Your first week on keto, day by day

The first week is the toughest part of keto, because your body is switching fuel from glucose to fat. You feel it in your energy and your hunger, and you see it on the scales.

If you have not yet read what keto is and what you eat on it, start with the guide The keto diet for beginners.

Before you start

Before day one, clear the cupboard of bread, biscuits, sweets and juices (with them in front of you the first days are much harder) and plan a week of meals, so you are not deciding what to eat at the moment you are hungry.

On the shopping list: eggs, meat, fish, vegetables, cheese, olive oil and some stock for the keto flu days; the food database shows what fits on keto. Measure your waist too. In the first week the scales mostly show water, whereas the tape measure will have more to tell you later on.

Anyone taking medication for diabetes or another long-term condition should talk to their doctor first: with few carbs the doses often need changing.

Days 1 and 2: the stores run down

For the first two days you are still burning the glucose your body has stored in the liver and muscles, called glycogen. Most of the time you feel almost normal, though you may miss bread and sweets, especially at the times you were used to having them.

The scales are already dropping and you go to the toilet more often. That is mostly water: glycogen is stored with three to four times its weight in water, so as it empties the water goes too.

From the first day drink plenty of water; don't cut back the salt in your food.

Days 3 to 5: the hardest stretch

Within three to four days the stores have run out and insulin drops. The liver starts making ketones from fat, so you gradually move into ketosis.

This is also when "keto flu" usually shows up, with headache, tiredness, dizziness, nausea, constipation and a kind of brain fog. In an Australian study that read through what 300 people had written on forums, most complaints had been posted in the first week.

On these days water and salt help above all. A mug of hot broth with salt, once or twice a day, covers both without any fuss. In the same study, salt was also the remedy mentioned most often by those who wrote about what had helped them.

Vegetables at every meal: they give you potassium and fibre, which also helps with constipation. Walk as normal, but leave hard exercise for a few days.

You can go through the symptoms one by one with the Keto flu checklist, while the Electrolyte calculator works out how much salt, potassium and magnesium you need.

Days 6 and 7: you start to adjust

Towards the end of the week the symptoms ease, your energy comes back and hunger turns up less often. For some people the discomfort lasts a bit longer; in the same study, though, complaints had almost stopped after the first month.

It is a good time to see what suits you. Keep the dishes you liked and swap the ones you got tired of. Check, too, whether the portions fill you up.

What the scales show

In the first week the scales often drop sharply. Most of that is water that was bound to glycogen, not fat. Fat comes off more slowly, so if the pace slows in the second week, don't be discouraged.

It works in reverse as well. One evening of plenty of carbs refills glycogen along with its water and the next day the scales have jumped up; nobody puts on fat in a single night.

How you know you are in ketosis

If you stay steadily below 20 to 50 grams of carbs a day, you have most likely moved into ketosis within the first few days, with no need to measure it.

For anyone who wants to see a number, urine strips are the cheapest option and a blood meter the most accurate. A strip shows the ketones leaving in your urine; the meter, the ones circulating in your blood.

When to talk to a doctor

Palpitations, marked weakness or a feeling that you are about to faint are not things to wait out. The same goes if the symptoms are severe or do not ease within a few weeks.

Frequently asked questions

Can I cut carbs gradually?

Yes. If keto flu worries you, step down over one to two weeks. Ketosis will take a little longer to arrive, but for some people the change is gentler that way.

Should I exercise in the first week?

Light exercise such as walking is no problem. Hard exercise may feel heavier in the first few days, so it is better to ease off until keto flu has passed.

Why am I hungrier in the first few days?

Usually because the meals have too little fat. If you cut out bread without putting something in its place, pour some oil on your salad or add an egg or a little cheese.