What do you think of intermittent fasting? Is it good or not?

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    This is a question that gets asked so often in group and one-to-one sessions. Rather than tell you what I think of intermittent fasting, let’s first go through what the science and the research say!

    Let’s understand what the research to-date outlines, what are the common types of intermittent fasting, the potential benefits and what the risks associated with it. Understanding those, you can then conclude for you and your lifestyle, if this approach to eating is for you or not!

    Fruit and vegetables

    What is intermittent fasting?

    Firstly, intermittent fasting (IF) involves alternating periods of fasting and eating. The focus here is the timing of when you eat, rather than, what you actually eat. 

    There are several common types of IF, a summary of these are in the table below (1).

    Common Types of IF (1).

    Several reasons why this way of eating is popular, is its simplicity and flexibility and also that you don’t necessarily have to restrict or monitor what you eat. Research is providing a greater understanding of the influence of meal timing on health outcomes, independent of the quality or quantity of food eaten, an area also known as chrononutrition (2). While we know that circadian rhythms affect many different functions in the body, we are only starting to understand the effect regarding meal timing.

    What are the potential benefits of eating this way?

    Research has shown in animal and human studies that fasting can improve metabolism, lower blood sugar levels, lessen inflammation and improve a range of health issues. Research relating to specific areas concludes that for:

    1. Weight loss – it is likely that IF can support weight loss with similar results to those pursuing a calorie restrictive diet (3). While one of the main benefits is not counting calories, the challenge is the sustainability of this approach over the longer-term to maintain a healthy weight loss. 
    2. Heart health –  there are mixed findings regarding heart health. Some studies show a reduction in cholesterol and blood pressure, whereas a 2021 review did not find sufficient evidence, to know whether intermittent fasting could prevent cardiovascular disease (4). A number of factors contribute to heart health and more research is needed to determine the direct impact of IF on cardiovascular health.
    3. Diabetes risk – a number of studies show positive improvements in fasting glucose concentrations. Results, which suggest that those at risk of type 2 diabetes may benefit from time restricted eating (5). 
    4. Gut health – most of the research in this area is on those fasting during Ramadan. Positive changes were seen in the diversity and composition of the gut microbiota. However, these reverted to baseline once the fasting stopped (6,7).

    In summary, the challenge with IF is that a lot of the research is animal based or on a specific group in the population. It needs to be determined if these benefits can be applied to the general population and if humans can maintain this approach for years to accrue the benefits (8).

    Are there risks associated with intermittent fasting?

    While the simplicity and flexibility of IF is appealing for many, it’s not without its risks.  Some of these to be mindful of are:

    • The challenge of maintaining this eating pattern over the longer-term. For many it does not fit with family life or their lifestyle and therefore the sustainability longer term is low.
    • IF is not suitable for everyone, and our individual response to it can vary. It is not advised for anyone with Type 1 diabetes, pregnant or breastfeeding, children or elderly or someone who has a history of disordered eating. 
    • Anyone on medication should check with their GP before commencing IF.
    • Side effects also observed include increased feelings of hunger, worse mood, heightened irritability, difficulty concentrating, increased fatigue, fear of loss of control and overeating during non-restricted days (9).
    • More research needs to be done to truly understand if the benefits are maintained over the longer term, including what the optimal fasting periods are for specific groups, length of fast and if what we eat also needs to change. 
    Fruit and vegetables

    Is it for me or not?

    IF certainly has its appeal for its ease to follow, not having to cut certain foods and studies showing its potential benefits. Nevertheless, it’s not without its challenges, lack of robust data for the longer-term benefits and the side effects of irritability, headaches, tiredness and potential to overeat. Like anything, you also need to consider is it something that fits with your lifestyle? Can you ensure that in the eating window you still consume nutrient dense meals to sustain your needs, and can you sustain it in the long-term?


    1. Patterson, R.E. et al. (2015) ‘Intermittent fasting and human metabolic health’, Journal of the Academy of Nutrition and Dietetics, 115(8), pp. 1203–1212. doi:10.1016/j.jand.2015.02.018.
    2. Panda, S. (2019) ‘The arrival of Circadian Medicine’, Nature Reviews Endocrinology, 15(2), pp. 67–69. doi:10.1038/s41574-018-0142-x.
    3. Seimon, R.V. et al. (2015) ‘Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials’, Molecular and Cellular Endocrinology, 418, pp. 153–172. doi:10.1016/j.mce.2015.09.014.
    4. Allaf, M. et al. (2021) ‘Intermittent fasting for the prevention of cardiovascular disease’, Cochrane Database of Systematic Reviews, 2021(3). doi:10.1002/14651858.cd013496.pub2.
    5. Sutton, E.F. et al. (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). doi:10.1016/j.cmet.2018.04.010.
    6. Zeb, F. et al. (2020) ‘Effect of time-restricted feeding on metabolic risk and circadian rhythm associated with gut microbiome in healthy males’, British Journal of Nutrition, 123(11), pp. 1216–1226. doi:10.1017/s0007114519003428.
    7. Kerndt PR, Naughton JL, Driscoll CE, Loxterkamp DA. Fasting: the history, pathophysiology and complications. West J Med. 1982 Nov;137(5):379-99. PMID: 6758355; PMCID: PMC1274154.
    8. de Cabo, R. and Mattson, M.P. (2019) ‘Effects of intermittent fasting on health, aging, and disease’, New England Journal of Medicine, 381(26), pp. 2541–2551. doi:10.1056/nejmra1905136.
    9. Harvie, M. and Howell, A. (2017) ‘Potential benefits and harms of intermittent energy restriction and intermittent fasting amongst obese, overweight and normal weight subjects—a narrative review of human and animal evidence’, Behavioral Sciences, 7(4), p. 4. doi:10.3390/bs7010004.