Intermittent fasting: What is it and how does it work?

Clock on table with bowl of salad and weights to symbolise intermittent fasting
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Intermittent fasting is a style of eating where you consume food in a specific window, and refrain from eating in others. Depending on the style of intermittent fasting you choose, these windows can range from several hours of fasting in a day, to fasting every other day in a week.

As a way to promote weight loss, intermittent fasting generally works by putting you into a calorie deficit, but many people do it for other associated health benefits. When the body goes into ‘starvation’ mode during a fast, due to low glucose levels, it then begins a homeostatic process called autophagy, which may be beneficial in the prevention of disease. 

But does intermittent fasting work for everyone? And are there any negative side effects? Read on to discover the science behind intermittent fasting. And if you decide to go ahead, we recommend you invest in one of the best water bottles to ensure you're getting enough fluid to stay hydrated. Plus, check out our tips for how to drink more water if you're still struggling to take on another liquids.

What is intermittent fasting?

Intermittent fasting is where you refrain from eating for a period of time, or eat in a specific window of time, with the aim to change body composition and improve health outcomes. A review in Nutrition Reviews indicates that fasting can be effective in reducing body weight in people of every size, with most studies focusing on alternate-day fasting or whole-day fasting trials. 

Dr Nirusa Kumaran, Medical Director & Founder of Elemental Health Clinic, explains the concept of intermittent fasting further. “Intermittent fasting means changing the timing of your meals so that you eat your foods and calories in a shorter eating window or you change your eating patterns so that there are prolonged periods where you are not eating,” she says. 

You can read our guide to intermittent fasting for women: is it safe for a more detailed look at why intermittent fasting may be different for women than it is for men.

Image of a clock and wooden knife and fork sitting on wooden plate

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Types of intermittent fasting

Time-restricted eating (TRE)

Time-restricted fasting is where you have windows of fasting and windows of eating over a 24 hour period. This is usually structured in 16/8 or 14/10 windows, where you fast for 16 or 14 hours and eat during the smaller window. As we tend to fast while we sleep, this tends to be the most practical option for most people and fits quite easily into a nine to five lifestyle.

Alternate day fasting

This style of intermittent fasting is probably the most intensive, with a fast day every other day of the week. On fast days you would cut your calorie intake right down to about 25%, although some people choose not to eat at all. You would then eat normally every other day. 

5:2 fasting 

The 5:2 method of intermittent fasting runs over an entire week, with two days of fasting and five days of regular eating. Fasting days may include small 200-300 calorie meals, but some people choose to fast completely for the two 24 hour periods. 

24 hr fasting

This style of intermittent fasting is usually done a maximum of a couple of times per week and involves not eating for 24 hours, usually from breakfast to breakfast or lunch to lunch. 

What happens during intermittent fasting?

One of the most notable consequences of intermittent fasting is that it can trigger the process of autophagy, which is where the body cleans out dysfunctional cells and recycles parts of them into new cells. It is a system that promotes regeneration of healthy cells, and clears out mutated cells that could develop into cancer, as well as toxic proteins that may be responsible for the development of neurological conditions such as Parkinson's and Alzheimer's disease, according to one study in the EMBO journal

Round wooden board with a quarter portion of vegetables symbolising intermittent fasting

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Autophagy begins after a period of fasting, and may be an evolutionary throwback from our hunter-gatherer days, where we would go for longer periods without eating due to the labor-intensive nature of acquiring food. Research in Autophagy Journal suggests putting your body into autophagy on a regular basis can help it to run optimally by giving it the opportunity to ‘reset’ and clear out cellular debris. Further research is needed, but a study in Carcinogenesis indicates that regular fasting triggering autophagy as a ‘starvation’ response might be protective of cancer and integral in tumor suppression, making it a promising area for cancer research. 

As well as autophagy, fasting causes the body to go into an alternative metabolic state, burning stored fat as there are no carbohydrates available for energy. A study in Cell Metabolism indicates that metabolizing fat produces ketones, which the body can use as an alternative energy source. Similar to the way the keto diet triggers this alternative metabolic state, intermittent fasting can put you into a state called ‘ketosis’ where you are using ketones as fuel. 

Is intermittent fasting safe for everyone?

If you are on any medication, are pregnant, trying to become pregnant, breastfeeding or have a chronic health condition, intermittent fasting is not recommended. 

Dr Kumaran advises particular caution for people in the following groups:

  • If you have a history of eating disorders
  • If you have a low BMI or are underweight
  • If you are prone to episodes of hypoglycaemia (low blood sugar)
  • If you are taking prescribed medications, particularly medications to lower blood sugar
  • If you have high stress levels or are suffering with insomnia
  • If you are trying to conceive, are pregnant or breastfeeding
  • If you have amenorrhoea (absent or irregular periods)
  • If you are a teenager or adolescent, as this is an important period of time for growth

If you fall into any of these categories, you can ask your doctor if intermittent fasting under their supervision would be suitable for your circumstances, but you will probably require careful monitoring.

If you don't think intermittent fasting is for you, you can have a look at our guides to what is the paleo diet and how does it work or the Mediterranean diet: everything you need to know to see if these diets would suit you better. 

This article is for informational purposes only and is not meant to offer medical advice.

Lou Mudge
Health Writer

Lou Mudge is a health writer based in Bath, United Kingdom for Future PLC. She holds an undergraduate degree in creative writing from Bath Spa University, and her work has appeared in Live Science, Tom's Guide, Fit & Well, Coach, T3, and Tech Radar, among others. She regularly writes about health and fitness-related topics such as air quality, gut health, diet and nutrition and the impacts these things have on our lives. 

She has worked for the University of Bath on a chemistry research project and produced a short book in collaboration with the department of education at Bath Spa University.