Intermittent fasting (IF) is an eating plan that switches between fasting and eating on a regular schedule. With intermittent fasting, you only eat during a specific time of the day (1). Over the last few decades, this diet style has been repackaged into different names, such as the 5:2 or the 16:8 which you may have already heard of.
Why is intermittent fasting popular?
The trend of intermittent fasting is unique in that it’s not strictly a diet per se, but more of an eating style. It doesn't stipulate what to eat, only the timing of your meals. This greatly benefits most people as no foods are necessarily ‘restricted’, hopefully meaning that it’s less likely that disordered eating habits arise during or after the ‘diet’ has ended. No foods are off limits, meaning your plate's composition is still entirely up to you. Even though there are well-known versions of intermittent fasting such as the 5:2, the ‘rules’ surrounding fasting time are flexible and can be determined by your personal preferences and needs.
What are the risks?
Intermittent fasting, in principle, makes logical sense. However, one of the key issues is the difficulty of maintaining this pattern on a long-term basis. A review of intermittent fasting studies noted that in many trials, there was a high drop-out rate (around 38%) of those in the fasting groups (2). The fact is that it's normal for us to want to eat when we're hungry! So we’re more likely to reach for ‘indulgent’ or feel-good foods when we're in this state in a bid to feel full again. This will have a knock-on effect on our energy supplies and could lead to us feeling sluggish and fatigued on fasting days.
Much of the interest in IF comes from animal studies currently, and there's no way of confirming that the results would be replicated in humans. Additionally, most of the human studies we do have aren't in the ‘general’ population. They tend to focus on a particular sector such as those with diabetes or obesity (3).
There are many groups within the population who should be discouraged from trying IF, such as those with a history of eating disorders, those under age 18, and women who are pregnant or breastfeeding (4).
Symptoms such as headaches and irritability have been reported when following strict fasting patterns. Your body has less overall ‘fuel’ so you may feel more tired than usual or have some sleep disturbances (5).
Intermittent fasting for weight loss?
Many people are led to believe that IF is superior for achieving weight loss. This idea seems to come from the belief that when we eat after an extended fast, we don’t fully compensate for the food/calories that we have gone without, subsequently putting us into an energy/calorie deficit and resulting in weight loss.
However, although there is evidence to suggest that IF may be an effective tool for facilitating weight loss (6), it has not been shown to provide any better results than any other approach to eating less. With IF, the reason for weight loss is simply due to consuming fewer calories overall as a result of reducing the ‘eating window’ during the day (7).
Unfortunately, with restrictive dietary approaches, many individuals tend to see short-lived weight loss results and often regain much of the weight that is initially lost (8). This is often linked to the diet being too restrictive to adhere to long-term.
If you are someone who leads an active lifestyle and are keen to maximise your exercise performance, IF also has the potential to negatively impact your ability to train. This could be the result of not consuming enough energy in order to optimise your performance during your training sessions, as well as not eating enough to support your overall exercise recovery. This can therefore potentially limit the effort you can input into your exercise session, as well as limit any potential training adaptation (the body’s physiological response to exercise). IF can also lead to muscle loss as the weight loss seen is usually a combination of both lean (muscle) mass and fat mass (9). Not only that but evidence suggests that regular ingestion of protein throughout the day can help support muscle mass maintenance/growth (10).
Optimising energy intake in order to support an active lifestyle is important for both males and females, however, fasting may have a greater negative impact on women and people with ovaries, as female hormones are more sensitive to the levels of energy available in the body (11). The stress hormone cortisol can increase in a fasted state (12) which can also negatively impact female reproductive hormones and, therefore reproductive health & fertility.
The evidence for the benefits of IF remains small and generally inconclusive. Some potential benefits in health markers are being highlighted in small research studies, however, it remains unclear as to whether these benefits are a result of the IF approach itself, or as a result of being in an energy deficit (13).
Overall, the findings are varied and inconclusive and further, longitudinal studies are required. If you are uncertain whether IF is the best approach for you, please seek individualised advice from a registered healthcare professional.
References
Barnosky et al (2014). Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. The Journal of Laboratory and Clinical Medicine. June, vol. 164, no. 4, pp. 302-311.
Rynders et al (2019). Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients. October, vol. 11, no. 10. [Accessed March 2023].
Anderson et al (2001). Long-term weight-loss maintenance: a meta-analysis of US studies. American Journal of Clinical Nutrition.January, vol. 74, pp. 579-584. [Accessed March 2023].
Escalante et al (2020). Fasted Versus Non-fasted Aerobic Exercise on Body Composition: Considerations for Physique Athletes. Strength and Conditioning Journal. May, vol. 00. [Accessed March 2023].
Schoenfeld et al (2018). How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. Journal of the International Society of Sports Nutrition. February, vol. 15, no. 10. [Accessed March 2023].
Loucks et al (1998). Low energy availability, not stress of exercise, alters LH pulsatility in exercising women. Journal of Applied Physiology. [Accessed March 2023]
Nakamura et al (2016). Systematic review and meta-analysis reveals acutely elevated plasma cortisol following fasting but not less severe calorie restriction. Stress. Vol. 19, no. 2, pp. 151-157. [Accessed March 2023]
Hody et al (2020). Intermittent Fasting and Metabolic Health: From Religious Fast to Time-Restricted Feeding. Obesity. July, vol. 28, pp.S29-37. [Accessed March 2023].