Young Ju, Ph.D.

Intermittent fasting (IF) is a diet method of energy deprivation that alternates periods of eating and fasting. IF has been practiced since ancient times in many cultures, especially in many religions. Research has shown that IF can be a way to manage weight and improve some metabolic parameters, e.g., blood sugar, blood cholesterol, waist circumference, and blood pressure. The purpose of this article is to help you better understand IF and how to practice it safely.

Types of IF (1, 2)

There are many variations of IF. The following methods are the most common:

• Time-restricted eating: This method sets fasting and eating windows for a 24-hour day. The 16/8 (16-hour fasting/8-hour eating), 14/10, and 20/4 methods are widely used.  Since most people already fast while they sleep, this type is convenient, safer to try, and can be repeated as you prefer.  It may take for a while to figure out the right fasting window for you. You can move your fasting window to fit into your work schedule or social events.

• Whole-day fasting: This means 1-2 days per week of complete fasting or reduced calorie restriction of about 75% on a  5:2 method.  Calorie intake is limited to 500 calories or less  for 2 days a week and a healthy and normal diet is followed for 5 days a week.  You can choose which 2 fasting days but with a non-fasting day between them.  It’s important to eat high-fiber and high-protein foods on non-fasting days.  You should eat the same amount of food you normally eat on non-fasting days.

• Alternate-day fasting: By fasting every other day, e.g., fasting on Mon-Wed-Fri and non-fasting on Tues-Thurs-Sat, you take only one meal (or 75% of calorie restriction) on fasting days and maintain a healthy and normal diet on non-fasting days.

How your body reacts to fasting (3, 4)

When we eat carbohydrates, carbohydrates are converted to glucose and used as energy. Excess glucose is stored as glycogen in the liver and muscles to use later. In the first 1-3 days of fasting, energy may come from stored glycogen and fat. During this period, the body tries to maintain its blood glucose level by converting stored glycogen to glucose. The body also tries to maintain its metabolic rate by burning fat as a source of energy. It appears that protein stores are not significantly (or less) affected during this period (Figure 1). It’s important to note that longer periods of fasting, such as 36-, 48- and 72-hour intervals, are not necessarily efficient in obtaining the health benefits of fasting. Actually, longer fasting may alter energy expenditure and may cause severe side effects including fatigue, low energy, headaches, irritability, and hunger.  

Reported health benefits of IF (5, 6)

Research evidence suggests consistently that IF could provide some health benefits including;

• improved lipid profile.

• lowered blood pressure.

• improved weight control and glycemic control in people with obesity and type 2 diabetes.

• favorable changes in the diversity of the gut bacteria.

IF and exercise performance (3, 7, 8, 9)

Available studies suggest that exercising while fasting makes the body break down fat or protein for fuel and may cause loss of muscle mass and fat mass. Exercising for longer hours while doing IF may also slow down your body’s metabolism. Therefore, it’s possible that the body may not perform at its best. Studies have examined the effects of long-time fasting (3-10 days) in athletes doing high-intensity exercise, endurance exercise, and resistance training. These studies have reported minor decreases in sprint performance, decreases in speed and power, and a shorter time to exhaustion. 

Studies that examined the effects of time-restricted fasting (20/4 or 16/8 method) reported decreased fat mass without changing muscle strength. When IF is combined with resistance training (also called strength training or weight training), it is more efficient in reducing body mass and body fat compared to regular diet intake.

Available studies also suggest that IF in combination with physical exercise has synergistic effects on improving metabolic parameters, preventing weight gain, improving insulin function, alleviating fatty liver, regulating lipid metabolism, and restoring healthy bacterial population in the intestine. Currently, no study has examined the effects of prolonged IF alone or combined with exercise.

What to do for safe IF and exercise

IF can be a safe method of obtaining health benefits while also being active if properly used.

• Decide your goal for IF–to improve metabolic parameters or to maintain fitness and activity levels.

• Choose the type of IF to suit your schedule and goals–you can modify the fasting/eating window depending on your schedule.

• Decide when to exercise; your exercise timing can be before, during, or after your eating window based on what seems efficient and safe for you. Some people prefer eating before moderate- to high-intensity exercise in order to have enough energy. Others prefer eating after exercise to maintain muscle mass.

• Stay hydrated–it is recommended that you  drink more water during fasting. Black coffee and tea are also allowed during fasting.

• Maintain your electrolytes.

• Eat a normal and healthy diet on non-fasting days

• Try diverse and nutritious food that supports good health; for example, follow a Mediterranean diet that includes leafy greens, healthy fats, lean protein, and complex/unrefined carbohydrates.

Potential risks of IF

IF may not be safe for certain populations including;

• women who are pregnant or breast-feeding

• children and teens: IF is not recommended for those in a rapid-growth stage.

• people with type 1 diabetes: type 1 diabetic patients take insulin. IF combined with insulin may additively lower blood sugar to unsafe levels (hypoglycemia).

• people with eating disorders: IF may increase binge eating.

• people with gastroesophageal reflux

 IF can cause side effects such as irritability, low energy, persistent hunger, unusual anxiety, headaches, nausea, temperature sensitivity, and poor work and activity levels, etc.

Overall

For most people, IF can be a safe life-style method for improving metabolic parameters while being physically active. Please keep in mind that different types of IF may have different effects on different people. Before you start IF, discuss your options and expected outcomes with your health care professional.  To borrow a quote from a famous movie, “Choose wisely.” Once you start, pay attention to your body’s responses. It can take 2-4 weeks for the body to adjust to IF. If you experience unusual anxiety, headaches, nausea or other symptoms, talk to your doctor.

References

1. Cleveland Clinic. Intermittent Fasting: How it works and 4 types explained. https://health.clevelandclinic.org/intermittent-fasting-4-different-types-explained/

2. Harvard T.H. Chan School of Public Health. The Nutrition Source. Diet Review: Intermittent fasting for weight loss. https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/intermittent-fasting/#:~:text=It%20may%20require%20complete%20abstinence,therefore%20prevent%20further%20weight%20loss.

3. Levy, E., Chu, T. Intermittent Fasting and Its Effects on Athletic Performance: A Review. Nutrition and Ergogenic Aids. 2019, 18(7):266.

4. Dote-Montero, M., Sanchez-Delgado, G., Ravussin, E. Effects of intermittent fasting on cardiometabolic health: An energy metabolism perspective. Nutrients 2022, 14:489.

5. Morales-Suarez-Varela, M., Sánchez, E.C., Peraita-Costa, I., Llopis-Morales, A., Soriano, J.M. Intermittent Fasting and the Possible Benefits in Obesity, Diabetes, and Multiple Sclerosis: A Systematic Review of Randomized Clinical Trials. Nutrients 2021, 13(9):3179.

6. Varady, K.A., Cienfuegos, S., Ezpeleta, M., Gabel, K. Cardiometabolic Benefits of Intermittent Fasting. Annu. Rev. Nutr. 2021, 41:333.

7. Correia, J.M., Santos, I., Pezarat-Correia, P., Minderico, C., Mendonca, G.V. Effects of Intermittent Fasting on Specific Exercise Performance Outcomes: A Systematic Review Including Meta-Analysis. Nutrients 2020, 12(5):1390.

8. Ashtary-Larky, D., Bagheri, R., Tinsley, G.M., Asbaghi, O., Paoli, A., Moro, T. Effects of intermittent fasting combined with resistance training on body composition: a systematic review and meta-analysis. Physiol. Behav. 2021, 237:113453.

9. Zhang, L., Wang, Y., Sun, Y., Zhang, X. Intermittent fasting and physical exercise for preventing metabolic disorders through interaction with gut microbiota: A Review. Nutrients. 2023, 15(10):2277.

This information is provided for your reference and you use at your own risk; you should rely on your medical professional for medical advice.