What is intermittent fasting and how does it work?

Fasting is gaining popularity as a 'bio-hacking' health and weight-loss regime
Fasting is gaining popularity as a 'bio-hacking' health and weight-loss regime

Endorsed by celebrities and CEOs, and our very own Prime Minister, historically, fasting was followed for cultural and religious reasons, but across the world it is gaining popularity as a “bio-hacking” health and weight-loss regime.

Intermittent fasting is free, and part of its appeal is that it is simple to follow. It works by focusing on when you eat, rather than what you eat, within a restricted time window.

So how safe is intermittent fasting, how exactly does it work and is it suitable for you?

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What is intermittent fasting?

In a nutshell, intermittent fasting is an eating regime that alternates between periods of fasting and eating. It does not generally specify which foods to consume or avoid, but rather emphasises when you eat.

“Intermittent fasting is not a fad diet; it does not impose restrictions on food selection or quantity,” says Mark Mattson, adjunct Professor of Neuroscience at Johns Hopkins School of Medicine and author of The Intermittent Fasting Revolution. “Intermittent fasting is an eating pattern that consists of numerous periods of time with little or no meals and it helps with weight loss.”

Scientific research on fasting started in laboratory experiments on mice in the 1940s but fasting rose to prominence in 2012 thanks to Dr Michael Mosley’s BBC2 documentary Eat, Fast and Live Longer, in which he explored fasting as not only a weight loss tool but also a way of extending your lifespan.

How does intermittent fasting work?

Worldwide, more than one billion people suffer from obesity and since 1975 the global obesity rate has tripled. The British Medical Journal reported in 2016 that in many nations, obesity is killing more people than smoking. According to the 2021 Health Survey for England, 25.9 per cent of adults are obese, while another 37.9 per cent are overweight, so finding a free, simple way to lose weight is bound to make intermittent fasting popular - it’s already the fourth most searched for weight-loss technique online.

A systematic evaluation of 40 trials concluded that intermittent fasting worked, with a typical weight loss of 7-11 pounds over 10 weeks. How? Intermittent fasting works by extending the time between when your body has burnt through the calories from your last meal and starts burning fat. It’s called metabolic switching.

“During fasting, your body switches from using glucose to burning fat for energy, which takes about 12 to 14 hours,” says Mattson. “With no energy intake for this long, it’s sufficient to deplete liver energy stores which is glucose and then switch to using fats and the ketones derived from the fats in your fat cells.”

This process is known as ketosis, he explains, and results in weight reduction. It also aids in the regulation of appetite hormones like leptin, which make you feel fuller and decrease food cravings - you don’t feel hungry and you feel more focused and energetic. Studies have found that intermittent fasting results in the reduction of the hunger hormone ghrelin, which suppresses appetite during a fasted state. Also when insulin is low, this allows the fat-burning process called lipolysis to pull energy from fat cells rather than from any sugar or food that has just been eaten.

Rachel Jones, 50, a property consultant in London, is a convert. “I’m naturally not hungry in the morning, so completing a fast from 8am to lunchtime or 1pm is easy. I’ve finally been able to shed the half stone I’d been struggling with for years, with no effort or even feeling like I’m on a diet. Fasting fits into my day and is free, requiring no meal preparation or special supplements - that’s why I like it and it’s become a way of life.”

The different types of intermittent fasting

  • 16 hour fast

  • The 5:2 diet

  • Fasting every-other-day

There are lots of variations of intermittent fasting, but the most basic is daily restricted eating, which entails compressing your eating window to 6-8 hours every day, resulting in a fast of 16-18 hours every day. Typically you might eat from 11am and 7pm, and abstain the rest of the time.

Another intermittent fasting regimen is the 5:2 diet, popularised by Dr Michael Mosley, which consists of eating your usual three meals per day for five days a week, but having no more than 600 calories on the remaining two days. Other options include “every-other-day” fasting regimens that alternate 300-400 calories per day with unrestricted eating days. Another strategy is to consume just 700 calories per day on five consecutive days per month. Read more about different fasting types here.

Does intermittent fasting work?

Mattson’s analysis of 1500 different peer reviewed journals was printed in the New England Journal of Medicine in December 2019 and showed that intermittent fasting has numerous health and cognitive benefits.

Not only can it help you lose weight, but it may lower your risk of Type 2 diabetes, heart disease, and cancer. How? Intermittent fasting has been shown to reduce insulin resistance by lowering blood sugar by 3-6 per cent and fasting insulin levels by 20-31 per cent, protecting against Type 2 diabetes. Some studies show that fasting reduces inflammation, which is the cause of many chronic diseases. It has also been shown to lower “bad” LDL cholesterol, blood triglycerides, inflammatory markers, blood sugar, and insulin resistance - all of which are risk factors for heart disease.

Animal studies suggest that intermittent fasting may be beneficial in the prevention of cancer. Intermittent fasting can also boost the brain hormone BDNF, a hormone that improves memory, mood and learning and aids in the creation of new nerve cells, which may be beneficial in the prevention of Alzheimer’s disease.

Fasting also may help you live longer. In a small study on 52 rats, the fasted ones lived 36-83 per cent longer than the rats that ate normally. In 2016, Dr Yoshinori Ohsumi, won the Nobel prize for his work on fasting and autophagy, which is the process that helps your cells “take out the rubbish”. Creating a state of autophagy in the body by fasting (it usually takes between 17 and 48 hours) helps remove damaged cells in order to generate newer, healthier cells, he found.

Is intermittent fasting safe?

In March this year, research from the Shanghai Jiao Tong University School of Medicine on longer-term outcomes of fasting has painted an alternative picture. After tracking nearly 20,000 adults in the United States, it found that those who said they ate within an eight-hour window were nearly twice as likely to die of heart attacks or strokes, compared with those who eat across 12 to 16 hours. The researchers stressed that the results must be treated with caution, and that there were limitations to the study with other factors, such as the age of the participants and their eating patterns on other days, potentially skewering the results.

Intermittent fasting is not suitable if you are pregnant or breastfeeding and for people with Type 1 diabetes. It is not advised for young children and teens; older adults who experience weakness; people with immunodeficiencies; people with a current or past eating disorder or anyone with a history of traumatic brain injury or post-concussive syndrome.

Seek medical advice before beginning a fast, and always check with your GP before starting a fasting lifestyle if you’re taking any medication. There is currently no authoritative research on the effects on the body long-term.

According to Mattson, it takes two to four weeks for the body to adjust to intermittent fasting. “You may feel irritable or ‘hangry’ as you adjust to your new routine,” he explains. “If you can make it through this adjustment period, I find that people stick with the plan because they notice they feel better - more focused and they lose weight.” Other possible side effects of intermittent fasting include headaches, tiredness, constipation and overeating during non-fasting periods, especially in the first few weeks.

Does intermittent fasting affect men and women differently?

“Male and female responses to intermittent fasting differ,” says Mindy Pelz, a nutrition and functional health expert and author of Fast Like A Girl. “Both sexes have a hormone called gonadotropin-releasing hormone (GnRH), which regulates the activities of our reproductive organs. In women, GnRH’s regulation functions decide the cycles of ovulation. But at the slightest alteration to a woman’s routine, GnRH is affected.” It can be affected by small things such as getting up later or stress, but the body is usually able to adapt better to these changes when you are eating normally.

Women need the hormones oestrogen and progesterone irrespective of their cycles. GnRH stimulates the production of these, but it needs insulin and glucose to fuel the process. “The problem is that fasting depletes the body’s insulin and glucose reserves,” Pelz says. She has observed that women may experience disrupted menstrual cycles if they fast later on in their cycle. Studies have also found that in menopausal women, fasting may increase hot flushes.

Pelz suggests women follow a fasting regime around their cycle. If a woman has an average 28-day cycle, she suggests intermittent fasting between day 1 and day 12. On days 12-14, when oestrogen production is at its peak, don’t fast. On days 21-28, when progesterone production is at its peak, increase your carbohydrate intake which will improve the production of progesterone in your body. “At this time, women need to fast less and eat more to bring your glucose up so it helps support progesterone production.” A study shows that fasting boosts GnRH production, but Pelz says this is only the case in women for days 1-12 when oestrogen is building, once you’ve ovulated and progesterone levels start to increase, it’s best to stop fasting.

It’s a different story for men, Pelz advises. “Male hormones are not as sensitive to spikes in insulin and cortisol so men can fast intermittently all month long if they wish.”

How to start intermittent fasting and what to eat

Before you start, Pelz says it’s best to do some preparation. “Spend the first day or two just cleaning up three things,” says Pelz. “Firstly, watch the quality of the fats you’re eating - eat good fats - avocado, nuts, and cod liver oils, not bad fats such as trans fats found in biscuits and processed food.”

Secondly, she advises cutting back on carbohydrates such as bread, cake, cookies and processed carbs, which raise blood sugar levels. Thirdly, cut out any toxins such as diet soft drinks, which may also raise your blood insulin levels, although researchers are unsure exactly why this is. “This will make your switch into the fat-burning mode (rather than the sugar-burning mode) much easier because you’re stabilising your blood sugar,” Pelz says. She also recommends we get used to the fasting time frame by pushing breakfast back an hour or bringing forward dinner an hour.

“My clinical observation has been that most people find their fasting groove within an eight-hour window that runs from around 11am to 7pm,” says Pelz. If you find it easy and feel good during the fast, then maybe try moving on to more advanced fasts like 24-hour fasts, one to two times per week.


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