What is an intermittent fasting diet?

Some popular intermittent fasting eating patterns include:

• Time-restricted eating – you select an eating window and refrain from eating outside of this. For example, the 16:8 diet involves fasting for 16 hours per day and eating within an eight-hour window. One of the most common ways to do this is by skipping breakfast and eating only from midday-8pm, so you are fasting for 16 hours (between 8pm and midday the next day). Other variations of the diet involve six-hour eating periods or shorter.
• Fasting days – you choose a regular day of the week or month during which to fast over a full 24-hour period. For example, if you finish dinner at 8pm one evening, you refrain from eating until 8pm the next day.
• Alternate day fasting – you choose certain days of the week to consume very few calories, while eating a normal number of calories during the rest of the week. For example, the 5:2 diet involves eating only 25 per cent of a normal calorie intake (500 kcals for women, 600 kcals for men) on two non-consecutive days of the week, then eating a normal and unrestricted amount on the other five.
• Fasting mimicking diet – this involves a reduced-calorie diet but with a specific macro and micronutrient breakdown and is conducted on five days of the month.

How does intermittent fasting work?

The theory behind the diet is that after a period of hours without eating, the body switches from the ready supply of energy from food to burning its fat stores. It’s worth noting that there is still a lot for us to learn about this metabolic switch, including what the optimal fasting pattern and calorie limit should be to optimise its effects.

The point at which you trigger the ‘switch’ will depend on how much energy you use, what your last meal was and the amount of stored glycogen you have – all of which means it may take as much as 12-36 hours without food.

What can I eat while intermittent fasting?

Unlike most diets, intermittent fasting focuses on when rather than what you eat. For this reason, there are no restrictions on foods during the scheduled eating window and you are encouraged to eat ‘normally’. However, you should try to avoid high-calorie junk food and instead aim for well-balanced nutritious meals by incorporating lean protein, healthy fats, leafy greens and unrefined carbs such as whole grains. Try not to use your eating window to consume calorific foods and treats.

Can you drink while fasting?

Drinks are restricted during your fasting period, when you should stick to water and zero-calorie drinks such as black, unsweetened tea, coffee or herbal teas. During your eating window there are no restrictions on what you can drink

Is intermittent fasting effective for weight loss and is it sustainable?

Studies suggest that intermittent fasting is equally as effective at promoting weight loss as a calorie-counted diet for those who are overweight and obese. Results vary, however, dependent on your individual circumstances and the amount of weight you have to lose. That said, the evidence to support intermittent fasting’s role as a weight loss tool is increasing, especially in the form of alternate day fasting. However, more knowledge is needed – especially on how effective IF is in the longer term. One small, short-term study suggested the weight lost by a group of male subjects, during the early stages of the fast, was predominantly a loss of water, protein and other lean tissue rather than fat loss.

The success of the approach may also depend on your ability to maintain this style of eating going forward. Some research suggests a weight plateau after six months, this may be because IF is not so much a ‘diet’ but more a programme of eating, which means sustaining the weight loss is down to how well you adapt to the eating patterns in the longer term.

What are the benefits of intermittent fasting?

So, is there any evidence that intermittent fasting works? As well as weight loss, various studies have linked intermittent fasting with:

• a reduced risk of cancer and heart disease
• an extended lifespan
• some protection against age-related diseases including chronic pain syndromes
• a protective effect against cognitive decline
• improved resistance to stress and disease
• an increase in the number of beneficial bacteria in the gut.

This is still an emerging area of research and these links are as yet inconclusive. It is important to note that some of these studies have only been carried out using animal models, and it is unclear at the moment whether all of these benefits would be replicated in humans.

Is intermittent fasting healthy and are there any downsides?

A nutritionist’s view

• Much of the emphasis is on the fasting stage but in order to be safe and healthy, the food consumed during your 'eating window' needs to be of high nutritional value.
• For this reason, you should aim to eat essential fats from oily fish, nuts and seeds, lean sources of protein, wholegrains and starchy carbs and plenty of fruit and vegetables to supply dietary fibre, vitamins and minerals.
• During the first two to four weeks, some followers report feeling hungry, irritable and experiencing migraines as their body gets accustomed to IF.
• It's worth bearing in mind there may additional social implications if you usually enjoy meals out with friends, as these might not fit with the strict timings, or you may find it difficult to fit into family and work life.
• Few studies have examined the long-term effects (more than six months) of intermittent fasting on humans, with much of the existing support based on animal models only.

If you’re considering this form of diet, you should first consult your GP or medical practitioner to confirm it is appropriate and safe for you. Many people find that a moderate fast, finishing all food by 7pm and then waiting until 7 or 8am the next day before eating breakfast can offer a more sustainable approach, while still reaping some of the perceived benefits.

Who should be careful when considering fasting?

If you are pregnant, breastfeeding, diabetic or have a condition that requires you to keep a close eye on your blood sugar levels, you should avoid fasting. In addition to this, some groups of people are at greater risk of the negative effects associated with fasting. Such effects might include, but are not limited to, headaches and dizziness, an inability to concentrate, flare-ups of a long-standing health conditions like gout, or an alteration in the way certain medications are absorbed and used by the body.

Vulnerable groups who should exercise caution may include:

• the elderly,
• the young (under 18 years of age),
• those who are on medication (especially if it needs to be taken with food at set times),
• those who have a low body mass index (BMI less than 18.5)
• and those with emotional or psychological issues around food, including any history of eating disorders.

For women of reproductive age, the timing of a fast may be best performed during the follicular (early) stage of the menstrual cycle. For those at other stages of their reproductive life, such as the peri-menopause, it may be best to minimise the length of each fast. Research to date, is limited in this area and more is needed before we can fully understand the implications of fasting on hormonal health.

Please note: if you're considering attempting any form of diet, please consult your GP first to ensure you can do so without risk to your health.

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Listen
The BBC Good Food podcast – fasting for weight loss

This article was last updated on 26 September 2023 by Registered Nutritionist, Kerry Torrens.

A nutritionist (MBANT) Kerry Torrens is a contributing author to a number of nutritional and cookery publications including Good Food. Kerry is a member of the The Royal Society of Medicine, Complementary and Natural Healthcare Council (CNHC), British Association for Applied Nutrition and Nutritional Therapy (BANT).

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