Intermittent fasting may seem like a recent trend, writes Hatty Willmoth, but it follows in ancient footsteps.

This article was originally published in the Autumn 2022 issue of the Optimum Nutrition magazine.

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Is fasting a fad? The number of internet searches for intermittent fasting (IF) has nearly tripled over the last five years, first taking off around a decade ago when Dr Michael Mosley popularised the 5:2 diet.

Meanwhile, emerging new evidence seems to suggest that IF helps with all manner of diseases.

Yet some question whether IF is safe in the long term; no long-term clinical trials appear to have been conducted.

Plus, most of the research has been limited to tackling obesity, rather than diseases and ageing processes more generally.

People have even said that IF regimes are eating disorders in disguise, or at least fleeting fads doomed to fail.

But author and nutritional therapy practitioner Amanda Swaine disagrees. Specialising in IF, she has practised fasting for the last decade or so.

She says: “‘Fasting’ and ‘intermittent fasting’ are umbrella terms covering a range of regimes that oscillate between periods of eating and not eating, without malnutrition.

“‘Intermittent fasting’ tends to focus on the shorter fasting programmes (less than 36 hours of not eating) whereas the term ‘fasting’ includes longer periods of fasting too.”

And ‘not eating’ is something humans have done since there were humans.

In times of food scarcity — for example between hunts or significant gathers, in the winter, or after a bad harvest — people have had no choice but to eat less food.

Religious and historical fasting

In many cultures, periods of ‘not eating’ have become ritualised and imbued with religious significance.

As depleting winter stores finally ran out in medieval Europe, for instance, Christians fasted for Lent.

To this day, the Catholic calendar is peppered with fast and feast days, and Christians of all denominations may feel called to fast by the Holy Spirit.

But Christian fasting doesn’t have to mean going without food and drink entirely; sometimes it just entails going without rich food such as meat, and/or simply eating less.

Fasting is a major part of countless other religions. In Islam, Ramadan is a holy month during which every practising Muslim (except in extenuating circumstances) must go without all food and drink from sunrise until sunset each day.

In Judaism, fast days include Yom Kippur — a day of repentance — and Tisha B’Av, when Jews remember tragedies endured by the Jewish people.

For Hindus, fasting practices depend on individual devotion to particular gods, as well as fasting on certain festivals. In Hinduism, fasting varies from total abstention, to eating one meal a day, or only eating certain food types.

Fasting for health reasons primarily started, in this country at least, in the nineteenth century, as part of the ‘nature cure’: an approach to health that stressed the importance of diet, exercise, sunshine, fresh air and positive thinking.

From the early twentieth century, it’s also been investigated as a treatment for obesity, with studies using ‘short-term starvation periods’ of between one and 14 days.

Fasting for health

According to Swaine, the reason fasting has been so common throughout history is because it’s natural.

“As humans, we have evolved to benefit from both eating and fasting modes, and to be optimally healthy we should be switching between both modes regularly,” she says.

“When we eat, we burn glucose for fuel and grow. When we fast, we gradually move over to using fat for fuel and repairing.

“Increasingly, modern life is associated with a disproportionate time spent in growth mode due to high eating frequencies, high calorie intake, sedentary lifestyles, chronic stress and longer life spans.

“This dysregulation is associated with increased incidences of obesity, metabolic disease and diminishing health spans.”

Swaine says the benefits of fasting relate to redressing this balance of fasting and feasting, and include weight loss, “metabolic flexibility” and ageing well.

It can also be useful, she says, for people needing help with “brain or gut health and autoimmune conditions”.

Studies have linked intermittent fasting to all manner of health benefits, including those related to: obesity (2017, 2018, 2019) diabetes (2017, 2017, 2019), cancer (2016, 2020, 2020), heart disease (2011, 2017), stem cell regeneration (2014, 2018), Covid outcomes (2022, 2022), epilepsy and fatty liver disease.

Different fasting patterns

Nowadays, there are three types of intermittent fasting. Time-restricted feeding is when fasters limit eating to a window of a few hours per day — typically between eight and 12.

Alternate-day fasting involves switching between a ‘fast day’ (no food or less than 25% of ‘normal’ intake) and a ‘feast day’ (as much food as the faster likes).

And finally, there’s periodic fasting, or whole-day fasting, which is any period of consecutive fasting for more than 24 hours. It could be a single day or several weeks of continuous fasting.

Again, ‘fasting’ here may mean totally refraining from food and drink, or eating less than a quarter of a ‘normal’ day’s food.

The 5:2 diet

Popularised in the UK by Dr Michael Mosley, the 5:2 diet is probably the most well-known fasting diet; involving fasting for two days per week, and eating ‘normally’ for the other five.

In 2012, Mosley presented a Horizon documentary in which he explored various methods of fasting, and eventually trialled the 5:2 diet.

In five weeks, he lost nearly a stone (over 5kg) in weight and was no longer pre-diabetic.

Since then, Mosley has promoted the diet through books, television, and journalism. He famously loves the diet, and says he finds it relatively easy to follow.

The 5:2 diet has been primarily marketed as a way to lose weight and improve metabolic health — reducing the risk of insulin resistance, diabetes, and heart problems — but many have tried it out with other goals in mind.

Indeed, Swaine herself gave it a try, but found that time-restricted eating suited her much better.

“I am not strict about my eating window,” she says, “but try to have gaps of not eating every day, usually by eating a later breakfast and limiting snacks. It roughly aligns with a 14:10 or 16:8 plan.”

That means Swaine generally fasts for 14-16 hours every day.

An eating window

It’s a pattern that seems to work for many people. Long-time faster Lyn Curtis, 70, experimented with various methods of fasting “many decades ago” before turning to time-restricted eating “full-time” in 2015.

She did so, she says, because she was concerned about her worsening memory and had heard that fasting could help.

Since then, she has regularly kept her eating to within a four- to eight-hour window each day: skipping breakfast, and eating a big lunch and early dinner. She says it’s helped keep her mind sharper.

“At first it felt weird,” she says, “like I hadn’t woken up properly…But I soon got used to being hungry for a few hours.

“I wouldn’t say I eat any less food, quantity-wise in the day, but am pretty okay with my weight, which has been the same for many years. I don’t snack, or rarely — if circumstances dictate a delayed lunch, for instance.

“I really enjoy my food, I think, more than I did before,” she adds. “I don’t feel too bad about having a big meal!

“I don’t have to think about being tempted into something, I just follow the rules, and don’t feel deprived at all.”

However, she says: “Sometimes I might be a bit under-par or grumpy, but then feel fine after a lovely lunch.”

Who is intermittent fasting for?

Fasting, however, is not for everybody. Swaine says: “I would consider it for anyone who needs support with building gaps into their eating regime.

“In particular for those experiencing metabolic issues, or who feel like they are ageing faster than they should be, they would ultimately get the most benefits…As fasting is a mild stressor, it is not right for everyone at all times.”

Swaine points out that it’s also important to find an eating pattern that works for you so you can continue long term.

“Intermittent fasting plans aren’t short-term diets,” she says. “They are ways of building in gaps of ‘not eating’ for life and these can, and should, evolve over time based on circumstances, age and overall health.”

Knowing why you’re fasting is also key, says Swaine. “What is your goal? By knowing that, you can adjust the regime to get those results you are after without being unnecessarily strict.”

She also warns against “being too strict, not listening to your body and taking things too far”.

If you’re fasting and you feel extremely tired, dizzy, faint or unwell, Swaine says you should break your fast immediately and eat something.

Who shouldn't fast?

Swaine warns fasting could be harmful for children, during pregnancy, for type 1 diabetics, and those who are underweight, malnourished, or suffering/recovering from an eating disorder or adrenal fatigue.

She also recommends that people with type 2 diabetes or a thyroid condition, or who are breastfeeding or over 70 years old, seek medical advice and take extra precautions if intending to fast.

The bottom line

In terms of timings, food intake and motivations, fasting patterns can vary significantly. For some, fasting is about getting closer to God; for others, it’s for weight loss, to feel sharper, for gut health, or something else.

And, while it is true that there are trends within fasting that may eventually fizzle into obscurity, the principle of ‘going without food for a bit’ is age old.

At the end of the day, what matters is what works for you.

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