Thinking About: Intermittent Fasting

Sep 2021

Thinking about: Intermittent Fasting article

I receive many questions about intermittent fasting (IF) and so I decided to dive in and take a look at the research and hopefully share some of my thoughts and sensible take-home points with you all.

Before I start though, it is worth mentioning that this is a big topic and the evidence is very mixed so it’s not possible to cover the entire body of research on this topic in one article. But I hope that it is at least a helpful start for those of you wishing to understand a little more about it, and to act as a springboard for your own research (particularly important if you are a fellow health or nutrition professional).

The bottom line, to my mind at least, is that there are no clear-cut answers around intermittent fasting – the potential benefits and risks likely vary hugely from person to person. Therefore, as always, please discuss any changes you are considering making to your diet and lifestyle with a qualified nutrition or healthcare professional beforehand to make sure that you are doing what is best for you and you have all the appropriate support in place. Intermittent fasting is not necessarily safe or appropriate for everyone.

What is Intermittent Fasting (IF)?

Intermittent fasting is a structured way of eating that limits your intake of food and drink during certain days of the week, or specified hours of the day. It is focused more on when you do or don’t eat, rather than what you do or don’t eat. Examples could include:

The 5:2 diet – where you eat normally for 5 days of the week, but significantly restrict your energy intake (usually to around 500-600 calories) for 2 days of the week.

The 6:1 diet – where you eat normally for 6 days of the week, but eat nothing for 24 hours on the final day of the week.

Alternate day fasting – where you eat normally one day, and then fast the next.

Time-restricted feeding – where you eat all of your meals over a 6-10 hour window, say 10am to 6pm, but fast overnight most evenings.

As you can see, there are lots of different ways to do intermittent fasting. And that in itself can be problematic, as all these different patterns are often grouped together to be analysed in the scientific literature, rather than looking at each one individually. It’s hard, therefore, to say whether one type of fasting programme might be better than another for various outcomes. More specific research is required.

What are the potential benefits?

Unfortunately, there is not (yet) a whole lot of good quality research examining the effects of intermittent fasting in humans. There are more studies in animals. Those studies that do exist tend to be quite small, and quite short. It’s therefore really important that we don’t try to draw conclusions from such studies and apply them to everyone, for life.

There are also not a lot of studies comparing intermittent fasting with other, perhaps more sustainable, approaches to healthy eating. So it’s hard to say it is better than another approach, such as following a generally healthy diet. As always, it is probably a case of finding the right approach for each individual, and so intermittent fasting is unlikely to be any sort of one-size-fits-all solution.

However, there are some possible benefits to IF, that warrant ongoing research:Intermittent fasting may help overweight adults to lose weight (compared to no diet), but according to a 2018 review on the topic, it was no more effective for weight loss than standard dieting approaches (Harris et al., 2018). Some people, however, report that intermittent fasting is easier to stick to than a more general, everyday shift in eating habits – at least in the short term (Varady et al., 2009). This is not the case with everyone, though, and it might in fact be harder to stick to in the longer term for some people.There is some evidence that intermittent fasting may help with blood sugar management and improve insulin resistance, but this outcome is not supported by all studies (Cho et al., 2019). There have been other benefits reported, such as effects on blood lipids, aging and brain health, but more high-quality research in humans is needed before using IF for specific health outcomes can be recommended(Mattson, Longo and Harvie, 2017).

What are some of the potential risks?

Potentially negative effects of intermittent fasting in the longer term are still unclear. In the short term, potential ‘side effects’ of fasting might include the following (Harvie and Howell, 2017);

  • Headaches
  • Fainting
  • Weakness
  • Constipation
  • Dehydration (although water is allowed during fasts – so if you are fasting, do make sure you drink plenty)
  • Hunger pangs
  • Irritability or worsening of mood
  • Difficulty concentrating
  • Fatigue
  • Eating-related thoughts
  • Fear of loss of control
  • Over-eating on non-restricted days

I would also put a possible risk of nutritional deficiencies into this list, particularly if intermittent fasting is followed long term without the support of a nutrition professional. If this is something you choose to do, it is therefore really important to ensure that on the times or days where you are not fasting, you get a very highly nutritious and balanced diet. Your body still needs all the essential micronutrients, high-quality proteins, fruits, vegetables, healthy fats and water you usually would, but consumed in fewer days or hours.

Who, generally, should not practice intermittent fasting?

As intermittent fasting can be quite a radical nutritional approach (especially if it is maintained for the longer term), it is highly recommended that you speak to your GP or an appropriately qualified nutrition professional before starting, to check whether it is safe and to help guide you through best practices. Below are some of the people or groups who are advised not to practice intermittent fasting, but note that this is most certainly not an exhaustive list of all risks and contraindications.

Pregnant or lactating women, or women trying to get pregnant

Those who have any history of eating disorder or any psychological or emotional struggles around food

Children or adolescents: Adolescent girls, for example, have been reported to potentially be more at risk of future eating disorders if they practice fasting (Stice et al., 2008)

People who are underweight, borderline underweight, or at risk of becoming underweight

Highly active people

Elderly people

People who are already feeling particularly run-down, exhausted, or stressed

Those with certain medical conditions, such as diabetes, or who are taking certain medications (particularly if they need to be taken alongside food).

Take-home points:

There is not a great deal of high quality, human evidence around IF yet – and not enough (in my opinion) to specifically recommend any specific type of IF for a specific goal or condition.

Always speak to your GP or a qualified nutrition professional before commencing a fasting programme, to see if it is suitable for you. Not everyone can safely fast, and it may make lead to a deterioration in both physical and/or mental health in some people.

If you are looking to start fasting for weight loss reasons, perhaps have a think about other lifestyle shifts first. I’ve written about this at length in my books and in articles online. The most important thing in weight loss is consistency – make changes that you can stick to, and try to tackle any root causes of weight gain too.

If you do choose to fast, think carefully about how fasting might impact day-to-day life on your low intake days. You are likely to be hungry, and possibly irritable and tired too – which could impact your work or family. It may also limit your ability to exercise, and you may well need to plan social engagements carefully.

Do make sure that you are still giving your body all the essential nutrients you need to function. Eating less food also means eating fewer nutrients. You will need to make up for this by eating a wide variety of really nutritious food on your non-fasting days. Don’t forget to drink plenty of water on both fasting and non-fasting days – set a timer if you are not great at remembering to do this to help reduce the risk of dehydration.

A sensible, physiological overnight fast of roughly 12 hours, however, I would not usually class as intermittent fasting. This is aligned to normal healthy eating habits – finishing your evening meal by 8pm and having breakfast the next day around 8am. It gives your digestion a break, and can reduce mindless evening snacking (without impacting significantly on your three main meals). This is a safe strategy for most otherwise healthy adults to follow.

Please be very wary of any apps or social media adverts that promote restrictive intermittent fasting programmes. I would not recommend using or following these, as they may not be safe or appropriate for you as a unique individual.


Cho, Y., Hong, N., Kim, K., Cho, S., Lee, M., Lee, Y., Lee, Y., Kang, E., Cha, B. and Lee, B. (2019). The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis. Journal of Clinical Medicine, 8(10), p.1645.

Harris, L., Hamilton, S., Azevedo, L., Olajide, J., De Brún, C., Waller, G., Whittaker, V., Sharp, T., Lean, M., Hankey, C. and Ells, L. (2018). Intermittent fasting interventions for treatment of overweight and obesity in adults. JBI Database of Systematic Reviews and Implementation Reports, 16(2), pp.507-547.

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.

Mattson, M., Longo, V. and Harvie, M. (2017). Impact of intermittent fasting on health and disease processes. Ageing Research Reviews, 39, pp.46-58.

Stice, E., Davis, K., Miller, N. and Marti, C. (2008). Fasting increases risk for onset of binge eating and bulimic pathology: A 5-year prospective study. Journal of Abnormal Psychology, 117(4), pp.941-946.

Varady, K., Bhutani, S., Church, E. and Klempel, M. (2009). Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. The American Journal of Clinical Nutrition, 90(5), pp.1138-1143.


Please note that the information on this website is provided for general information only, it should not be treated as a substitute for the medical advice of your own doctor or any other health care professional providing personalised nutrition or lifestyle advice. If you have any concerns about your general health, you should contact your local health care provider.

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