Intermittent fasting has been one of the most popular (and most googled) diets for the past few years.
Although there is some interesting evidence emerging related to this, there are also risks related to this and it isn’t appropriate for everyone.
Let’s take a look at the latest evidence related to this diet, as well evidence and risks related to intermittent fasting and disordered eating specifically.
What is Intermittent Fasting?
Intermittent fasting means fasting for shorter periods of time, as compared with longer fasts.
There are lots of different types of intermittent fasting, such as:
- The 5:2 diet: This involves limiting inatke to 500-600 kcals per day on two days each week, and eating freeling on the remaingin 5 days.
- 6:1 diet (A.K.A. Eat Stop Eat): This means a 24 hour fasts on one day each week.
- Alternate day fasting (ADF): 24 hour fasts (or limiting intake to 500 kcals max) every other day.
- Fasting mimicking diet (FMD): This is a low carb, low protein, high unsaturated fat diet that limits intake to 34–54% of normal calorie intake.
- Spontaneous meal skipping: Randomly skipping meals.
- Time-restricted feeding (TRF): Where the window of time where eating occurs can vary, like the 16:8 diet (A.K.A. Leangains protocol) that includes a 16 hour fast each day. This feeding window can also occur early in the day (i.e. early TRF) or later in the day, from around noon and onwards (late TRF).
How Could Intermittent Fasting Impact Health?
There are different mechanisms by which intermittent fasting could impact health. These vary in terms of how much evidence is behind them, and some of them are more theoretical.
These possible mechanisms include (1):
- Autophagy i.e. removing and recycling damaged parts of cells to boost cell repair
- Reduced nutrient signalling pathways may impact stress response and longevity genes
- Reduced inflammation and oxidative stress
- Syncronizing with our body clock
- Ketosis i.e. a starvation mechanism where the body starts to rely on fat for fuel which increases levels of ‘ketone bodies’
- Release of brain-derived neurotrophic factor (BDNF) which is a protein involved in brain repair and developing new connections in the brain
- Impacting immune system function
- Impact on gut bacteria and metabolic markers like blood glucose and cholesterol
Risks vs. Benefits of Intermittent Fasting
There have been some positive findings related to intermittent fasting and certain health outcomes. But there are varying levels of evidence as well as pros and cons to consider.
There’s some evidence that intermittent fasting may reduce inflammation, boost cell and brain cell repair (1-3). However, most of the research related to this has been carried out in petri dishes or animals, so ongoing human research is needed.
On the other hand, there’s a link between missing breakfast and an increased risk of early death as well as a higher risk of type 2 diabetes (4-5).
Intermittent fasting can also lead to muscle loss, although there have been some mixed findings about this. Low muscle mass in older age is associated with a higher risk of falls, fractures and mortality as well as lower quality of life (6-7).
There’s some interesting research into the impact of intermittent fasting on diabetes. There have been some positives and negatives found related to this, which as a whole may point to the importance of the timing of eating and fasting (8-9).
For example, a study from 2018 involving men on men at risk of diabetes found that eating between 9am to 3pm reduced diabetes risk markers as compared to a 12-hour eating window; importantly this was unrelated to weight loss and may have had more to do with the impact of the body clock (10).
But fasting can cause issues with blood glucose levels for those with diabetes. Those taking insulin or certain diabetes medication can be at higher risk of low blood glucose levels. Long periods without food can also lead to consuming larger amounts in one go, hence a higher risk of high blood glucose levels.
There have been mixed findings when it comes to intermittent fasting and heart disease risk (11-12).
A Cochrane review from 2021 that examined concluded that ongoing research is needed in order to understand the safety and risks vs. benefits analysis of intermittent fasting in different groups groups (they specifically mentioned those with eating disorders) and the long-term heart health risks (12).
Intermittent fasting may be a bad idea for those at risk of gallstones. For example, in a study of women who fasted for 14 hours overnight, as compared with 8 hours, the risk of gallstones almost doubled (13).
Intermittent fasting is usually not a good idea for those who are more nutritionally vulnerable or those who particularly need a regular supply of energy and nutrients.
This is why it isn’t recommended for children, pregnant women, those who are malnourished or unwell and of course those with an eating disorder or a difficult relationship with food. Most athletes benefit from a regular supply of nutrition to maximise their performance as well.
Fasting may also be worse for women and people with ovaries, as female hormones are sensitive to the levels of energy available in the body (14-15). The stress hormone cortisol can also increase in a fasted state (16), this can negatively impact female reproductive hormones and therefore reproductive health, fertility and period regularity.
Those with certain medical conditions or vulnerabilities may need to be careful as intermittent fasting has been linked with a reduction in white blood cells which are need for a strong immune defence (17). Similarly, fasting for 24 hours or longer has been linked with kidney and heart issues (18).
On an individual level, some people don’t feel good when they fast and may experience fatigue, headaches, dizziness, mood and sleep issues.
There are still a to of unknowns when it comes to the effect of intermittent fasting on different groups, as ongoing human research is needed.
For more information about the research related to intermittent fasting, check out this article I wrote for The Food Medic Educational Hub.
What About Intermittent Fasting and Disordered Eating?
Dietary restriction is a well-established risk factor and symptom of eating disorders and disordered eating.
This often includes fasting, rigid rules about meal timing and ‘pushing meals out’ so there’s less time to consume enough food overall.
In fact, fasting is listed as an example of an “inappropriate compensatory behaviour in order to prevent weight gain” in the DSM-5 eating disorder diagnostic criteria (19).
Intermittent fasting poses the risk of contributing to or masking disordered eating, especially as it’s quite a socially acceptable form of restriction.
Over the past few years intermittent fasting has consistently been one of the top most googled diets, which shows just how popular it is.
Some of the physical risks of intermittent fasting outlined above, can be even riskier for those struggling with disordered eating.
For example, this could compound issues that are already common in disordered eating such as:
- Muscle wasting
- Weakened immune system
- Hormonal issues
- Low energy
- Sleep difficulty
- Mood issues
Therefore, intermittent fasting is usually not recommended for those who have a disordered relationship with food.
Those who may be at risk of developing disordered eating, and those with a history of this should also be cautious about this, and individually weigh up the pros and cons.
What Does the Research Say About Disordered Eating and Intermittent Fasting?
There’s a lack of research overall when it comes to intermittent fasting and disordered eating. But a few interesting studies have looked into this – some of these are related to fasting rather than intermittent fasting specifically.
A study from 2008 that included 496 adolescent girls over 5 years found that fasting was a very strong and consistent predictor of recurrent binge eating disorder and subclinical bulimia nervosa (20).
Fasting was an even stronger predictor for these conditions than dietary restraint (actively restricting calories or intake).
A study 8 years later that included 1,272 young women (with an average age of 18.5) again found fasting to be a significant predictor of a number of eating disorders including bulimia nervosa, subclinical bulimia and subclinical purging disorder (21).
In 2021 Cuccolo et al. carried out a study including 44 women and 20 men who followed the 16:8 intermittent fasting diet i.e. fasted for 16 hours each day (22).
They found intermittent fasting to be linked with higher eating disorder symptom scores, and just under a third of participants scored at or above the cut-off level that indicated a clinical eating disorder. This is a very worrying result.
As with all areas of nutrition, there are pros and cons when it comes to intermittent fasting which varies between different people and situations.
Although there’s some promising research related to diabetes in particular, there are a number of risks related to fasting and a lot of unknowns due to the limited long-term human research available.
Intermittent fasting is a form of restriction, so it can be both a trigger and symptom of disordered eating. This makes it very risky for those with a disordered relationship with food, or those at risk of developing disordered eating.
If you’re in doubt as to whether it’s a good idea for you, please seek individual advice from a health professional who has experience or awareness of disordered eating. You can find information about the Dietetically Speaking Clinic and our dietitians here.
- Longo et al. (2021) “Intermittent and periodic fasting, longevity and disease” [accessed May 2022 via: https://www.nature.com/articles/s43587-020-00013-3]
- Wang, X., Yang, Q., Liao, Q., Li, M., Zhang, P., Santos, H. O., … & Abshirini, M. (2020). Effects of intermittent fasting diets on plasma concentrations of inflammatory biomarkers: a systematic review and meta-analysis of randomized controlled trials. Nutrition, 79, 110974. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/32947129/]
- Jamshed, H., Beyl, R. A., Della Manna, D. L., Yang, E. S., Ravussin, E., & Peterson, C. M. (2019). Early time-restricted feeding improves 24-hour glucose levels and affects markers of the circadian clock, aging, and autophagy in humans. Nutrients, 11(6), 1234. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/31151228/]
- Ofori-Asenso, R., Owen, A. J., & Liew, D. (2019). Skipping breakfast and the risk of cardiovascular disease and death: A systematic review of prospective cohort studies in primary prevention settings. Journal of cardiovascular development and disease, 6(3), 30. [accessed May 2022 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6787634/]
- Ballon, A., Neuenschwander, M., & Schlesinger, S. (2019). Breakfast skipping is associated with increased risk of type 2 diabetes among adults: a systematic review and meta-analysis of prospective cohort studies. The Journal of nutrition, 149(1), 106-113. [accessed May 2022 via: https://academic.oup.com/jn/article/149/1/106/5167902]
- Tinsley, G. M., & La Bounty, P. M. (2015). Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition reviews, 73(10), 661-674. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/26374764/]
- Larsson, L., Degens, H., Li, M., Salviati, L., Lee, Y. I., Thompson, W., … & Sandri, M. (2019). Sarcopenia: aging-related loss of muscle mass and function. Physiological reviews, 99(1), 427-511. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/30427277/]
- Crupi, A. N., Haase, J., Brandhorst, S., & Longo, V. D. (2020). Periodic and intermittent fasting in diabetes and cardiovascular disease. Current diabetes reports, 20(12), 1-14. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/33301104/]
- Munhoz, A. C., Vilas-Boas, E. A., Panveloski-Costa, A. C., Leite, J. S. M., Lucena, C. F., Riva, P., … & Carpinelli, A. R. (2020). Intermittent fasting for twelve weeks leads to increases in fat mass and hyperinsulinemia in young female Wistar rats. Nutrients, 12(4), 1029. [accessed May 2022 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7230500/]
- Sutton, E. F., Beyl, R., Early, K. S., Cefalu, W. T., Ravussin, E., & Peterson, C. M. (2018). Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell metabolism, 27(6), 1212-1221. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/29754952/]
- Dong, T. A., Sandesara, P. B., Dhindsa, D. S., Mehta, A., Arneson, L. C., Dollar, A. L., … & Sperling, L. S. (2020). Intermittent fasting: a heart healthy dietary pattern?. The American journal of medicine, 133(8), 901-907. [accessed May 2022 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7415631/]
- Allaf, M., Elghazaly, H., Mohamed, O. G., Fareen, M. F. K., Zaman, S., Salmasi, A. M., … & Dehghan, A. (2021). Intermittent fasting for the prevention of cardiovascular disease. Cochrane Database of Systematic Reviews, (1). [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/33512717]
- Sichieri, R., Everhart, J. E., & Roth, H. (1991). A prospective study of hospitalization with gallstone disease among women: role of dietary factors, fasting period, and dieting. American journal of public health, 81(7), 880-884. [accessed May 2022 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1405175/]
- Loucks, A. B., Verdun, M., & Heath, E. M. (1998). Low energy availability, not stress of exercise, alters LH pulsatility in exercising women. Journal of applied physiology. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/9451615/]
- Kumar, S., & Kaur, G. (2013). Intermittent fasting dietary restriction regimen negatively influences reproduction in young rats: a study of hypothalamo-hypophysial-gonadal axis. PloS one, 8(1), e52416. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/23382817/]
- Nakamura, Y., Walker, B. R., & Ikuta, T. (2016). Systematic review and meta-analysis reveals acutely elevated plasma cortisol following fasting but not less severe calorie restriction. Stress, 19(2), 151-157. [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/26586092/]
- Gasmi et al. (2018) “Time-restricted feeding influences immune responses without compromising muscle performance in older men” [accessed May 2022 via: https://www.sciencedirect.com/science/article/abs/pii/S0899900718300364]
- Kerndt, P. R., Naughton, J. L., Driscoll, C. E., & Loxterkamp, D. A. (1982). Fasting: the history, pathophysiology and complications. Western Journal of Medicine, 137(5), 379. [accessed April 2022 via: https://pubmed.ncbi.nlm.nih.gov/6758355/]
- Berkman, N. D., Brownley, K. A., Peat, C. M., Lohr, K. N., Cullen, K. E., Morgan, L. C., … & Bulik, C. M. (2015). Management and Outcomes of Binge-Eating Disorder [Internet]. [accessed May 2022 via: https://www.ncbi.nlm.nih.gov/books/NBK338301/table/introduction.t1/]
- Stice et al. (2008) “Fasting Increases Risk for Onset of Binge Eating and Bulimic Pathology: A 5-Year Prospective Study” [accessed May 2022 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2850570/]
- Stice et al. (2016) “Risk factors that predict future onset of each DSM-5 eating disorder: Predictive specificity in high-risk adolescent females” [accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/27709979/]
- Cuccolo et al. (2021) “Intermittent fasting implementation and association with eating disorder symptomatology” accessed May 2022 via: https://pubmed.ncbi.nlm.nih.gov/34191688/]