Headaches and fatigue in week one

The headache usually arrives on day two or three, sits behind the eyes and behind the forehead, and is gone inside a week. The tiredness outlasts it, closer to a fortnight, and is the symptom that convinces people the diet is not working.

Both numbers come from trials rather than forums. They have different causes, and only one of the two is the diet.

What the trials recorded

The most complete summary is a systematic review in BMC Nutrition in 2026 [1]. It pooled 36 studies covering 42 ketogenic interventions and 1,162 participants, across classic ketogenic ratios, modified Atkins, MCT-based versions and low glycaemic index treatment. At least one adverse event appeared in 43% of participants across the pooled studies, with a range from 0 to 89%, and an average of 0.91 events per person.

Neurological events, which is the bucket headache lands in, appeared in 60% of the 42 interventions. General symptoms, mostly fatigue, appeared in 45%. Constipation was the single most common individual symptom, at 24%.

Duration was recorded far less often: seven of the 36 studies, 19%, said how long anything lasted. Across those seven, events ran from one week to six, mean 3.5 weeks. Two of the seven split it by symptom, and those are the useful ones. A 2024 pilot in 21 people with bipolar disorder or schizophrenia recorded headaches lasting one week and fatigue averaging 1.86 weeks, with constipation running past two [2]. A 2023 pilot in bipolar disorder recorded fatigue continuing beyond two weeks [3].

The review puts the general symptoms in the adaptation phase, the first two to seven days, which is the cluster called keto flu outside clinical settings.

Who those percentages describe

Not you, most likely. Fifteen of the 35 intervention groups with a named condition were in epilepsy and nine in cancer, mean intervention length was 19.45 weeks, and the diets were supervised. Percentages from that population describe a therapeutic regimen given to people with a diagnosis.

What transfers is the shape rather than the risk: headache early and short, fatigue longer and vaguer, both concentrated in the opening fortnight. That shape repeats across every study that reported symptom-level durations, and it matches what the site's own cramp and loose-stool pieces describe for the same period.

Three things happening at once

Sodium and water. The review lists a shift in fluid and electrolyte balance among the explanations for early symptoms [1]. The mechanism is ordinary: lower insulin means the kidney reabsorbs less sodium, water follows the sodium out, and total body water falls by a litre or two. The scale drop of 1 to 3kg in week one is mostly that, not fat.

Caffeine. This is the one most people miss, because changing how you eat usually means changing coffee at the same time, along with the milk and the sugar in it. A review of 57 experimental and 9 survey studies found headache in 50% of people withdrawn from caffeine under experimental conditions, with distress or impairment severe enough to matter in 13% [4]. Onset was 12 to 24 hours after the last dose, peak intensity 20 to 51 hours, duration 2 to 9 days. Symptoms appeared at doses as low as 100mg a day, about one mug of filter coffee.

That timeline is diagnostic. A headache that starts the afternoon after your last cup, is worst the next morning, and has gone by day four is withdrawal. One that arrives on day three and fades across the week fits the trial pattern better.

Missed meals. The NHS lists not eating regular meals among the common causes of headache [5], and appetite often drops in the first week before it settles. Two eggs at eleven and nothing until seven is a headache with a cause you can name.

The week, roughly

Days 1 and 2: thirst, more urination, no headache yet. Days 2 to 4: the headache, light-headedness on standing, a flat mood and poor patience. Days 4 to 7: the headache eases and the fatigue stays. Week 2: energy comes back for most people.

That sequence is the trial data plus the mechanism plus judgement, in that order of confidence. Nobody has measured it day by day in healthy adults, which is the gap at the bottom of this piece.

What to do about it

Salt your food properly for the first fortnight. If the headache has already started, a quarter teaspoon of salt in 500ml of water twice a day is the usual starting point. No trial has tested salt against week-one headache, so treat it as a judgement call built on the sodium shift above: often enough to be worth trying, not reliable enough to be a rule.

Drink to thirst rather than to a target, and check the colour of your urine, which should be pale straw. Drinking several litres without sodium dilutes what you have left and makes the light-headedness worse.

Separate the caffeine change from the carbohydrate change by a fortnight, in either direction, or taper the coffee over a week. Doing both on the same Monday leaves you unable to tell which one is causing what, and the two need different responses.

Eat on a clock for the first seven days even when hunger has gone. Painkillers are fine at the label dose for a few days, but the NHS lists taking too many painkillers as a cause of headache in its own right, so a fortnight of daily paracetamol is a conversation with a GP rather than a salt problem [5].

When it is not adaptation

If you take insulin, fatigue and headache are also symptoms of diabetic ketoacidosis, which usually builds over about 24 hours and needs hospital care [6]. Home ketone readings come with action bands attached: below 0.6mmol/L is normal, 0.6 to 1.5mmol/L means retest in two hours, 1.6 to 3.0mmol/L means ring the diabetes team, and above 3.0mmol/L means call 999. Insulin is not the thing to reduce because you are eating less, since that is how the condition starts.

For everyone else the red flags are specific. Call 999 or go to A&E for a headache that came on suddenly and is extremely painful, one with a high temperature, stiff neck or dislike of bright light, one with numbness or weakness in the body or face, a seizure or fit, being drowsy or confused, difficulty speaking, balancing, walking or remembering, loss of vision, a rash that does not fade under a glass, or any headache within three months of a head injury. Ask for an urgent GP appointment or ring 111 for a headache with eye or vision problems, one made worse by coughing, sneezing, bending or exercise, one with vomiting, or one with jaw pain on eating or a tender scalp [5].

The gap

No trial has measured headache and fatigue in healthy adults starting a very low carbohydrate diet for their own reasons, and none has tested salt, water or caffeine tapering against either. Everything above is pooled from clinical populations or reasoned from a mechanism, and the confidence differs sharply between those two.

That matters when someone tells you the first week is definitely three days, or definitely three weeks. The published range for a recorded adverse event is one to six weeks, mean 3.5, and that includes events far more persistent than a headache.

One week for the headache and two for the tiredness is the working rule. Past the second week, stop calling it adaptation and start looking for the reason.