Have you been wondering why everyone is talking about intermittent fasting? It seems like everywhere you turn someone is telling you how it has helped them lose weight or how much healthier they are thanks to avoiding food from time to time.
Is intermittent fasting really as useful as people are proclaiming it to be? Or is it just another fad?
It can be difficult to keep up with the latest trends in diet and nutrition. These trends just add to the confusion out there – we’ve discussed how the food & pharmaceutical industries influence healthy eating guidelines.
It seems as soon as you have wrapped your head around the latest hot topic, there is another one nipping at its heels. That’s why we’ve create this guide to intermittent fasting.
Intermittent fasting became popular as a way to lose weight in 2012 with broadcast journalist Dr. Michael Mosley’s TV documentary Eat, Fast, Live Longer and his book The Fast Diet1. It looked set to become a fad diet people would eventually lose interest in.
Ten years later, intermittent fasting has gained in popularity and a significant amount of research has been done on the topic. It’s a dietary practice that’s been used by people for various reasons for centuries. Many cultural and religious practices include fasting in one way or another, and even our ancient ancestors were forced to go without food when there was none available. So, perhaps there is more to it than meets the eye.
What’s intermittent fasting?
For the last two to three decades conventional nutritional guidelines have told us to eat three meals per day – breakfast, lunch and dinner. And snacks between meals have been promoted as being necessary and beneficial.
We have been encouraged to eat according to our body’s calorie requirements to ensure we have enough energy to get us through the day and to avoid malnutrition.
The practice of intermittent fasting (IF) goes against these rules and suggests restricting your calorie intake by 60 to 100% for a stipulated period of time2.
The authors of a study examined “The impact of IF on health and disease processes” and defined intermittent fasting as an umbrella term for “eating patterns in which individuals go extended time periods (e.g., 16–48h) with little or no energy intake, with intervening periods of normal food intake, on a recurring basis3.”
In other words, when you follow an intermittent fasting regimen you will alternate between eating as you normally do and refraining from food altogether, or significantly limiting your intake.
The exact time periods and calorie restriction depend on which method of intermittent fasting you choose to follow.
Four types of intermittent fasting
One of the reasons you may find intermittent fasting confusing is because there are four different recognised forms of this dietary practice.
As we mentioned before, it is used as an umbrella term to talk about implementing fasting in your routine. intermittent fasting may be referring to any one of the following applications4:
- Complete alternate day fasting: Completely restricting calorie intake every second day.
- Alternate day fasting: Restricting your calorie intake to 20 to 25% of your daily requirements on scheduled fasting days such as the 5:2 method where you limit your food intake on two consecutive days.
- Time-restricted feeding: You are only allowed to eat within a specified “eating window”, most commonly 8 hours, followed by a 16-hour fast during which no food is allowed.
- Religious fasting: The time and level of restriction vary depending on the religion and the reason for fasting.
Benefits of intermittent fasting: more than weight loss
The exciting thing about intermittent fasting is it offers you so much more than simply being able to lose a few kilograms to fit into your favourite pair of jeans.
Research shows it has both short-term and long-term health benefits.
Scientists may not be 100% certain of how fasting brings about changes in the body, but they do have some ideas that may explain how it works5.
It is thought the short-term benefits of fasting are a result of ketosis. When your calorie intake is significantly restricted, your metabolism shifts towards burning fat for energy and your insulin sensitivity improves.
This combination means more fat is used by the body and less fat is stored. The benefits we see are5:
- Weight loss
- Reduced waist circumference
- Lower body fat percentage
Research suggests the long-term outcomes of intermittent fasting are attained through a reduction in oxidative stress. The low-calorie intake as a result of limiting how much you eat causes a decrease in the production of reactive oxidative species in the mitochondria – the powerhouses of our cells.
It also increases the activity of antioxidants and improves the regulation of circadian rhythms. The knock-on effect of these metabolic changes is reduced inflammation at a cellular level and an increase in autophagy – the removal of damaged cells. The long-term health benefits of intermittent fasting include5,6:
- Reduced risk of metabolic syndrome
- Reduced risk of non-alcoholic fatty liver disease
- Reduced risk of neurodegenerative disorders
- Improved cardiovascular health
- Lower blood pressure
- The prevention of some cancers
- Improving metabolic health
The true advantages of intermittent fasting lie in its effect on our metabolism. When your cells become more efficient at burning fat for energy and less damage is caused by oxidative stress, all bodily processes work better. It becomes easier to lose weight and create (and maintain) good health.
The best foods for intermittent fasting
Numerous intermittent fasting studies show it is possible to reap the weight and health rewards of intermittent fasting by changing when you eat, not what you eat. The practice is more effective, though, if you stick to a healthy diet in between fasting periods7.
If you indulge in high-calorie, ultra-processed foods during your feeding window you will be undoing all the good you did by restricting your intake. It makes sense, then to avoid foods such as:
- Fast food
- Refined sugar
- Refined carbohydrates
- Soft drinks
For further information, read our guide on what are ultra processed foods and how to avoid them.
Instead, you should focus on eating lower-calorie foods that provide the body with the protein, fat, fibre, vitamins and minerals it requires to function optimally and promote health. Such foods include:
- Meat – i.e. high-quality proteins
- Fibre-rich carbohydrates such as oats, brown rice and sweet potatoes
- Healthy fats from avocados, nuts, seeds and olive oil
- Plenty of water
Fasting can be difficult, especially if your diet is based on calorie-dense, fatty, sugary, salty food. Choosing to do intermittent fasting does not mean you can go ahead and eat whatever you like. You still have to nourish your body with healthy food – your nutrition is an important first step to feel your best.
What intermittent fasting is best for me?
With so many different ways to adopt intermittent fasting, how do you know which one is best for you?
The two most common methods, aside from religious fasting, are alternate day fasting and time-restricted eating. Both have been proven to offer weight loss and health benefits. It is said the best diet for you is the one you can stick to; the same can be said for intermittent fasting.
Alternate day fasting, or the 5:2 method, means you only have to restrict your food intake on two days during the week. They don’t have to be consecutive days, but the benefits are increased if they are. If the idea of extended daily fasting periods makes you worry about being constantly hungry, this method may be the best one for you. It is also easier to plan your schedule to incorporate your fast.
On the other hand, time-restricted eating in the form of a 16-hour fast followed by an 8-hour feeding window, can fairly easily become routine. All it means is extending your natural overnight fast from about ten hours to sixteen. You don’t have to go directly from ten to the full sixteen hours. You can slowly extend your fast over time.
For example, if you normally eat your dinner at eight o’clock at night and eat your breakfast at six in the morning you already have a ten-hour fast. The first step is to delay your breakfast by two hours, only eating it at eight, to give you a twelve-hour fast. Continue extending your overnight fast until you are eating the first meal of your day at midday.
Whichever method you choose, a good suggestion is to ease your body into it. Start by improving your overall diet and eating regular meals. If your body is well nourished and your insulin levels are under control, it becomes easier to restrict your calorie intake7.
FAQs about intermittent fasting
Who should not try intermittent fasting? Intermittent fasting can result in a significant calorie restriction. Thus, it is not recommended for pregnant and breastfeeding women, children, and people who are underweight or struggling to gain weight.
Can you do intermittent fasting if you have a chronic condition? You should talk to your doctor before you decide to take up intermittent fasting if you have a chronic disease that is being managed with medication. This is especially true if you have diabetes that is controlled with insulin.
Is intermittent fasting suitable for someone with a history of eating disorders? No. If you have a history of anorexia or bulimia intermittent fasting is not recommended for you.
Can you exercise while doing intermittent fasting? It is generally safe to exercise while doing intermittent fasting, especially if you are practising time-restricted eating. If you are using alternate day fasting, you may not be able to exercise at your normal intensity on your fasting days.
How do you control your hunger while fasting? Your body adapts well to intermittent fasting and you should find you don’t feel too hungry, even while you are fasting. Ketosis helps to keep hunger under control.
Are there any side effects of intermittent fasting? Some people struggle with side effects when they start intermittent fasting. You may feel light-headed, dizzy, or nauseous and migraines and insomnia are also common.
- Staff HHP. Intermittent fasting: The positive news continues – Harvard Health [Internet]. Harvard Health. 2018 [cited 2022 Jul 27]. Available from: https://www.health.harvard.edu/blog/intermittent-fasting-surprising-update-2018062914156
- Rynders CA, Thomas EA, Zaman A, Pan Z, Catenacci VA, Melanson EL. Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients [Internet]. 2019 Oct 14 [cited 2022 Jul 27];(10):2442. Available from: http://dx.doi.org/10.3390/nu11102442
- Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Research Reviews [Internet]. 2017 Oct [cited 2022 Jul 27];46–58. Available from: http://dx.doi.org/10.1016/j.arr.2016.10.005
- Nowosad K, Sujka M. Effect of Various Types of Intermittent Fasting (IF) on Weight Loss and Improvement of Diabetic Parameters in Human. Current Nutrition Reports [Internet]. 2021 Apr 7 [cited 2022 Jul 27];(2):146–54. Available from: http://dx.doi.org/10.1007/s13668-021-00353-5
- Mandal S, Simmons N, Awan S, Chamari K, Ahmed I. Intermittent fasting: eating by the clock for health and exercise performance. BMJ Open Sport & Exercise Medicine [Internet]. 2022 Jan [cited 2022 Jul 27];(1):e001206. Available from: http://dx.doi.org/10.1136/bmjsem-2021-001206
- Ahmed A, Saeed F, Arshad MU, Afzaal M, Imran A, Ali SW, et al. Impact of intermittent fasting on human health: an extended review of metabolic cascades. International Journal of Food Properties [Internet]. 2018 Jan 1 [cited 2022 Jul 27];(1):2700–13. Available from: http://dx.doi.org/10.1080/10942912.2018.1560312
- Templeman I, Gonzalez JT, Thompson D, Betts JA. The role of intermittent fasting and meal timing in weight management and metabolic health. Proceedings of the Nutrition Society [Internet]. 2019 Apr 26 [cited 2022 Jul 27];(1):76–87. Available from: http://dx.doi.org/10.1017/S0029665119000636