Ketosis: what it is and how long it takes

Most of the difficulty in the first fortnight is not the food. It is the arithmetic. You cut carbohydrate, the body runs down a small store of glycogen, water follows it out, and for a few days you feel worse than you did before you started. Knowing what is actually happening makes that week easier to sit through.

What ketones are

The liver turns fat into three molecules. Acetoacetate is made first and can be used directly. It converts to beta-hydroxybutyrate, usually written BHB, which is what blood meters read and what most of the research measures, in millimoles per litre. Acetone is the third, a waste product you breathe out, which is where the smell comes from.

BHB is a fuel, not a toxin. The brain, heart and skeletal muscle all take it up and burn it in the same machinery they use for glucose. A few tissues still need glucose: red blood cells have no mitochondria at all, and the kidneys use a fair amount. The liver makes that glucose from protein in a process called gluconeogenesis, which is why a diet with no carbohydrate in it does not produce a blood glucose of zero.

Why the switch happens

An adult carries roughly 400 to 500g of glycogen in muscle and around 100g in the liver. Muscle glycogen cannot be released back into the blood, because muscle lacks the enzyme needed to free the phosphate group from glucose-6-phosphate. Only the liver can top up blood glucose from its store, and that store runs low within a day of cutting carbohydrate.

As carbohydrate falls, insulin falls with it. Lower insulin releases fat from fat cells and takes the brake off the liver's fat-burning pathway. Ketone production climbs over the next two to four days until the liver is putting out more than the muscles and brain are using, and the surplus spills into the blood. That is nutritional ketosis: commonly quoted as 0.5 to 3.0 mmol/L of BHB.

Diabetic ketoacidosis is a different thing wearing a similar name. It usually sits above 10 mmol/L of BHB together with a low blood pH, and it is a medical emergency. Anyone with type 1 diabetes who is unwell, vomiting or has high blood glucose should be testing ketones and following their sick-day plan rather than reading about nutrition.

The first week, hour by hour

Every gram of glycogen holds about three grams of water. Empty the store and the water leaves with it, which is why the scales drop 1 to 3kg in the first week and why so much of that is not fat. Sodium follows the water. Low insulin also makes the kidney shed more sodium, so within two or three days you can be down a few grams of salt, and you will feel it as a headache, dizziness when you stand up quickly, cramp in the calves at night and a flatness that is easy to mistake for the diet not suiting you.

Salt is the first thing to fix. A little more on the food, or a quarter teaspoon in a glass of water twice a day, settles most of it within an hour or two. If you are on blood pressure medication, ask your GP before adding salt, because the dose that was right for you on a higher carbohydrate intake may not be right now.

Acetone on the breath is real and usually temporary. Loose stools in the first three or four days are common and usually settle. Constipation is the opposite and often arrives in week two. Both are worth reading about before they happen rather than after.

Weeks two to six

What people call keto flu is mostly the salt and water story, plus the ordinary tiredness of changing the way you eat in a house full of people who are not changing theirs. It is not a stage you have to earn.

Behind it, adaptation continues. The muscles get better at using fat and ketones for fuel, which is measurable as a lower respiratory exchange ratio during exercise, and it takes weeks rather than days. Early exercise feels heavy. By week four or five it usually does not.

Appetite is the other thing that moves. Protein and fat slow gastric emptying and protein in particular is the most satiating of the three macronutrients in acute feeding studies. What that means in practice is that meals get smaller without effort, which is the mechanism most people describe when they explain why they stopped counting.

How to measure it, and whether to bother

A blood ketone meter gives BHB in mmol/L and is the only measurement that maps onto the numbers in the research. Strips are expensive, around 1 pound each, and most people stop testing after a few weeks.

Urine strips measure acetoacetate. They work well in the first fortnight and then often fade even when blood BHB is unchanged, because the body gets better at converting acetoacetate onward instead of spilling it. A pale urine strip after a month is not a sign that you have fallen out of ketosis.

Breath acetone meters exist and are the least reliable of the three in ordinary use.

The honest answer on chasing numbers is that there is nothing useful to do with a reading of 2.4 that you would not do with a reading of 1.1. Appetite, energy through the afternoon, stool and sleep tell you more. Ketones are a fuel gauge, not a score.

What ketosis does not do

Ketones do not burn fat for you. Weight change on a low carbohydrate diet is driven by how much you eat, and the advantage most trials find is that protein and fat make eating less feel less like a sacrifice, which is a smaller and more boring claim than the one on the packaging.

Ketones do not clean anything out. The liver processes waste continuously whether or not you are making ketones, and no second pathway opens when carbohydrate falls.

If you take insulin or a sulfonylurea, cutting carbohydrate changes your medication requirement, and the risk of a hypo is real. That is a conversation with your GP or diabetes team before you start, not after.

Where to start

Pick a fortnight where nothing important is happening. Keep salt high from day one rather than adding it after the headache arrives. Do not judge the experiment until week three, because weeks one and two are mostly water, sodium and adjustment. Then look at the things you actually care about: hunger, energy, mood, stool, sleep.