Intermittent fasting refers to a diet schedule that cycles between not eating and eating. Cycles of intermittent fasting can be hourly or daily.
People may choose to fast for reasons such as religion, diet, or potential health benefits.
During intermittent fasting (IF), people may wish to continue or begin exercising. It is possible to exercise while fasting, but precautions are necessary.
In this article, we explore exercising while fasting, different types of IF, how to plan workouts during fasting, and safety tips to consider.
People who are fasting can partake in exercise. Some people exercise while IF as they believe it has potential health benefits. These include:
When people eat carbohydrates, the body converts this into a type of sugar known as glucose. The body stores glucose as glycogen.
Research suggests that during periods of fasting, glycogen stores are empty. This means the body starts to burn fat for energy during exercise, which may help weight loss.
One study found that exercising in a fasted state also led to a higher fat loss than in people exercising after a meal.
However, other studies have produced different results.
A 2014 study found that people who fasted overnight and exercised did not lose more weight than people who ate before exercise. Another study in mice concluded that IF with or without exercising led to effective weight loss in mice.
To lose weight, the calories someone consumes must be less than those they burn each day. IF may help a person lose weight as it controls the number of calories they eat.
A study that compared IF with a calorie restricted diet that does not include periods of fasting found no major differences in the amount of weight each group lost.
Findings from a review of research looking at exercise and fasting highlight that it may increase autophagy.
Autophagy is a process that helps destroy unwanted or damaged cells to regenerate newer and healthier ones.
A 2018 study found that IF and exercise may slow down aging and disease processes. This is because IF and exercise may cause changes in metabolism.
Research has also identified some potentially negative side effects to exercise while fasting. These can include
- Poorer performance: Research suggests that IF may impair exercise performance, particularly in athletes that are highly trained.
- Struggle to build muscle: A 2018 randomized controlled trial found that males who were IF put on less muscle in comparison to those who ate meals as usual. However, IF did not negatively affect their muscle retention. Another study supports this, highlighting IF may be effective at maintaining muscle mass
- Lightheadedness: Both IF and exercising can lower blood pressure. By combining the two, a person may experience lightheadedness because of the drop in blood pressure.
- Blood Sugar Levels: IF and exercising may cause blood sugar levels to drop. If sugar levels drop too low, this can cause a person to faint.
The research examining fasting and exercise seems to have conflicting information. Depending on a person’s exercise goals, they may want to try IF for fat loss. However, if someone wants to build muscle, they may wish to use alternative diets.
There are a number of different types of intermittent fasting. These include:
- 16:8: During 16:8 fasting, a person will not eat anything for 16 hours and then have an 8-hour window in which they consume food. During the 16 hours of fasting, people can consume noncalorie beverages, such as black coffee, black tea, and water.
- 5:2: The 5:2 diet is a type of fast where a person eats normally for 5 days and allocate 2 days to fasting.
- Nightly: A nightly fast involves having a prolonged period, lasting from the evening to the following morning, without food. A 2016 study found that a prolonged fast of 13 hours overnight can potentially reduce the risk of breast cancer in females.
- Up-to-the-ninth-hour: During this fast, a person will not consume food for the first 8 hours of their waking day. From the 9th hour, the person can then eat.
- One meal a day (OMAD): OMAD fasting involves picking one meal a day to eat and spending the rest of the day fasting.
- Alternate Day: As the name suggests, alternate day fasting is when a person eats food as normal one day and fasts on the following day.
If people want to change their usual eating habits, it is worth discussing this with a doctor or nutritionist, especially for a change such as fasting.
It is important to plan workouts during IF to stay safe. Some considerations are:
- Type of exercise: There are two types of exercise, aerobic and anaerobic. Aerobic exercise, or ‘cardio,’ is exercise over a sustained period, such as running, walking, and cycling. Anaerobic is exercise that requires maximum effort over a short period, such as weight lifting or sprinting.
Which type of exercise a person does will likely depend on the type of fast they do. For example, a person doing 16:8 or nightly fasts can do either aerobic or anaerobic exercise during their periods of eating.
However, if someone is doing alternate days and wants to exercise during their day of not eating, they should probably stick to less intense aerobic exercise.
- Timing of the exercise: Although a person can exercise in a fasted state, it may be better to time exercise for after meals.
- Type of food: If exercising during periods of eating, it is important to consider what to eat.
Pre-workout nutrition should consist of a meal 2–3 hours before exercise rather than just before. It can be rich in complex carbohydrates, such as whole grain cereal and protein.
A post-workout meal should consist of carbohydrates, high quality proteins, and fats to help recovery.
After planning the workout, it is also worth considering the following tips to stay safe.
- Exercising after periods of eating: This will provide a person with the energy they need to complete a workout
- Sticking to low-intensity exercises: If in a fasted state, a person may wish to try and do low intensity aerobic exercise. However, if exercising after eating, it is usually safe to do any type of exercise.
- Listening to what the body is saying: If someone is starting to feel unwell during exercise while on IF, they should stop.
- Staying hydrated: Even when not IF, it is essential to keep hydrated during exercise. As most of the human body is water, it is vital to replace fluids lost during exercise.
For some people fasting and exercising may be more dangerous, including:
- people with diabetes
- people with low blood pressure
- people who have previously had disordered eating
- pregnant women
- women who are breastfeeding
If a person has underlying health conditions but wishes to try IF and exercise, it is best they discuss this with their doctor.
People may fast for a variety of reasons, such a religion, diet, or other potential health benefits. It is possible to exercise during IF safely.
Although IF and exercise may help weight loss, there is conflicting evidence saying whether it is more effective than other means, such as calorie restricted eating.
Possible benefits to fasting and exercise are an improvement in the autophagy process and possible anti-aging effects.
However, people may not be able to build muscle during IF and may find they perform poorer in their workouts. A person may want to consider another diet if they wish to build muscle
There are many types of IF. Whichever a person chooses to do, they should plan the workout accordingly and consider potential safety measures.