Intermittent fasting is all the rage at the moment, with lots of studies currently being undertaken about its potential health benefits, whether it be to lose fat, prolong lifespan or generally enhance our health. But before embarking on this diet, it is important to know what intermittent fasting is, how to fast and more about this way of eating.

What is fasting?

Fasting refers to any abstention from eating, however there are lots of different types of fasting. The widespread use of fasting for health benefits was first raised in the 1930s, when an experiment on rodents in which their calories were restricted resulted in a longer lifespan.

How does fasting work?

Intermittent fasting involves not eating for a period of time, and then eating freely for the remainder of the day/week. Fasting works due to the caloric restriction. As a result of not eating for a particular amount of time, people are less likely to consume the number of calories they otherwise would have done in a restricted timeframe. The habit of fasting can even lead those to eat less on non-fast days out of habit. 

There are states that the body goes through when fasting:

  • The postprandial period: this is the period from when you start eating, up to 4 hours after, while the food is digested.
  • The post-absorptive state: this is after the postprandial period and lasts 6 hours. The glycogen stores are broken down and fat is used for energy.
  • The fasted state: this does not officially begin until 10 hours after your last meal, when both the postprandial and post-absorptive states have ended.

Put simply: the body stores energy from food as fat when you constantly eat, however when fasted, the body is more likely to use its own fat stores for energy.

As the fasted state does not begin until 10 hours after eating, some people may never be in a fully fasted state if they finish eating late at night and eat breakfast early in the morning. As a result, they are not allowing the body to use fat as fuel.

Additional changes that happen within the body when fasted are:

  • Human Growth Hormone (HGH) levels increase, benefitting fat loss and muscle gain
  • Insulin sensitivity improves, with levels dropping. This makes stored body fat more accessible for use as fuel by the body, as opposed to food

What are the benefits of fasting?

  • Reduction in the risk of diseases such as diabetes and cardiovascular disease
  • Reduction of ghrelin levels (the hormone that stimulates hunger)
  • Reduction of inflammation
  • It can help with fat loss. Fat loss is essentially about calories in vs calories out, and when you fast you are restricting the hours within with you can eat, restricting your caloric intake and making you less likely to consume as many calories.
  • It simplifies your day of eating. You don’t have to prepare every meal, you can simply skip either breakfast or dinner and only have to prep the meals you will be eating.
  • Live longer: there have been studies to suggest that restricting calories in an intermittent fasting pattern could prolong life.
  • Could reduce cancer risk: there have only been a few studies on this, but the results appear positive.

What are the types of fasting?

Traditional intermittent fasting involves days of intuitive eating followed by fast days. This has numerous benefits, such as not needing to calorie count on fast days and easily fitting socialising into your diet, however fasting for 36 hours at a time can be difficult to maintain. An example of this way of eating would be finishing dinner at 8pm and fasting until 8pm the next day.

Another form of fasting is limiting your food intake on “fast” days, to around 50% of the Basal Metabolic Rate (BMR). The intake on these days is low enough to still reap the metabolic benefits of fasting, but are easier to adhere to. An example of this is the popular 5:2 diet in which you fast 2 out of 7 days of the week. This allows you to eat flexibly on the non-fast days, making it more maintainable, while also allowing for some food to be eaten on the fast days. However, this can be difficult to maintain long term and could still restrict activities on the fast days.

Another form of fasting is time-restricted feeding (TRF), which involves manipulating the meal pattern within the day. The most widely-known approach is the 16:8, or the “Leangains” approach, in which you fast for 16 hours out of the day, and eat in the remaining 8 hours. This approach creates around 6 hours of actual fasting state every 24 hours, given the states the body goes through. This is popular as people find it easier to adhere to, just having to skip either breakfast or dinner, or bringing one of the two meals earlier or later. As a result of this, people tend to eat less in their free eating hours. This allows you to eat every day, means you don’t necessarily have to calorie count or restrict food in the 8 hours you do eat (depending on your goals). However, this can make socialising difficult due to restricted eating hours, and could potentially cause overeating in the 8 hour window due to the desire to fit in as much food as possible. An example of this way of eating would be eating from 12pm to 8m, and then fasting for the rest of the day and night.

What are the potential drawbacks of intermittent fasting?

  • Potential lower energy.
  • Hunger when starting intermittent fasting. For the first couple of weeks, you might feel hunger when fasting, however your body really quickly adapts so this won’t last long.
  • If you have a medical condition, you should consult with your doctor before attempting intermittent fasting, particularly if you:
    • Have diabetes.
    • Have problems with blood sugar regulation.
    • Have low blood pressure.
    • Are underweight.
    • Are a female who is trying to conceive.
    • Are a female with a history of amenorrhea.
    • Are pregnant or breastfeeding.


Can I drink liquids while fasting?

Yes. Any liquids which don’t contain calories can be consumed, such as water, tea and coffee.

Can I work out while fasted?

Yes. Some people recommend taking branched-chain amino acids BCAAs before and during a fasted workout. However, fasted training is quite controversial. It has been claimed that, without glycogen stores from food, the body is forced to use the fat stores in cells to fuel the workout, therefore burning more body fat. However, if you feel lightheaded or low on energy while working out, make sure you fuel yourself before – it all depends on the person. Fasting also increases the body’s insulin sensitivity, meaning you’re more likely to use the food you consume after the workout more efficiently, potentially helping with fat loss.

Can I lose weight with intermittent fasting?

Weight loss is commonly why people decide to try out intermittent fasting. By eating fewer meals per day, your daily calorie intake is reduced. However, the other benefits of fasting, such as hormone level changes and insulin sensitivity, can also facilitate weight loss. It is really important to remember that the main reason people lose weight when fasting is due to a lower calorie intake, so if you eat in a caloric surplus during the eating periods, you will not lose any weight.

Should Women Fast?

There is evidence that intermittent fasting may not be as beneficial for women as for men, as well as reports of women who suffered from amenorrhea (loss of period) when they started fasting. However, some women have great results with intermittent fasting. It is important to take caution with intermittent fasting and stop immediately if you experience issues such as amenorrhea.

Does fasting cause muscle loss?

All weight loss methods can cause muscle loss if you are not fuelling yourself and training adequately, so ensure you weight train and keep protein intake high.

How can I start Intermittent Fasting?

The chances are that you’ve actually practiced intermittent fasting before. If you’ve eaten dinner, then slept in and not eaten until around lunchtime the following day, you’ve already done a 16 hour fast. Many people actually instinctively eat like this! It is generally found to be the case that the 16/8 method is the simplest way to start intermittent fasting. Another approach is to simply fast whenever it is convenient for you: skip meals when you’re not so hungry or don’t have time to cook.

Is Intermittent Fasting necessary?

Intermittent fasting is not something that you need to do. It is just one of many ways of living that can have health benefits. As always, the most important thing to ensure you do is eat whole foods, exercise regularly and sleep enough. It can certainly be beneficial for the body BUT it’s not the only way to eat and is by no means necessary. It is important to remember that intermittent fasting is not a diet, but a way of eating, so you can still gain fat or lose fat.