A Guide to the Possible Side Effects and Symptoms of Intermittent Fasting, By Stage 

Intermittent fasting has exploded in popularity in recent years. While there is research to support the potential benefits, there is also concern about possible side effects. Let’s take a deep dive into intermittent fasting and discuss the potential side effects and symptoms.

Is intermittent fasting healthy? 

Intermittent fasting is an eating plan that alternates between fasting and eating on a regular schedule. There are various intermittent fasting schedules that people can follow, but some of the most popular include:

• Daily time-restricted fasting: The 16:8 eating pattern is the most common version of this approach. A person fasts for 16 hours each day (usually skipping breakfast) and eats normally during an 8-hour window.

• Alternate-day fasting: A person eats normally one day and then does a full fast (or only eats a small meal) the next day.

• 5:2 fasting: A person eats normally five days per week and then fasts two days per week.

With intermittent fasting, there is no requirement to count calories or follow a certain type of meal plan. Although eating a healthy, balanced diet would be ideal, some say that the simplicity of intermittent fasting makes it easy for someone to fit into their life.

There is a growing body of research regarding the potential benefits of intermittent fasting. One of the most popular uses of this eating pattern is for weight loss. This is because this type of eating pattern can help people eat less calories over the course of a day or week. 

Other research has looked at the benefits of intermittent fasting for people with type 2 diabetes and obesity. It has been shown that intermittent fasting may help lower blood sugar and insulin levels. 

Other potential benefits of intermittent fasting include improved brain health, decreased inflammation, and increased longevity.

With that being said, intermittent fasting has not necessarily been shown to be better than other eating patterns. When it comes to following a healthy diet, the most important aspect is whether or not it’s sustainable. It depends on the person, their lifestyle, and whether or not the eating pattern works for them in the long-term. 

If you’re interested in trying intermittent fasting, it’s essential to speak with your healthcare provider before starting. 

There are a number of different groups of people who should not try intermittent fasting. These include:

• Children and teens under age 18

• People who are pregnant or breastfeeding

• Those with type 1 diabetes who take insulin

• Anyone with a history of eating disorders

Another concern is a potential negative impact on female hormones. Therefore, some experts suggest that pre-menopausal women should stick to less intense time restriction if using intermittent fasting.

Does fasting have side-effects? 

If you’ve ever gone a long time without eating, you may be familiar with some of the potential side effects of fasting, including: 

• Hunger

• Fatigue

• Headache

• Nausea

• Insomnia

These side effects can occur due to low blood sugar during the fasting period, lack of nutrients, and overall undereating.

Another unintended side effect of intermittent fasting is overeating during the eating period. Intermittent fasting patterns are intended to reduce total calorie intake, but there’s also a chance that people may eat more as a rebound effect from food restriction. If this progresses into a pattern of restricting followed by binging, intermittent fasting should be stopped.

Time-restricted fasting may not have as many side effects as alternate-day or 5:2 fasting methods. This is because the fasting period in the time-restricted method is typically around 16 hours compared to a full one to two days of fasting with the other patterns.

If you are giving intermittent fasting a try and you are experiencing concerning side effects, you should speak to your healthcare professional.

Side effects in the early fasting state

For someone trying intermittent fasting for the first time, it’s helpful to know what to expect. 

When a person eats their last meal before a fasting period, they may consume more than usual because they are anticipating not eating for a span of time. If they are going to be following a time-restricted pattern, they may be skipping their traditional bedtime snack and could even feel some hunger that night. This could negatively impact their sleep.

The next day, they may feel hungry when they wake up if they are used to eating breakfast right away. For those following a time-restricted eating pattern who plan to break their fast at lunch, staying hydrated and shifting focus can help. For those who are fasting for an entire day, this first 24-hour period can be more of a challenge.

Blood sugar levels drop after not eating for a few hours, which causes the brain to send hunger signals to the body. Low blood sugar can also cause headaches and irritability.

Intermittent fasting researchers say that it can take two to four weeks for these initial side effects to go away. Sticking to a shorter fasting period (like 12 hours) to start may help people ease into intermittent fasting.

Even though you’re not eating during a fasting period, it’s essential to still drink fluids to avoid dehydration and other complications.

Side effects in the fasting state

For those following a time-restricted fasting pattern, they are typically going to break a fast after 12-16 hours of not eating. But alternate-day fasting, or 5:2 fasting, includes fasting for 24 hours or more. This may cause fatigue or even dizziness due to low blood sugar and lack of calories. 

Fasting can cause the body to go into a metabolic process called ketosis. When the body is low on glucose, it starts making ketones by burning stored fat. 

The benefits of intermittent fasting may be, in part, due to the potential benefits of ketosis. However, ketosis also comes with its own potential side effects like electrolyte imbalances and constipation. It’s important to ensure you’re staying hydrated and getting enough electrolytes through your diet. Zero-calorie electrolyte drinks may also be helpful during the fasting period.

It’s essential to monitor yourself if you plan to include longer fasting periods into your routine. If you're concerned about side effects, consider starting with shorter fasting periods after discussing with your healthcare provider

Side effects in the long-term fasting state 

As discussed, most intermittent fasting patterns include a fasting state of as little as 12 hours to as long as 48 hours. But how about long-term fasting states?

Long-term fasting can cause serious side effects such as nutrient deficiencies and muscle loss. It can also negatively impact the immune system and the body’s metabolism. Another major concern is the potential to trigger eating disorders or disordered eating.

After a period of long-term fasting, there is a concern for refeeding syndrome once the person returns to a regular diet. Lack of nutrients leads to a change in metabolism; once nutrients are reintroduced, there can be severe shifts in electrolytes. Refeeding syndrome can have a negative effect on the brain, heart, lungs, and muscles. Food should be slowly added back into the diet to help prevent refeeding syndrome. 

While some people may not have significant issues from longer fasting periods, it could cause big problems for others. The longer the fasting period, the bigger the concern for negative outcomes. Prolonged fasting should not be taken lightly and should ideally be done under the approval and supervision of a healthcare provider.

Is experiencing diarrhea while fasting normal? 

While it’s not a common side effect, some people may experience diarrhea during fasting. This could be a result of drinking beverages containing sugar substitutes such as sugar alcohols or stevia. 

Diarrhea may also be caused by drinking caffeine. For example, someone might drink more coffee during their fasting window to fight off fatigue. Additionally, the act of drinking coffee on an empty stomach can be enough to trigger a gut response.

Fasting itself may lead to diarrhea due to electrolyte imbalances or changes in the gut microbiome, but more research is needed to show that there is a direct connection between fasting and diarrhea.

It’s important to figure out your potential triggers for diarrhea during fasting. If diarrhea persists or worsens, seek help from your healthcare provider.

Can intermittent fasting cause heart attacks? 

As of March 2024, there has been a concern circulating over intermittent fasting causing cardiovascular disease (which could include or lead to heart attacks). This is in response to preliminary research presented at an American Heart Association (AHA) conference.

The researchers found that a less than eight hour time-restricted eating pattern was linked to a higher risk of death from cardiovascular disease when compared with a regular eating pattern. According to the AHA, these findings “are considered preliminary until published in a peer-reviewed scientific journal.”

Given that other researchers have found potential heart health benefits from intermittent fasting, these results would point to the need for more research into this topic.

It’s important to be aware of the potential side effects of intermittent fasting before trying this popular eating pattern. Your healthcare provider can help you determine if this method is appropriate for you or if you should avoid it. If you need a more personalized approach to your eating plan, consider working with a Season Dietitian for expert guidance!