The keto diet, explained

How keto works

Your body normally runs on a mix of glucose (from carbs) and fat, with a small glucose store called glycogen in your liver and muscles. Eat very few carbs for a few days and that store runs down. Your liver then turns fat into ketones, which your brain and muscles can burn. That state is called nutritional ketosis.

For most people it takes two to four days of eating under about 50 g of carbs to get there. Eat a big bowl of pasta and you'll usually drop out of it within a day.

This is not diabetic ketoacidosis, a medical emergency that mostly affects people with type 1 diabetes. In a healthy person, insulin keeps ketones in a safe range. Keto began in the 1920s as an epilepsy treatment and is still used that way under medical supervision.

Keto macros in grams

Most keto plans aim for about 70% of calories from fat, 20% from protein and 5-10% from carbs. On an 1,800-calorie day that works out to:

Many people count net carbs (total carbs minus fibre). The term isn't regulated in the US, and UK and EU labels already leave fibre out of the carb figure, so check which number you're reading. New to macronutrients? Start there.

What you eat: a sample day

Keto cuts out bread, pasta, rice, potatoes, beans, lentils, breakfast cereal, sugar and most fruit. To put it in numbers, one medium apple has about 25 g of carbs and a cup of cooked rice about 45 g, so either one can use up your whole day.

You build meals from meat, fish, eggs, cheese, nuts, olive oil, avocado and green vegetables like spinach, broccoli and courgette (zucchini). A realistic day:

• Breakfast: two or three eggs scrambled in olive oil with a handful of spinach and a few cherry tomatoes.
• Lunch: a big salad with grilled chicken thigh, half an avocado, cucumber, a little feta and an olive oil and lemon dressing.
• Snack: a small handful (30 g) of almonds, or celery with a tablespoon of peanut butter.
• Dinner: a salmon fillet with roasted broccoli and cauliflower, tossed in olive oil and garlic.
• If you want something sweet: plain Greek yogurt with a few raspberries.

That lands at roughly 20-30 g of net carbs. Vegetarian keto is possible with eggs, dairy and tofu, but harder.

The first two weeks and the side effects

Days 1-3: you'll probably lose a couple of pounds fast. That is mostly water, because each gram of stored glycogen holds water with it.

Days 2-7: this is when many people get the so-called keto flu. Think a dull headache, tiredness, brain fog, irritability, leg cramps and feeling dizzy when you stand up. Part of the cause is losing sodium and water. Salting your food, drinking plenty of water, and eating leafy greens, avocado and nuts for potassium and magnesium usually helps (see electrolytes).

Week 2: for most people the fog lifts. Hunger often settles and energy evens out. Workouts that need bursts of speed, like sprinting or heavy sets, may still feel harder for a few weeks.

Other side effects to watch for:

• Constipation, because you've dropped most of your fibre sources.
• Bad breath with a fruity or nail-polish smell, from a ketone called acetone.
• Higher LDL cholesterol. Some people see big rises, some see none. You won't know without a blood test.
• Boredom and awkward meals out, the main reason people stop.

How to tell if it's working

People like keto because the rules are clear and they feel less hungry, so they eat less without counting. Weight loss still comes from a calorie deficit; keto is one way to get there. After the first month, signs it's working:

• You're losing roughly 0.5-1 kg (1-2 lb) a week after the first water drop, and your waist measurement is shrinking.
• You're not thinking about food all day, and you rarely feel shaky between meals.
• You can keep it up without dreading every meal.

You don't need to test ketones to lose weight. If the scale stalls for three or four weeks, portions of cheese, nuts and oil have usually crept up.

Common mistakes, and a better way to do it

• Treating it as a licence for bacon and butter. Processed meat and lots of saturated fat aren't good for your heart. Lean on olive oil, avocado, nuts and oily fish like salmon.
• Skipping vegetables. Leafy greens are your main fibre source now. Eat some at every meal.
• Living on keto snack bars and "fat bombs". Packaged keto products are often ultra-processed and high in calories. The label says low carb, not healthy.
• Not salting food in week one. Much of the keto flu is avoidable.

Two keto diets with the same carb count can be very different for your health. Diet quality still counts.

How to stop keto without regaining

Decide before you start what keto is for. Is it a three-month kick-start, a long-term way of eating, or a medical diet your doctor has prescribed? People who stop suddenly often slide back to old habits and regain the weight.

A gentler exit: add back about 10-15 g of carbs a day each week, starting with beans, lentils, whole fruit, oats and potatoes. Keep the habits that helped, like vegetables at every meal and no sugary drinks. Expect the scale to rise a little at first as water comes back. That isn't fat.

Many people end up somewhere around a moderate low-carb diet or a Mediterranean diet, which are easier to live with for years.

Talk to a doctor first if this is you

• You take insulin or diabetes tablets. Cutting carbs can drop your blood sugar fast, and doses often need changing within days. SGLT2 inhibitor tablets such as empagliflozin or dapagliflozin carry a risk of ketoacidosis that a very-low-carb diet can raise.
• You take blood pressure pills. Keto can lower blood pressure, and with your medicine it may go too low.
• You're pregnant, breastfeeding or trying to conceive.
• You have kidney disease, liver or pancreas problems, or you've had your gallbladder removed.
• You've had an eating disorder. Strict food rules can pull you back into it.

Get urgent medical help if you have repeated vomiting, severe weakness, confusion, trouble breathing or signs of serious dehydration.

Who keto suits, and who it doesn't

Still unsure? See how to choose a diet or is there one best diet?

Keto questions people ask

Sources

• Harvard Nutrition Source: Ketogenic Diet Review
• NIH StatPearls: Ketogenic Diet
• Epilepsy Foundation: Ketogenic Diet
• Harvard Health: Should You Try the Keto Diet?