What is Intermittent Fasting

What is intermittent fasting?

Intermittent fasting is an eating pattern that cycles between fasting and eating. When fasted, people abstain from food and calorie-containing drinks. People fast anywhere from 12-24 hours. Unlike diets that tell you what to eat intermittent fasting focuses on when to eat.



How does intermittent fasting work?

Having less time to eat each day may help you eat less food and consume fewer calories. Recent research into time-restricted feeding found that participants who ate within a 4 or 6-hour window ate on average 550 kcal a day less than the control group. Studies show that you can lose between 3-8% of your weight within 3 months by intermittent fasting.

Even though researchers have not found much difference in the degree of weight loss achieved with intermittent fasting compared with other dietary regimens it appears you can achieve metabolic benefits with intermittent fasting even without weight loss. Fasting promotes changes in metabolism, cellular processes and hormonal secretion, all of which result in improvements in insulin sensitivity and reductions in glucose levels, blood pressure and lipids (blood fats). In humans, 12-24 hours of fasting significantly reduces blood sugar levels and causes the body to switch to a ketogenic mode, using fatty acids and fat-derived ketones for energy. 

There have been dozens of animal studies which have reported that when fasted, cells initiate processes resulting in increased DNA repair, autophagy (where cells digest and remove damaged protein and cells, in order to regenerate newer, healthier cells) and reduced inflammation, all key drivers to many chronic diseases and ageing. Cells throughout the bodies and brains of animals maintained on intermittent fasting regimens show improved function and resistance to a wide range of potentially damaging stressors. However, all these results need to be confirmed in human trials. 


What is circadian biology?

The circadian rhythm is a 24-hour internal clock that regulates our sleep-wake cycle. It also controls many of our body’s functions including hormone levels, metabolism and even how we respond to medication. The master biological clock is controlled by light and dark stimuli. Food is an important signal for the rhythms of the biological clocks, including those that control our metabolism. In the morning, in response to daylight and food, insulin sensitivity increases and melatonin decreases, making you feel alert and energized. Meanwhile, in the evening, melatonin levels go up and insulin sensitivity decreases, preparing your body for rest and cell repair.

Eating at night, outside the normal eating time, may alter our biological clock and disrupt energy balance, resulting in weight gain and metabolic trouble. Researchers found that nightshift workers have disrupted biological clocks and have an increased rate of obesity, metabolic disease and cancer. One study showed that forcing people to stay up late just a few nights in a row resulted in quick weight gain and reduced insulin sensitivity, changes linked to type 2 diabetes. In another study, when women who were overweight or obese were put on a diet for three months, those who ate most of their calories at breakfast lost two and a half times more weight than those who ate most of their calories in the evening, despite similar caloric intake and physical activity levels.

So it appears that when you eat is as important as what you eat when it comes to human health and risk of disease. It is possible that some of the benefits of intermittent fasting are mediated through circadian biology and so intermittent fasting may be used to counteract disruptions to the circadian rhythm caused by jetlag and night shift work for example.


What are the different types of intermittent fasting?


  • Alternate-day fasting


With alternate-day fasting, you alternate fasting and feeding days. You fast for 24 hours every other day.

During fasting days no calories are consumed. You can still drink calorie-free drinks such as water, unsweetened black coffee and tea. During feeding days, you can consume food and drinks as usual with no restrictions.


  • Modified alternate-day fasting


Modified alternate-day fasting involves alternating very low-calorie days and feeding days.

During the very low-calorie days you eat only 25% of your calorie needs (between 400 and 600 kcal a day). During feeding days you can consume foods and drink as usual with no restrictions.


  • The 5:2 diet


The 5:2 diet involves fasting on 2 days per week. On the fast days, you consume between 500 -600 kcal per day. During feeding days you can consume food and drink as usual with no restrictions.



Time-restricted feeding involves shortening the time you eat every day to an eating window that ranges from 4-10 hours.

The most popular time-restricted feeding methods are the 16:8. There is no intentional calorie restriction during eating windows.


Is intermittent fasting safe?

Evidence suggests that intermittent fasting is safe and tolerable for healthy adults. However:

  • If you are underweight or have a history of eating disorders you should not fast.
  • People under the age of 18 years should avoid fasting.
  • People with type 1 diabetes or people on insulin should consult their health professional first.
  • People with any major health condition should not start a fasting regimen without consulting their doctor.


What can you drink while fasting?

  • You can consume water freely
  • Unsweetened tea and black coffee
  • Artificial sweeteners and diet drinks can be consumed however some studies have shown that they may alter blood sugar so it is best to avoid artificial sweeteners if you can.
  • However adding milk, honey, cream or sugar would break the fast.


Is it better to skip dinner than to skip breakfast during time-restricted feeding?

Studies have shown that eating earlier rather than later in the day would be more beneficial for health. Insulin sensitivity appears to decrease throughout the day. This means that you may not respond as well to carbohydrates later in the day. It would, therefore, be better to skip dinner than breakfast.

However, it is better to set your eating window according to your own preferences so it is sustainable. If you get hungry later in the day rather than earlier, then set up your window for later. 


How do I break my fast?

Avoid breaking your fast with simple sugars because doing so could result in a rapid rise in blood sugar and insulin. Instead tried starting with a meal high in fibre and protein and low in simple carbs.


Am I at higher risk of experiencing a rebound effect?

Rebound effect happens when you are metabolic rate decreases after severe calorie restriction. This can happen if you fast for 30 hours or more. Fasting between 12 and 24 hours is unlikely to affect your metabolic rate. Try to eat enough protein in your eating window and exercise regularly to maintain your metabolic rate.



Intermittent fasting is an effective tool for weight loss, at least as effective as other dietary regimens. It may have health benefits that are independent of weight loss, such as improving insulin sensitivity, blood glucose, blood pressure and blood fat levels but larger, longer-term clinical trials are needed to confirm this. The best way of achieving weight loss and improving health is different for each individual so a personalised approach is recommended – what is key is to find a healthy balanced dietary programme that you are able to stick to.


By Dr Mohgah Elsheikh