The first four weeks: a plan you can actually follow

Four weeks is long enough to learn something and short enough to survive. This is the version I would run again, written as a schedule rather than a list of rules, because the thing that goes wrong is rarely the food.

Before day one

Get the blood tests done while you are still eating normally. A full blood count, ferritin, HbA1c, a lipid profile, kidney and liver function, vitamin D, B12 and folate. Ask your GP. If coeliac disease has ever been mentioned as a possibility, ask for the coeliac blood test now, because it stops working once you have been off gluten for a few weeks.

Clear the diary of anything that needs you at your best: a job interview, a wedding where you are speaking, a long drive at the end of a hard week. Weeks one and two are not the weeks to schedule those.

Weigh yourself once, take a waist measurement, and skip the progress photography. There is a rule on this site about it and it exists for good reason.

Book a date four weeks out to review. Put it in the calendar with a reminder.

Week one: food, salt, water

Two or three meals a day, at times you would normally eat. Beef, salt, water, and nothing else if you want the cleanest read on how you respond.

Rough portions for an adult: 250g of beef mince or a 250 to 300g steak per meal. Add fat rather than subtracting it, because lean meat with no fat and no carbohydrate leaves most people hungry within two hours. Beef dripping, butter or the fat already in a 20% fat mince all work.

Salt from day one, not after the headache. A quarter teaspoon in a glass of water twice a day, or the same amount spread over the food, is a reasonable start for most people. If you take medication for blood pressure, ask your GP first.

Water: 2 to 2.5 litres a day, more if it is warm or you are training.

Expect, in this order: a drop on the scales of 1 to 3kg that is mostly water, a headache around day two or three, cramp in the calves at night, loose stools for two or three days, and a flatness in the afternoons that lifts in week two.

Do not add anything to fix the way you feel in week one except salt and sleep. Adding cheese on day three because you feel rough teaches you nothing.

Week two: settle, and start looking

The water drop is done and the scale should be moving slowly or not at all. Appetite usually falls this week, which is the interesting signal, so eat to hunger rather than to a number.

Constipation often arrives now. Water, salt, fat at meals, a walk after eating, and magnesium if you want it. There is a longer piece on this in the gut section.

Sleep is the thing to watch. If you are waking at three in the morning and cannot get back down, that is usually hunger or sodium, not a mysterious metabolic event.

If you are training, keep the sessions but expect them to feel heavy. Do not judge fitness this month.

Week three: the honest assessment

This is the week where the experiment becomes testable. Week one was water and week two was adjustment; week three is closer to what this way of eating actually feels like.

Write down four things every evening. Hunger from 0 to 10. Energy from 0 to 10. Stool form on the Bristol scale. Sleep quality from 0 to 10.

Then, on the review date, compare week three and four against the week before you started. Not against how you hoped you would feel.

Week four: decide, and write it down

Three outcomes are possible and all three are useful.

It is clearly better than before, in the specific things you logged. Then the question becomes how long to continue and what to reintroduce first, which is a separate guide.

It is mixed: some things better, some worse. Then change one variable, not five. Usually fat intake or total quantity.

It is worse or the same. Then stop, and be honest about it. Four weeks of a diet that did not work is a result, not a failure.

The things that make people quit in week two

Eating the same meal six times running, which is a cooking problem and not a diet problem.

Being hungry because the portions were too small, which is the commonest one.

Not salting the food, which produces the headache, the dizziness, the cramp and the flat afternoons, all of which get blamed on the diet.

Being cold. Fat loss and low insulin both reduce the sensation of warmth; wear more and give it a fortnight.

Household friction. There is a separate piece on that, and it is worth reading before you announce anything.

When to stop and call someone

Palpitations that do not settle. Dizziness on standing that persists past a minute or two despite salt and water. Fainting. A resting heart rate that stays unusually high for several days. Vomiting that stops you keeping fluids down. Any of these is a GP conversation.

If you take insulin or a sulfonylurea, or medication for blood pressure, the plan needs adjusting before you start rather than after.