Intermittent fasting side effects

Are you interested in trying intermittent fasting? You aren’t alone. Intermittent fasting has quickly become one of the most popular and effective ways to achieve weight loss and improve health conditions such as type 2 diabetes, high blood pressure, and metabolic syndrome.¹

As with many lifestyle interventions, some people may experience unwanted side effects when fasting. This guide will explain what you may encounter and how to prevent any undesirable effects.

We’ll also cover who may not be the right candidate for fasting. Make sure you continue reading to determine into which category you fall.

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What is intermittent fasting?

Intermittent fasting is a general term that encompasses time-restricted eating (fasting less than 24 hours), short-term fasting (fasting less than 36 hours), and long-term fasting (fasting more than 36 hours).²

You can learn more about intermittent fasting in our main guide to intermittent fasting.

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Potential fasting side effects and tips for preventing them

Most people tolerate intermittent fasting without any serious trouble. However, mild side effects are common, especially when starting.³⁴ A general rule for fasting is that mild symptoms are expected, but severe or disabling symptoms are not typical and indicate you should immediately break your fast. Fortunately, these are very rare.

It is also essential to plan ahead and anticipate how you will respond if you do experience side effects while fasting. Knowledge is power! Be prepared so that you can increase your chances of success.

Hunger

Hunger is likely the most common side effect of fasting.⁵ The key is to recognize that most hunger is benign and can be ignored.⁶

Many find that hunger comes and goes in waves; simply ignoring it will help it disappear.⁷ Others find distractions helpful. Going for a walk, engaging in conversation, or engaging your brain in a challenge ( e.g. a work project, a puzzle, or a game) are all activities that can help. Others find that sparkling water, hot water with lemon, or water with salt can also reduce hunger feelings.

The most important thing is to anticipate that you might feel hungry and realize this is okay. Plan ahead how you will respond to this sometimes nagging feeling, so you aren’t caught off guard.

Weakness

Some people find they become a bit weak while fasting. It may require decreasing the intensity and/or duration of exercise, but time-restricted eating should not prevent you from exercising at all.⁸

Those with physically demanding jobs, however, may find this to be an issue. Staying well-hydrated and supplementing with salt are two potential ways to prevent feelings of weakness.⁹

While mild weakness is normal, you should not feel extreme weakness or difficulty walking or standing. If that is the case, you should break your fast and contact your healthcare professional.

Headache and dizziness

As with weakness, mild headaches or dizziness are relatively common and may be prevented with hydration and salt intake.¹⁰

However, severe, debilitating headaches or severe dizziness are not typical and indicate it is time to break the fast.

Confusion

Some may feel less mentally sharp while fasting, but this practice should not lead to outright confusion. If that is the case, break the fast and consult your healthcare professional. Conversely, many people feel mentally sharper as they go deeper into ketosis with fasting. It may be worth experimenting to see how you respond.

Constipation

It makes sense that if you are eating less, you will naturally have fewer bowel movements. Going to the bathroom less frequently is a normal consequence of fasting, and does not mean you are constipated.

If you do not feel bloated or uncomfortable and you don’t have to strain to have a bowel movement, then you likely are not constipated. However, if you experience these symptoms, you may want to increase your hydration or try magnesium supplements.¹¹

Heartburn

For some, fasting can relieve heartburn symptoms. For others, it can worsen symptoms. Again, experimenting with fasting will help you determine how you respond.

Elevated blood sugar

While fasting may benefit overall blood sugar control, some find their morning blood sugar rises.

This is called the “dawn phenomenon.” It occurs when the liver releases glucose into the bloodstream in response to receiving signals that the body may need more energy after a long rest.¹²

Those who follow a low-carb diet and experience elevated blood sugar with fasting usually find their highest levels are upon waking (almost always less than 120mg/dl), and the rest of the day, they have normal, flat blood sugar levels.¹³

As we discuss in our guide on the dawn effect, these mildly elevated fasting blood sugar levels may not be of clinical concern, especially if they are only present while fasting. However, if someone’s fasting blood sugar increases regularly above 130, they may want to consult their physician.

Low blood sugar

Low blood sugar in the absence of glucose-lowering medications is rare.¹⁴

If someone feels sweaty, extremely weak, nauseous, or overly lightheaded, it is possible – but not likely – they are suffering from hypoglycemia. In this situation, it’s helpful to check a blood sugar reading— if it can be done safely. Otherwise, you should break your fast right away and consult your healthcare provider.¹⁵

Refeeding syndrome

This is an extremely rare but very serious condition that can occur after prolonged fasting in those who are underweight or malnourished.¹⁶ These people can experience potentially fatal shifts in fluids and electrolytes during refeeding.

Most practitioners of intermittent fasting will never have to worry about this, and it is one reason why fasting is contraindicated for underweight or malnourished individuals.

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Who should not fast?¹⁷

The following categories of people should not practice intermittent fasting.

1. Anyone who is underweight or malnourished: Fasting could deprive underweight or malnourished individuals of the nutrition and energy they need.
2. Anyone with a history of eating disorders (such as anorexia or bulimia): Fasting may trigger a relapse of these disordered eating behaviors.
3. Children and adolescents: Children and adolescents have greater growth demands and therefore need a steady supply of nutrition and energy. We recommend overweight or obese children and adolescents focus on healthier eating patterns without fasting.
4. Women who are pregnant or breastfeeding: Pregnancy and breastfeeding increase the mother’s metabolic and nutrient needs, and fasting can result in a failure to meet those needs.
5. Women who are trying to conceive: Fasting can trigger a woman’s body to ovulate irregularly or completely stop ovulating, thereby decreasing the chance of pregnancy.
6. Those who find fasting triggers cravings and overconsumption: Some people don’t have any contraindications to fasting, but they find fasting makes them much more likely to binge eat, choose higher-carb foods, or eat more calories than they normally would. This counteracts the benefits of fasting and therefore they should avoid fasting.

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Who needs special guidance?¹⁸

Individuals in the following categories may be able to successfully fast, but they should seek care from an experienced healthcare clinician to ensure safety. If you need help finding someone with knowledge in this area, please see our clinicians map.

1. Those who take diabetes medication: Someone with either type 1 or type 2 diabetes who takes medications that lower blood sugar — such as insulin, sulfonylureas, and others — may be at risk of dangerously low blood sugars when fasting. This applies equally to long- and short-term fasts.
   
   These individuals may still be able to fast, but they will need careful coaching and monitoring from an experienced professional. You can see our informational resource meant to educate clinicians and help them guide their clients to safely start and maintain a fasting routine.
2. Those who take blood pressure medications: While the effect of fasting on blood pressure is much less dramatic than the effect on blood sugar, some may find their blood pressure drops too low when fasting. This is more relevant for longer-term fasts, but can also be an issue with short-term fasts. Close supervision with a physician is recommended.¹⁹
3. Those who take other medications: Some medications — such as aspirin, ibuprofen, other NSAIDs, metformin, beta blockers, and others — may cause digestive upset when taken on an empty stomach.
   
   If you are considering fasting, that does not mean you should simply skip these medications. It is important to first have a discussion with your physician regarding your options when it comes to taking your medications regularly, skipping them, or changing their timing.
4. Those with serious medical conditions: Any serious liver, kidney, heart, or other condition may negatively impact one’s ability to fast safely. Anyone with these conditions should get clearance from their physician first.

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How to break a fast

How you break a fast can be just as important as the fast itself.

For time-restricted eating (fasts less than 24 hours) and short-term fasts (less than 36 hours), there are only a few things to consider:

1. Make sure you have control over what you will eat when you break your fast. The last thing you want is to be hungry and looking for nutritious food, only to find yourself at a work event with pizza, bagels, and doughnuts.
2. Don’t “make up for lost calories.” Remember, part of the benefit of time-restricted eating is a natural reduction in calories. Eat your normal meals as if you hadn’t fasted.
3. The only exception to rule #2 is that you may need to add a little extra protein to your meals to ensure you reach your daily targets. This is especially important for those eating one meal a day (OMAD). Learn more about protein needs in our guide: “How much protein should I eat?”

For longer-term fasts (greater than 36 hours), you may need to take extra precautions to avoid side effects such as bloating, heartburn, or diarrhea. The following suggestions can help minimize these risks:

1. With longer fasts, it is even more important to make sure you have complete control over when and what you will eat to break the fast. Don’t get caught off guard!
2. Start with a snack, not with a full meal. You can start with bone broth, low-carb soup, or a small snack. You don’t want to eat anything too heavy for your first food intake.
3. Two or three hours after your snack, you can have your first meal. Again, keep it relatively simple. Avoid heavy or complex sauces. Go for a small amount (no bigger than the palm of your hand) of simply prepared protein, low-carb veggies cooked in butter, or EVOO and a quarter avocado.

With longer fasts, it’s important to remember to have a transition day. For example, a five-day fast is a six-day protocol including five days of fasting and one transition day. You can then resume regular eating on the seventh day. For more information, see our guide on breaking a fast.

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Key takeaways

Intermittent fasting can be a helpful addition to any healthy lifestyle, and most people can safely and effectively incorporate fasting into their lives. Make sure you are aware of potential side effects before you start fasting and anticipate your response. That way you won’t get caught off guard and you will be prepared to succeed.

/ Dr. Bret Scher, MD