What to Know About Fasting Headaches, According to Dietitians

There are many reasons why 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 told you to fast. You may be celebrating a religious holiday that includes fasting, such as Ramadanor maybe you’ve accidentally been fasting because it’s a wild day and you haven’t had a second to eat (been there, did that!).

Regardless of the reason why If you haven’t eaten, there’s a real chance you’ve run into one of the not-so-good side effects of fasting: headaches.

Fasting-related headaches tend to be more common among people who have headaches in general, says Jennifer Maeng, RD, a consultant for a custom supplement company. twin lab and founder of Chelsea Nutrition. These headaches usually occur after fasting for more than 16 hours. However, it’s certainly possible to experience a fasting-related headache when you’ve been abstaining from food for less than that.

That’s partly because there are a few different reasons fasting headaches occur. Here’s what you need to know about fasting headaches, including what’s behind them and how to get rid of them, according to dieticians.

What causes fasting headaches?

There are a few likely culprits for fasting headaches, from dehydration and blood sugar swings to caffeine withdrawal.

“When your body becomes dehydrated, your blood vessels constrict,” says Reda Elmardi, RD, CSCS, owner of the gym goat. That causes the tissues in your body, including your brain, to shrink or contract. And as your brain shrinks, it separates from your 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 their water needs from food, according to the Academy of Nutrition and Dietetics. So if you’re not eating, you may need to drink more water than usual to make up for the fluid you’re not getting from food.

If you’re drinking enough water, it’s still possible to become dehydrated during fasting due to an electrolyte imbalance. electrolytes are minerals (for example, 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 that your nerves, muscles, heart, and brain can function properly, according to with the National Library of Medicine (NLM). Your electrolyte levels fluctuate naturally, but 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 drinks, but if you don’t eat (or drink) for long periods of time, there’s a chance your electrolyte levels could become unbalanced. “Fasting more than 12 hours or following a diet that mimics fasting (such as the ketogenic diet) will put you at a higher risk of dehydration and electrolyte imbalance,” because it causes your kidneys to expel sodium and potassium, says Maeng. If your body is expelling those two electrolytes, it could cause an imbalance, which can lead to dehydration or overhydration, the which can give you a headache, according to the NLM.

Also, “when your body doesn’t get enough water, it starts to produce less serotonin, a neurotransmitter that helps regulate mood and sleep patterns,” says Elmardi. serotonin changes are believed to trigger tension headaches as well as migraines, a neurological disease that often includes headache as a hallmark symptom. So that drop in serotonin caused by dehydration can cause a migraine, headache, or even mild depression during the day, says Elmardi.

In addition to serotonin, changes in other hormones like cortisol (commonly known as the stress hormone) are also a common cause of headaches, says Lisa Powell, MS, a registered dietitian in canyon ranch in Tucson, AZ, and investigate shows that fasting increases both the level and frequency of cortisol secretion.

Then there is blood sugar. One of the main reasons behind fasting headaches is fluctuations in blood sugar or hypoglycemia (also known as low blood sugar), says Maeng. The brain’s two most important sources of energy are oxygen and sugar (also known as glucose, which comes from food and is transported to the brain through the blood, hence the name “blood sugar”), according to the National Headache Foundation. When these energy sources are too low (such as when you are fasting and therefore not giving your body glucose to use), the brain reacts to try to restore them, which can cause headaches, as well as dizziness , weakness, headache and perspiration. You may even experience throbbing headaches from fasting, which “is due to minor swings in blood sugar affecting pain receptors,” says Maeng.

Finally, a fasting headache could be due to caffeine withdrawal if you’re used to drinking caffeine in the morning and now you’re skipping it because of fasting, says Maeng. Investigate has shown that caffeine withdrawal headaches are triggered by a change in blood flow to the brain. Because caffeine causes blood vessels to constrict, stopping caffeine allows blood vessels to open up, which increases 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 you’re preparing to eat, do so strategically. “Your first meal after fasting shouldn’t be a single large carbohydrate meal, like a large bowl of pasta, bread, rice, or noodles,” she says. “Eating a large amount of simple carbohydrates can raise your blood sugar and potentially lead to overproduction of insulin and cause another episode of hypoglycemia. If this continues throughout the day (eating simple carbohydrates all day), it can affect energy levels, mood, bowel movements, sleep, and even cause brain fog.” Instead, focus on eating high fiber carbohydratesvegetables, protein, and fat to provide many nutrients and support blood sugar balance.

You should also prioritize rehydration. “It’s important to maintain adequate hydration, especially when fasting,” says Powell. “I recommend drinking half your current body weight in fluid ounces per day as a baseline. So, for example, a 150-pound person would need about 75 fluid ounces per day, not including alcohol, which is significantly dehydrating. Some people may even need more fluids than this, depending on weather, exercise, and personal needs.”

As for caffeine withdrawal, if you know ahead of time that you’ll be fasting and avoiding caffeine, cut back to avoid headaches when you start, he says. Mariam Eid, RDN, LDN, in Houston. This way, your body will have already reduced its reliance on caffeine and will (hopefully) suffer less when you cut it out.

A Word About Fasting Safety

“Fasting is certainly not for everyone,” says Maeng. Specifically, it is not recommended if she has diabetes or if she is currently pregnant or trying to conceive. If she has a history of eating disorders, low weight or low blood pressure, fasting is strongly discouraged, she says. And more importantly: “At any time, if you feel dizzy, nauseous, irritable or unable to concentrate or sleep, it is best to break the fast.”

— Additional reporting by Emilia Benton