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Everything you need to know about intermittent fasting

Although intermittent fasting has been found to aid weight loss due to the calorie deficit involved, it is less of a diet and more of a lifestyle choice which comes with several potential health benefits.


When you think of fasting, you may associate it with religious practiceor as a requirement prior to a medical procedure.

Fasting is something people have practiced throughout history and has played a key role in most of the world’s major religions, being associated with penitence and other forms of self-control. Muslims fast during the holy month of Ramadan, Judaism has several annual fast days including Yom Kippur, the Day of Atonements, while Roman Catholics observe a 40 day fast during Lent.

Our hunter-gathererancestors spent most of their lives fasting, feasting in times of plenty and then facing long periods of scarcity often out of circumstances rather than choice. So it makes sense that our bodies can perform well under the harsh conditions of feast and famine. In recent years the practice of fasting has evolvedintoone of the world’s most popular health and fitness trends. People tend to find this regimen easier to follow than traditional calorie restriction approaches.

What is intermittent fasting?

Intermittent fasting is a process in which people alternate between periods of eating and fasting. Contrary to most eating plans, intermittent fasting does not limit what you eat, instead, it restricts when you eat. Whilst we sleep each night, we are in a state of fasting for several hours, dependent on how much sleep we get. Intermittent fasting simply prolongs that period of fasting either when we wake up in the morning or before we go to bed. For example, when intermittent fasting you may have your first meal at 12pm and your last mealat 8pm,fasting again until 12pm the next day. However, there are numerous different intermittent fasting methods, all of which spilt the day or week into periods of fasting and eating.

Intermittent fasting methods

During the period of fasting no calories are to be ingested, though coffee, tea, calorie-free sweeteners and water are ok. Here are some of the most popular methods;

The 16/8 method – fast for 16 hours of the day dependent on your schedule, for example you’re eating period could fall between 1pm and 9pm and for the remaining 16 hours you will fast. Eat-stop-eat method –involves not consuming any calories for a full 24-hour period, once or twice a week. For example, not eating from 12pm on Tuesday until 12pm on Wednesday. The 5:2 diet – consists of eating only 500-600 calories for 2 days of the week. The reduction of calories involved in these methods often leads to weight loss and is one of the main reasons people follow intermittent fasting.

Intermittent fasting benefits

Although many people use intermittent fasting to lose weight, it has been reported to have other benefits for your body and brain. Here are 4 evidence-based health benefits of intermittent fasting.

Changes to the function of cells, genes and hormones

During a period of fasting, several things happen to your body on a cellular and molecular level. For example, your body adjusts your hormones levels to make your fat stores more accessible often resulting in fat loss. Here are some changes that happen to your body when you are in a fasting phase; Insulin levels: levels of insulin drop considerably, which has been found to aid fat burning (1). Insulin is the hormone which regulates our blood sugar levels and is released into our bloodstreams in a response to eating. Once released, insulin causes the body to move glucose into storage. Human growth hormone (GH): levels of GH have been found to increase dramatically, as much as 5-fold. GH is a peptide hormone that stimulates growth, cell reproduction and cell regeneration in humans. Increased height in childhood is the most widely known effect of GH, yet in adulthood it has been found to increase muscle mass and stimulates the growth of all internal organs excluding the brain (2). Low levels of insulin-like growth factor (IGF-1): IGF-1 levels decrease significantly and stay down after returning to a normal eating pattern. IGF-1 is a growth hormone produced by the liver which plays an important role in childhood growth and stimulates psychological well-being and physical performance in adult life. Low levels of IGF-1 have been found to reduce the risk of colorectal, breast and prostate cancer (3). Cellular repair: cells initiate a natural regeneration process, known as autophagy. The cell removes unnecessary or dysfunctional proteins that build up inside. This can reduce the likelihood of contracting some diseases as well as prolonging lifespan (4).

Lowers your risk of Type 2 diabetes

In the UK alone almost 9 out of 10 people who are diagnosed with diabetes have Type 2 (5). It is estimated that up to three in five cases of Type 2 diabetes can be prevented or delayed by making healthier choices (5). The main attribute of Type 2 diabetes is high blood sugar levels in the context of insulin resistance. Interestingly, a study analysing the impact of intermittent fasting on blood sugar and insulin has shown fasting blood sugar dropped by 3-6%, while fasting insulin reduced by 20-31% in humans (6).

Intermittent fasting for weight loss

The main reason that intermittent fasting aids weight loss is that it helps you to reduce your calorie intake. By skipping breakfast, you automatically lose 20-30% of your daily calorie consumption. A recent study found that intermittent fasting can lead to substantial weight loss. Participants involved in the study sawa 3-7% body weight reduction over a period of 3-24 weeks (7). As well as reducing insulin and increasing growth hormone levels, intermittent fasting can amplify the release of the fat-burning hormone norepinephrine. An increase in this hormone may speed up your metabolic rate by 3.6%-14% (8). However, bear in mind when following intermittent fasting, weight loss may only occur when you are consuming fewer calories overall. If you overcompensate by eating more during your eating periods, you may not lose any weight at all.

Good for your brain

While human research is limited, it has been found that intermittent fasting can improve cognitive function. A study on mice showed that fasting stimulates the production of a protein in nerve cells called brain-derived neurotrophic factor, or BDNF (9). This protein is essential in learning, memory and the generation of new nerve cells. Safety and side effects

Initially, hunger can be the main side effect of intermittent fasting. Fasters can also feel low on energy and occasionally the brain may not function as well as it usually does. This is usually only temporary and should surpass as the body adapts to a new eating cycle. If you have any medical conditions, it is important to consultwith your doctor before beginningintermittent fasting. This is particularly important if you: Have diabetes. Have problems with blood sugar regulation. Have low blood pressure. Take medications. Are underweight. Have a history of eating disorders. Are a woman who is trying to conceive. Are a woman with a history of amenorrhea. Are pregnant or breastfeeding.

Although a vast amount of research already exists on intermittent fasting and its effect on the body there is a lack of long-term research studies, including studies that specifically investigate the impact of intermittent fasting on men and women. It is too early to fully understand the long-term benefits or drawbacks of fasting.


1. Heilbronn, L., Smith, S., Martin, C., Anton, S. and Ravussin, E. (2005). Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. The American Journal of Clinical Nutrition, 81(1), pp.69-73. 2. Ho, K., Veldhuis, J., Johnson, M., Furlanetto, R., Evans, W., Alberti, K. and Thorner, M. (1988). Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. Journal of Clinical Investigation, 81(4), pp.968-975. 3. BBC (2014). How fasting changed my body for the better. [online] BBC News. Available at: [Accessed 22 Aug. 2019]. 4. BBC (2018). Can autophagy be good for your health?. [online] BBC News. Available at: [Accessed 22 Aug. 2019]. 5. Diabetes UK (2018). Number of people living with diabetes doubles in twenty years. [online] Diabetes UK. Available at: [Accessed 22 Aug. 2019]. 6. Barnosky, A., Hoddy, K., Unterman, T. and Varady, K. (2014). Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Translational Research, 164(4), pp.302-311. 7. Zauner, C., Schneeweiss, B., Kranz, A., Madl, C., Ratheiser, K., Kramer, L., Roth, E., Schneider, B. and Lenz, K. (2000). Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. The American Journal of Clinical Nutrition, 71(6), pp.1511-1515. 8. Amigo, I. and Kowaltowski, A. (2014). Dietary restriction in cerebral bioenergetics and redox state. Redox Biology, 2, pp.296-304. 9. Mattson, M., Moehl, K., Ghena, N., Schmaedick, M. and Cheng, A. (2018). Intermittent metabolic switching, neuroplasticity and brain health. Nature Reviews Neuroscience, 19(2), pp.80-80.