What is intermittent fasting?

Niamh Hennessy
Lead Dietitian, Cromwell Hospital
11 January 2023
Next review due January 2026

Intermittent fasting can be a controversial topic. Some people think it helps with weight loss, while others are more sceptical. But what does the evidence say? Here, I’ll explore the different types of intermittent fasting and discuss any potential health benefits it might have.

woman eating breakfast from a bowl

Is intermittent fasting a new thing?

Intermittent fasting (IF) is when you go for extended periods of time without eating. This is followed by a period of eating normally. Your ancestors would have eaten like this, not because of choice, but because of food availability. They would have only eaten when they could find food, either by hunting or foraging. Nowadays you no longer have to hunt for your food. So, you’re unlikely to fast unless you choose to.

Intermittent fasting is a way of mimicking more traditional eating patterns.

What are the different types of intermittent fasting?

There are different forms of intermittent fasting, some of which are more extreme than others.

5:2 diets

Here, you consume only 500 to 600 calories for two days each week. On the other days you would eat a normal, healthy, and balanced diet with your usual calorie intake.

16:8 plan

This involves eating during an eight-hour window and fasting for 16 hours. So, you could eat from 10am to 6pm and then drink water, milk, tea or coffee for the remaining time.

Alternate day fasting

For this form of IF, you’d fast every other day, which can be very difficult to maintain over the longer term.

24 hour fast

People following this diet would fast for an entire 24-hour period, perhaps monthly or weekly.

How does intermittent fasting work?

The idea behind intermittent fasting is that it gives your body a break from digesting food. This can potentially lead you to have less calories than if you were eating more regularly. Also, eating less often can have positive effects on your blood sugar levels which can be useful if you have prediabetes or type two diabetes.

Intermittent fasting may also trigger a process called autophagy. This is where your body works to remove damaged cells from your body. Your body does this when it doesn’t have to focus on digesting food. There is some evidence to show that autophagy can reduce your risk of cancer and other chronic diseases, but more evidence is needed.

Other potential benefits of IF include:

  • improved gut health
  • better sleep
  • reduced cardiovascular risk factors, including lower blood pressure
  • lower inflammatory markers (linked to many chronic diseases)

Does intermittent fasting lead to weight loss?

It might do. In several short-term studies, people did lose weight when they tried intermittent fasting. This is because you would likely be having less calories overall. But for some people, intermittent fasting can lead to overeating. This is because when you go without eating for a while your body makes more of the hunger hormone ghrelin. This can lead you to eat lots when you break your fast. So, eating healthily and being aware of how many calories you have is still important if you choose to try IF.

Is intermittent fasting safe?

Research suggests that it can be safe for healthy adults. But there’s not enough evidence to show what would happen if you did this type of fasting for a long time. There are also some circumstances where IF would not be suitable.

  • if you are pregnant
  • have type 1 diabetes
  • have a history of disordered eating
  • have anxiety or depression

So, it’s worth checking with your doctor if you have any health issues or concerns.

How to start intermittent fasting

If the idea of alternate day or 24-hour fasting doesn’t appeal, there are other options. For example, most people could manage to eat within an 8-hour window. This can stop all day-grazing. You can choose the time of your 8-hour window to suit your lifestyle. If you get up early you could eat between 9am and 5pm. But if you prefer to eat later in the evening, you could start at 11am and finish your meal by 7pm.

There’s also some evidence that sticking to a 12-hour eating window is better than eating late at night and then having an early breakfast. So, try experimenting with different timings to see what is realistic and achievable for you.

What should I consider?

  • Make sure that your diet is still healthy and balanced. So, focus on eating a wide range of vegetables, fruits, lean proteins, healthy fats, and wholegrains.
  • You should eat enough to give you the energy you need to carry out your daily tasks, including exercise. Some people find that IF patterns such as alternate day fasting can make them struggle to concentrate, so the 16:8 pattern could be easier to manage.

Overall, you should eat a healthy diet by reducing your intake of added sugars, saturated fat, and highly processed foods. But if you do choose to try IF then make sure it’s suitable for you and prioritise a balanced diet when you do eat.

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Niamh Hennessy
Niamh Hennessy
Lead Dietitian, Cromwell Hospital

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    • Jamshed H, Beyl R, Manna D et al. Early time-restricted feeding improves 24-hour glucose levels and affects markers of the circadian clock, aging, and autophagy in humans. Nutrients, 2019;11(6): 1234. Doi:10.3390/nu11061234.

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