Headache while fasting: why it happens and what helps

Why fasting can give you a headache

Doctors have a name for it: fasting headache. It's usually felt across the forehead or the whole head, is mild to moderate, and tends to start after at least 16 hours without food. It becomes more likely the longer you fast, and people who often get headaches are more prone to it. Caffeine withdrawal and low blood sugar are the two main suspects, though much is still unknown ¹.

Water matters more than most people expect. When young women lost just over 1% of their body weight in water, they reported more headaches and more fatigue ³. About 1 in 10 people recognise a water-deprivation headache, and for most it passed within 30 minutes of drinking around half a litre of water ⁴. On longer fasts, falling insulin also makes your kidneys let go of more salt and water ⁵.

Likely cause	Clues	What helps
Too little water	Thirst, dark urine, a dull ache	Half a litre of water, then sip through the fast 4
Missing your coffee	Around the time of your usual cup; more likely if you usually drink a lot 2	Your usual black coffee or plain tea
Salt loss on longer fasts	After 18 hours or more, with lightheadedness	A pinch of salt in water 5
Too long a fast, too soon	In the first days, late in a long fast	A shorter fast, such as 12 or 13 hours

What helps

Our guide to your first week explains why the first days are the hardest, and coffee and fasting covers how much is fine.

MYTH

A fasting headache is toxins leaving your body.

FACT

There's no evidence for this. The causes that have been studied are ordinary ones: caffeine withdrawal, low blood sugar and too little water ^1,4. All of them are easy to fix.

When to stop and eat

• The headache doesn't ease after water, your usual coffee and some rest.
• It comes with dizziness, shakiness, sweating, confusion or a racing heart, which can be signs of low blood sugar ⁸.
• You get migraines and fasting seems to set one off: eat, and talk to your doctor about whether fasting suits you ¹.

Sources

1. Torelli P, Manzoni GC. Fasting headache. Curr Pain Headache Rep. 2010;14(4):284-91. doi.org/10.1007/s11916-010-0119-5
2. Magdy R, El Desouky ED, Hammad EM, et al. Prevalence, characteristics, and factors associated with caffeine-withdrawal headache during the first day of Ramadan. Headache. 2023;63(3):360-367. doi.org/10.1111/head.14483
3. Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. J Nutr. 2012;142(2):382-8. doi.org/10.3945/jn.111.142000
4. Blau JN, Kell CA, Sperling JM. Water-deprivation headache: a new headache with two variants. Headache. 2004;44(1):79-83. doi.org/10.1111/j.1526-4610.2004.04014.x
5. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
6. Wikoff D, Welsh BT, Henderson R, et al. Systematic review of the potential adverse effects of caffeine consumption in healthy adults, pregnant women, adolescents, and children. Food Chem Toxicol. 2017;109(Pt 1):585-648. doi.org/10.1016/j.fct.2017.04.002
7. He FJ, Li J, Macgregor GA. Effect of longer term modest salt reduction on blood pressure: Cochrane systematic review and meta-analysis of randomised trials. BMJ. 2013;346:f1325. doi.org/10.1136/bmj.f1325
8. National Institute of Diabetes and Digestive and Kidney Diseases. Low Blood Glucose (Hypoglycemia). U.S. National Institutes of Health. www.niddk.nih.gov/health-information/diabetes/overview/preventing-problems/low-blood-glucose-hypoglycemia
9. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873