydrate in a single slice of bread plus a small portion of vegetables. Fibre is often subtracted to give 'net carbs', but total carbohydrate is the safer number to count, especially in the first weeks.
What is keto flu?
The headache, fatigue, irritability and cramps many people feel in the first week of keto. It is mostly caused by water and salt loss as glycogen empties — each gram of glycogen holds about 3 g of water. Drinking water and eating salt helps, and symptoms usually pass within a week. If they do not, the diet may not suit you.
Can I build muscle on keto?
Yes, but it is harder than on a higher-carbohydrate diet. Protein can be adequate (1.6 g/kg), but glycogen depletion reduces high-intensity performance, which limits training volume and progression. Most lifters find muscle gain slower on keto, and many use a targeted approach — adding carbohydrate around training — or a different diet altogether for a gaining phase.
Is keto safe long term?
The long-term evidence is limited. Keto is a medical treatment for epilepsy with decades of supervised use, but as a weight-loss diet the evidence beyond two years is thin, and LDL cholesterol rises in a significant minority of people. Anyone considering keto long term should have blood lipids checked and discuss it with a clinician, especially if they have heart disease risk factors.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.