Nutrition

Keto Diet Guide: How Ketosis Works, What to Eat & Safety Tips

Understand how the ketogenic diet triggers ketosis, which macros to target, how to manage electrolytes, and what the science says about safety and effectiveness.

March 27, 2026 · 11 min readLast updated: May 21, 2026

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What is the ketogenic diet?

The ketogenic diet is a high-fat, adequate-protein, very-low-carbohydrate eating pattern that forces the body to burn fat as its primary fuel source instead of glucose. By drastically reducing carbohydrate intake, typically to 20-50 grams of net carbs per day, the body enters a metabolic state called ketosis in which the liver converts fatty acids into ketone bodies. These ketones then serve as the main energy source for the brain, heart, and muscles. The diet was originally developed in the 1920s as a therapeutic intervention for drug-resistant epilepsy in children, and it remains a clinically recognised treatment for that condition today. Over the past two decades it has gained widespread popularity for weight management, metabolic health, and athletic performance.

Ketosis at a glance

Net carbs

20-50 g/day

Keeping net carbohydrates (total carbs minus fibre) within this range is the primary driver of ketosis. Most people need to stay at or below 30 g/day during the first few weeks to reliably enter ketosis.

Fat intake

60-75% of calories

Fat becomes the dominant macronutrient, providing the substrate for ketone production. Sources should emphasise monounsaturated and saturated fats from whole foods such as avocado, olive oil, nuts, and fatty fish.

Adaptation period

2-4 weeks

The body requires two to four weeks to fully upregulate the enzymes and transport proteins needed to efficiently burn ketones. During this transition, energy levels and exercise performance may temporarily dip.

Primary fuel

Ketone bodies

Beta-hydroxybutyrate (BHB) is the most abundant circulating ketone. Blood BHB levels of 0.5-3.0 mmol/L indicate nutritional ketosis. Levels can be measured with a fingerstick blood meter.

Types of ketogenic diets

Three keto approaches

Standard (SKD)

75% fat / 20% protein / 5% carbs

The classic ketogenic ratio used in most clinical research. It maximises fat intake to ensure deep, sustained ketosis and is the best starting point for beginners. Protein is kept moderate to prevent excess gluconeogenesis.

High-Protein (HPKD)

60% fat / 35% protein / 5% carbs

A variation that raises protein to support muscle retention during weight loss or resistance training. The higher protein intake may slightly reduce ketone levels but still maintains nutritional ketosis in most people.

Modified (MKD)

65% fat / 25% protein / 10% carbs

A more flexible approach that allows 40-60 g of net carbs per day. Easier to sustain long term and often used as a maintenance diet after reaching a target weight. Some individuals may cycle in and out of mild ketosis.

How ketosis works

When carbohydrate intake drops below the threshold needed to replenish glycogen stores, blood glucose and insulin levels fall. In response, the body mobilises fatty acids from adipose tissue and transports them to the liver, where they undergo beta-oxidation and are converted into three types of ketone bodies: acetoacetate, beta-hydroxybutyrate (BHB), and acetone. BHB is the most metabolically active ketone and can cross the blood-brain barrier, providing the brain with an efficient alternative to glucose. The heart and skeletal muscles also preferentially oxidise ketones when they are available. Full metabolic adaptation to ketosis typically takes two to four weeks. During this period the body upregulates mitochondrial enzymes, increases ketone transporter expression, and shifts from relying on glucose to running primarily on fat-derived fuel. Once adapted, most people report stable energy levels, reduced hunger between meals, and improved mental clarity.

The keto flu

During the first one to two weeks of carbohydrate restriction, many people experience a cluster of symptoms commonly called the 'keto flu': headache, fatigue, irritability, dizziness, nausea, and muscle cramps. These symptoms are primarily caused by the rapid loss of water and electrolytes that accompanies glycogen depletion. Each gram of stored glycogen holds roughly 3 grams of water, so as glycogen stores empty, the kidneys excrete large volumes of fluid along with sodium, potassium, and magnesium. The keto flu is temporary and can be largely prevented by proactively increasing electrolyte intake and staying well hydrated during the transition period.

Electrolytes on keto

Electrolyte management is one of the most overlooked aspects of a ketogenic diet, yet it is often the difference between feeling terrible and feeling great. On a standard mixed diet, insulin signals the kidneys to retain sodium. When insulin drops on keto, the kidneys excrete more sodium, and potassium and magnesium follow. If these minerals are not replaced through food and supplementation, symptoms such as fatigue, headaches, muscle cramps, heart palpitations, and brain fog can persist well beyond the initial adaptation phase. Many people who 'fail' on keto are actually suffering from inadequate electrolyte intake rather than a fundamental incompatibility with the diet.

Daily electrolyte targets

Sodium

3000-5000 mg

Add salt liberally to meals, drink bone broth, or dissolve a pinch of salt in water. This is significantly more than standard dietary guidelines recommend, but the increased renal excretion on keto demands it.

Potassium

3000-4700 mg

Avocados, spinach, mushrooms, and salmon are keto-friendly potassium sources. A potassium citrate or potassium chloride supplement can help fill any remaining gap. Deficiency causes muscle weakness and cramps.

Magnesium

300-500 mg

Magnesium glycinate or citrate are well-absorbed forms. Dark chocolate (85%+), pumpkin seeds, and almonds are good dietary sources. Low magnesium contributes to sleep disturbances, cramps, and anxiety.

What to eat on keto

Keto-friendly foods

Fatty fish (salmon, sardines, mackerel): rich in omega-3 fatty acids and high-quality protein with zero carbs. Aim for two to three servings per week.
Eggs: one of the most nutrient-dense foods available, providing fat, protein, choline, and fat-soluble vitamins. Eat the whole egg, as the yolk contains most of the nutrition.
Avocado: an excellent source of monounsaturated fat, potassium, and fibre. Half an avocado contains about 2 g of net carbs and 15 g of healthy fat.
Olive oil and coconut oil: use extra virgin olive oil for dressings and low-heat cooking, and coconut oil for higher-heat cooking. Both provide beneficial fats that support ketone production.
Non-starchy vegetables (spinach, broccoli, cauliflower, zucchini, kale): provide fibre, vitamins, minerals, and antioxidants with minimal carb impact. Fill half your plate with these at every meal.
Nuts and seeds (almonds, walnuts, chia seeds, flaxseeds): offer healthy fats, fibre, and micronutrients. Watch portions, as carbs can add up if you eat large quantities.
Cheese and full-fat dairy (butter, cream, Greek yoghurt): provide fat, protein, and calcium. Choose unprocessed varieties and be mindful of lactose content in milk and soft cheeses.
Meat and poultry (beef, pork, chicken thighs, lamb): prioritise fattier cuts for energy and satiety. Grass-fed and pasture-raised options offer better fatty acid profiles.

Foods to avoid

Sugar and sweets (soft drinks, juice, candy, pastries, ice cream): these are the most concentrated sources of carbohydrates and will immediately knock you out of ketosis.
Grains and starches (bread, pasta, rice, cereal, oats): even whole grains are too carb-dense for a ketogenic diet. A single slice of bread contains 12-15 g of net carbs.
Starchy vegetables (potatoes, sweet potatoes, corn, peas): while nutritious in other contexts, their high carbohydrate content makes them incompatible with ketosis.
Most fruits (bananas, apples, oranges, grapes): fruit sugar (fructose) counts toward your carb limit. Small portions of berries (strawberries, blueberries, raspberries) are the exception.
Legumes (beans, lentils, chickpeas): despite being high in fibre and protein, their net carb content is too high for standard keto. A cup of cooked lentils contains about 25 g of net carbs.
Processed low-fat products (low-fat yoghurt, fat-free dressings, diet snacks): manufacturers typically replace the removed fat with sugar or starch, making these products high in carbohydrates.

Safety considerations

For most healthy adults, a well-formulated ketogenic diet is safe and can be followed for months or even years under appropriate guidance. Systematic reviews have found that keto diets are effective for short-to-medium-term weight loss and can improve markers of metabolic syndrome, including fasting glucose, HbA1c, triglycerides, and HDL cholesterol. However, the diet is not suitable for everyone. People with type 1 diabetes are at risk of diabetic ketoacidosis (DKA), a dangerous condition distinct from nutritional ketosis. Pregnant and breastfeeding women should not follow a ketogenic diet because the developing foetus requires a steady supply of glucose for brain development. People with kidney disease may be harmed by the higher protein load associated with some keto variants. Children and adolescents under 18 should only follow a ketogenic diet under direct medical supervision, typically for epilepsy management. Anyone taking medication for diabetes or hypertension should consult their doctor before starting keto, as drug dosages often need to be adjusted as blood sugar and blood pressure improve.

When to consult a doctor

Seek medical advice before starting a ketogenic diet if you have type 1 or type 2 diabetes, kidney disease, liver disease, a history of eating disorders, gallbladder issues, or if you are pregnant or breastfeeding. Also consult your doctor if you take insulin, SGLT2 inhibitors, blood pressure medication, or any other drug whose dosage depends on dietary intake. A healthcare professional can monitor your blood work, adjust medications, and ensure the diet is safe for your specific situation.

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