Keto flu: why it happens and how to get through it

The body stores carbohydrates as glycogen, and every gram of it holds water. When there are few carbohydrates in the daily diet, the supply is used up, and the water leaves with it.\n\nSalts leave along with the fluid. This is why headaches, lethargy, and dizziness when standing up quickly occur — it is more often a water-and-salt issue than a case of the “body being deprived of sugar.”\n\nThe second part is metabolic adaptation. The body is used to glucose as its main fuel and works in transition for some time.

For some people, the first days pass unnoticed, and this is not a sign that “nothing is working.”\n\nIt is usually easier for those who previously ate little sweets and did not snack constantly. It is harder for those whose day revolved around fast carbohydrates: baked goods, sweet coffee, and snacks every two hours.

Food. The most common cause of difficult first days is not carbohydrates but hunger: a person removes familiar foods and does not replace them with an equal volume of other food.\n\nDrink enough: about two liters of fluid per day, more in hot weather and on training days. Keep salt in the range of 3-5 g per day.\n\nAnd do not combine a new way of eating, a strict calorie deficit, and a new training routine in one week. Otherwise, it is unclear what exactly is making you feel bad, and the method is usually blamed.

From several days to several weeks; there is no exact timeframe.\n\nUsually, by the end of the first or second week, the initial headaches and weakness subside, if they occurred at all. But there is no calendar rule such as “day three is the turning point,” and you should not wait for one.

Marked deterioration should not be dismissed as adaptation.\n\nFainting or a near-fainting state, confusion, unusual weakness, repeated vomiting or being unable to drink normally, symptoms of hypoglycemia, and severe or worsening abdominal pain are all reasons to stop and seek medical help rather than trying to endure it.

Do not start on your own during pregnancy or breastfeeding, with type 1 diabetes, or with type 2 diabetes if you use insulin or medications that carry a risk of hypoglycemia.\n\nChronic kidney disease, gallbladder removal, eating disorders, and adolescence should be discussed separately with a doctor. For older adults, sufficient calories and protein become the priority.

Completely prevent it? No. Make it milder? Yes.\n\nThree things work in advance: do not cut calories at the same time as changing the composition of your food, establish fluid and salt intake right away, and do not start a new training routine in the same week.\n\nThe fourth is gradual change. If you transition in five steps instead of one day, difficult days usually do not occur at all. It takes longer, but is noticeably easier to tolerate.

Yes, and this is the simplest way to address both fluid and salt at once.\n\nA cup of warm salted broth during the first days often relieves a headache within an hour. Both regular meat broth and vegetable broth work — the salt matters, not the recipe.\n\nThis is useful specifically during the first weeks, when water is being lost. Later, the water-and-salt balance normalizes on its own, and there is no need to drink broth deliberately.

This no longer looks like adaptation, and it should not be dismissed as such.\n\nThe most common cause is undereating: a person removes familiar foods and does not replace them with an equal volume of other food. The second is dehydration combined with severe salt restriction. The third is lack of sleep, which itself causes weakness and brain fog.\n\nIf food intake, water, salt, and sleep are adequate but your condition is worsening, see a doctor rather than trying to endure it.