Intermittent fasting – a guide

Reviewed by our clinical team

What is intermittent fasting?

Intermittent fasting is an eating schedule with prolonged periods of not eating (or fasting). The main difference between intermittent fasting and traditional diets is that fasting diets focus on ‘when’ a person should eat rather than ‘what’ they should eat. 

The reason many people try intermittent fasting is because some researchers claim that the human body is designed to go without food for long periods, for example, when we were hunter gatherers. It is also linked to weight loss, improved metabolic health and decreased insulin resistance. Insulin is a driver of fat storage and is triggered by eating. If released all day over a prolonged period, insulin resistance occurs, and the body has to make more. Fasting provides the opportunity for insulin levels to drop. 

Does intermittent fasting work for weight loss?

There is evidence to suggest that people lose some weight when intermittent fasting. However, studies show that there is no significant difference between fasting groups and continuous calorie restriction groups. Calorie restriction is a regimen where calories are reduced from what is ‘normal’ without causing malnutrition. 

The different types of intermittent fasting

Time restricted eating

Time-restricted eating is a form of intermittent fasting. It means that you eat during certain hours of the day and fast, or don't eat, the rest of the time. By limiting your eating window, you naturally reduce your caloric intake, which can lead to weight loss. 

The easiest way to do this for most people is to extend the natural fast that happens when they sleep. You can pick which eating window best suits you and your lifestyle. For effective weight loss you should choose to eat mainly nutrient dense foods and avoid processed foods in the time frame.

If you fast during a 14 hour window and eat during the remaining 10 hours, you're following a 14/10 time-restricted eating pattern. This could mean that you eat your first meal at 8am and start your fast at 6pm, after your last meal. Or you might eat your first meal at midday and start your fast at 10pm. There are also 16/8 and 20/4 versions of time restricted eating.

During the fasting period, it’s important to stay hydrated. Drinking plenty of water is advised – this can be sweetened with a slice of lemon or lime. People often drink tea or coffee in their fasting period too but avoid using milk or sugar. 

Before starting a fasting plan, if you have a medical condition or special dietary needs, talk to your GP. 

5:2 fasting schedule

During a 5:2 fast a person should restrict calorie intake to 500-600 calories on two days of the week and follow a healthy diet on the remaining five days.

The benefits of intermittent fasting

Several benefits have been reported, including:

• A sharper mind – people may experience an improved memory 
• Weight loss – ‘a systematic review of 40 studies found that intermittent fasting was effective for weight loss, with a typical loss of 7-11 pounds over 10 weeks’
• Protection against some chronic diseases - such as type 2 diabetes, heart disease, some cancers, and IBS

Can you eat anything whilst intermittent fasting? 

Technically, yes. Intermittent fasting focuses on sticking to eating windows rather than prescribing what should be eaten. 

However, to maximise the benefits of intermittent fasting, during an eating window, we recommend you maintain a healthy and varied diet, which means eating foods across the food groups. For example, make sure you’re getting enough protein, eating fruits and vegetables, wholegrains and avoiding overly processed foods. 

How long should a fasting plan last?

If weight loss if the goal, intermittent fasting is not a quick fix. As mentioned above, across 40 studies, participants lost weight gradually over a 10-week period with a typical loss of 7-11 pounds. 

Who shouldn’t fast?

Drastically reducing calories and skipping meals can be dangerous for some people, including those people with:

• Diabetes
• Digestion issues, for example, stomach ulcers
• Weakened immune system
• Cancer
• Eating disorders
• Some other chronic conditions. 

Fasting is also not recommended during pregnancy because of the importance of mother and baby getting the nutrients they need. If you choose to fast whilst breastfeeding it’s important to discuss with a health professional first, such as a GP or midwife. 

Tips for success with intermittent fasting

• Start slowly (don't start off straight away with a 16 hour fast. Start with 10/12 hour fast and slowly increase and build up to 16:8)
• Stay hydrated
• Plan meals in advance (planning healthy/nutritious meals)
• Avoid high-intensity workouts during fasting days/period
• Get enough rest (sleep) and make sure you have time to relax
• Avoid late night eating
• During eating window make sure meals are rich in nutrients form all food groups

Our lifestyle hub 

To read more expert diet and lifestyle advice visit our lifestyle hub. We’re here for you with simple and clear expert advice on losing weight, stopping smoking and leading a more balanced lifestyle.

References

https://patient.info/news-and-features/is-intermittent-fasting-a-healthy-way-to-lose-weight
https://www.diabetes.co.uk/diet/5-2-intermittent-fast-diet.html 
https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/intermittent-fasting/
https://diabetesmyway.nhs.uk/keeping-healthy/different-dietary-approaches-for-weight-loss/intermittent-fasting/
https://www.nejm.org/doi/full/10.1056/NEJMra1905136
https://www.health.harvard.edu/heart-health/not-so-fast-pros-and-cons-of-the-newest-diet-trend
https://www.tommys.org/pregnancy-information/im-pregnant/nutrition-in-pregnancy/fasting-pregnancy