Keto: what changes in the body when you cut carbs

On a carb-rich diet, every meal raises insulin. It is known for its effect on blood sugar, far less for another one: it tells the kidneys to keep sodium.

The effect was measured as early as 1975. In healthy volunteers, an insulin infusion, with blood sugar held steady, cut urinary sodium excretion from 401 to 213 microequivalents per minute: almost half, in under two hours.

That retention is not a malfunction. It is a normal regulation, and a useful one: it spares a salt that was scarce and precious for most of our history.

On keto, insulin stays low all day. The kidneys therefore let sodium go, and the water that goes with it. What that regulation used to keep on its own now has to be supplied on purpose.

General guidelines aim the other way: under 5 g of salt a day according to the WHO, that is 2 g of sodium. They are written for people who eat carbs, whose kidneys therefore hold on to sodium.

For a ketogenic diet, Stephen Phinney, one of the researchers who has studied the subject most, describes in his studies a total intake of 3 to 5 g of sodium a day, 3 g of it added as broth. That is 7.5 to 12.5 g of salt.

Keep the conversion in mind: 1 g of sodium is 2.5 g of salt. European labels give salt, studies often give sodium.

It is worth being clear about how solid these figures are. The insulin mechanism is established. The amounts come from the practice of a few research teams, not from trials comparing several salt doses in people on keto.

This is the keto paradox: it often starts by giving people headaches.

A 2020 study went through the “keto flu” accounts posted on 43 online forums. Of 101 people describing their symptoms, a quarter mentioned headaches, alongside fatigue, nausea and dizziness. Complaints peak in the first week and fade after four weeks.

The authors acknowledge it: online accounts are not a representative sample, and nothing confirms these people were actually in ketosis. The loss of sodium and water in the first days remains the most common explanation, and it is to prevent it that Phinney added broth from day one.

Later on, many people report the opposite: fewer headaches than before. For migraine, there are studies. A 2023 meta-analysis pooled ten of them, covering about 270 adults: every form of ketosis tested reduced how often attacks occurred, and depending on the study, 58 to 83% of participants saw their attacks drop by at least half.

Its limits are significant. Most of these studies have no control group, nearly all come from the same Italian team, many test very low-calorie diets in overweight people, and one participant in five drops out along the way. The authors conclude to a possible benefit, not a proven one.

The proposed mechanisms are plausible: ketones acting as a backup fuel for the brain, less inflammation, less excitable neurons. And other changes may play a part: less sugar and processed food, no more blood-sugar dips after meals, often less alcohol.

Sodium does not leave alone. When its loss lowers blood volume, Phinney explains, the kidneys start excreting more potassium. And glycogen, which melts away in the first days, releases the potassium it was holding.

Hence the cramps, typical of the first weeks. The European Food Safety Authority sets adequate intakes at 3,500 mg of potassium a day, and 300 mg of magnesium for women, 350 mg for men.

What potassium does. It is sodium's partner: the gap in concentration between the two, on either side of the cell membrane, is what makes nerves and muscles work, the heart included. Enough of it also helps the kidneys clear sodium and lowers blood pressure, which is why the WHO recommends at least 3,500 mg a day, with a measured effect on stroke risk. Its alkaline salts finally limit calcium loss in urine, and so kidney stones.

Several low-carb foods are rich in both. Values are per 100 g, three to four times the handful actually eaten in the case of seeds. Carbs here are net, with fiber already taken out, unlike the figures on US labels:

Avocado stands out: hardly any carbs, plenty of potassium, and 100 g is an easy portion. These figures, and many more, are on the Nuts & seeds page. What each mineral does, and what drains it, is set out in Minerals.

The body stores carbs as glycogen, in the liver and muscles. Every gram of glycogen is stored with three to four grams of water.

Once carbs are cut, that glycogen empties within a few days, and its water goes with it. Together with the sodium the kidneys release, it explains the weight lost very quickly at the start: it is mostly water, not fat.

This is neither bad news nor good news: it is normal, and it is reversible. A carb-rich meal sends the scale back up the next day, for the same reason.

On an ordinary diet, much of the fiber comes from grains, legumes and fruit, precisely what keto cuts back. Digestion often feels it. The European authority recommends 25 g of fiber a day.

Fiber does not raise blood sugar, which is why the carbs below have it taken out. It simply has to come from elsewhere:

Two tablespoons of chia, about 25 g, already provide 8 to 9 g of fiber. For vegetables, the lowest in carbs are on the Vegetables page.

Almost everything that surprises people at the start of keto comes from a single cause: low insulin, which lets sodium, water and potassium go.

All of this describes a change of fuel, not a case against carbs. The body runs on both, and these adjustments only say what has to be supplied differently once you change yours.

Salt. On keto, the need genuinely goes up. Not without medical advice if you have high blood pressure, or are followed for your kidneys or heart.

Headaches. Those of the first days often come from there. The benefit for migraine is a serious lead, not yet a proof.

Concentration. A fog in the first days, then a clarity many people describe and no trial has yet measured: Keto and focus.

Weight. What is lost in the first week is mostly water.

Fiber, potassium and magnesium. They are found in seeds, nuts and avocado.

Physical performance. It drops during adaptation, then endurance gains and strength is unchanged, while short repeated efforts stay carb territory: Keto and exercise.

For the role of insulin itself, see What happens after a meal.