There are many reasons you may find yourself fasting for an extended period of time. Maybe you're trying intermittent fasting or you have an upcoming blood test and your doctor instructed you to fast. You might be celebrating a religious holiday that includes fasting, such as Ramadan, or maybe you've accidentally been fasting because it's a wild day and you haven't had a second to eat (been there, done that!).
Regardless of the reason why you haven't eaten, there's a real possibility you've encountered one of the not-so-great side effects of fasting: headaches.
Fasting-related headaches tend to be more common among people who get headaches in general, says Jennifer Maeng, RD, advisor for personalized supplement company Twinlab and founder of Chelsea Nutrition. These headaches typically occur after fasting for more than 16 hours. However, it's certainly possible to experience a fasting-related headache when you've abstained from eating for less time than that.
That's in part because there are a few different reasons why fasting headaches happen. Here's what you need to know about fasting headaches, including what's behind them and how to get rid of them, according to dietitians.
What Causes Fasting Headaches?
There are a few likely culprits behind fasting headaches, from dehydration and changes in blood sugar to caffeine withdrawal.
"When your body becomes dehydrated, the blood vessels become constricted," says Reda Elmardi, RD, CSCS, owner of The Gym Goat. That causes tissues in your body — including your brain — to shrink or contract. And as your brain contracts, it pulls away from the skull, putting pressure on nerves and causing pain, according to the Cleveland Clinic. For that reason, even mild dehydration can cause a headache.
And drinking water might not be enough. People typically get about 20 percent of the water they need from food, according to the Academy of Nutrition and Dietetics. So if you're not eating, you'll likely need to drink more water than usual to offset the fluid you're not getting from food.
On the off chance that you are drinking enough water, it's still possible to be dehydrated while fasting due to an electrolyte imbalance. Electrolytes are minerals (ex: sodium, potassium, magnesium) in your body that are important for balancing the amount of water in your body, moving nutrients into your cells, and ensuring your nerves, muscles, heart, and brain can work properly, according to the National Library of Medicine (NLM). Your electrolyte levels naturally fluctuate, but they can become too low or too high as a result of changes in the amount of water or electrolytes in your body. Most people get enough electrolytes through their food or beverages, but if you aren't eating (and/or drinking) for long periods, there's a chance your electrolyte levels could be thrown off balance. "Fasting more than 12 hours or following a fast-mimicking diet (such as the ketogenic diet) will put you at a higher risk of dehydration and electrolyte balance" because it causes your kidneys to expel sodium and potassium, Maeng says. If your body is expelling those two electrolytes, it could cause an imbalance, which can lead to dehydration or overhydration — both of which can give you a headache, according to the NLM.
Additionally, "when your body doesn't get enough water, it starts producing less serotonin, a neurotransmitter that helps regulate moods and sleep patterns," Elmardi says. Serotonin changes are thought to potentially trigger tension headaches as well as migraines, a neurological disease that often includes headache as a hallmark symptom. So that dip in serotonin caused by being dehydrated can cause a migraine, headache, or even some mild depression during the day, Elmardi says.
Besides serotonin, shifts in other hormones such as cortisol (commonly known as the stress hormone) are also a common cause of headaches, says Lisa Powell, MS, a registered dietitian at Canyon Ranch in Tucson, AZ — and research shows that fasting increases both the level and frequency of cortisol secretion.
Then there's blood sugar. One of the main reasons behind fasting headaches is fluctuations in blood sugar or hypoglycemia (aka low blood sugar), Maeng says. The brain's two most important sources of energy are oxygen and sugar (aka glucose, which comes from food and is transported to the brain via your blood — hence, the name "blood sugar"), according to the National Headache Foundation. When these energy sources are too low (such as when you're fasting, and thus not giving your body any glucose to use), the brain reacts to try to restore them, which can cause headaches as well as lightheadedness, weakness, headache, and sweating. You may even experience pulsing headaches from fasting, which is "due to minor changes in blood sugar that affect pain receptors," Maeng says.
Finally, a fasting headache could be due to caffeine withdrawal if you are used to drinking caffeine in the morning and now you're skipping it due to fasting, Maeng says. Research has shown that caffeine-withdrawal headaches are triggered by a change in blood flow in the brain. Because caffeine causes blood vessels to constrict, stopping your caffeine consumption allows blood vessels to open up, increasing blood flow. This sudden change can cause those painful withdrawal headaches.
What Can You Do About a Fasting Headache?
There's good news: "It usually goes away once you eat," says Maeng. That said, if you're suffering from a fasting headache and are getting ready to eat, dig in strategically. "Your first meal after fasting should not be a large simple carbohydrate meal such as a large bowl of pasta, bread, rice, or noodles," she says. "Eating a large amount of simple carbohydrates can spike your blood sugar and potentially lead to overproduction of insulin and lead to another episode of hypoglycemia. If this continues to happen throughout the day (eating simple carbs all day), it can impact energy levels, mood, bowel movements, sleep, and even cause brain fog." Instead, focus on eating high-fiber carbohydrates, vegetables, protein, and fat to provide lots of nutrients and support blood sugar balance.
You should also prioritize rehydrating. "It's important to maintain adequate hydration, especially when fasting," Powell says. "I recommend drinking half of your current body weight in fluid ounces per day as a baseline. So, for example, a 150-pound person would need about 75 ounces of fluid per day, not including alcohol, which is significantly dehydrating. Some people may even need more fluids than this, depending on climate, exercise, and personal needs."
As for the caffeine withdrawal, if you know ahead of time that you'll be fasting and avoiding caffeine, taper down your intake to prevent headaches when you do start, says Mariam Eid, RDN, LDN, in Houston. This way, your body will already have reduced its reliance on caffeine and you'll (hopefully) suffer less when you nix it.
A Word on Fasting Safety
"Fasting is certainly not for everyone," Maeng says. Specifically, it's not recommended if you have diabetes or if you're are currently pregnant or trying to conceive. If you have a history of disordered eating, being underweight, or have low blood pressure, fasting is strongly discouraged, she says. And even more importantly: "At any point, if you feel lightheaded, nauseous, irritable, or unable to focus or sleep, it's best to end your fast."
— Additional reporting by Emilia Benton