intermitting fasting

Fasting has been used for different reasons over the years, mainly for religious/spiritual purposes, but also for political reasons like the 1981 Irish Hunger Strikes.

Intermittent fasting is currently one of the most popular weight loss trends worldwide (Gunnars, 2020).  It involves cycles of fasting and eating.  It doesn’t specify which foods you can or cannot eat, just when you can eat them.  Like the Paleo diet, one theory behind intermittent fasting is that we should eat more like our ancestors, but while the Paleo diet looks at what our ancestors ate, intermittent fasting looks at how they ate.  It is believed that our ancestors would have only eaten when they could get food which probably wasn’t every day. 

There are different methods of intermittent fasting (Gunnars, 2020). 

  • 16/8 – This method is the most popular as it’s easiest to maintain.  It involves skipping breakfast and restricting your eating period to 8 hours.  For example, you would eat from 12pm – 8pm, then fast from 8pm – 12pm.
  • Eat, Stop, Eat method – you fast for 24hours once or twice per week.
  • 5:2 diet – You only consume 500kcals on 2 non-consecutive days but eat normally for the 5 other days in the week

Any of these methods should lead to weight loss if you don’t binge during the ‘eat’ periods.  Below are a few health ‘Pro’s’ and ‘Con’s’ associated with following this type of diet.


  • Can be less time consuming as you’ve fewer meals to prep, cook and clean up after (unless you have children, intermittent fasting is NOT suitable for children or teenagers)
  • It can be an effective weight loss tool if followed properly (no binging).  Short term it may help boost metabolism and loose harmful abdominal fat.  One reason for this is that fasting leads to an increase in the production of human growth hormone which has been shown to increase fat loss and muscle gain (Ho et al., 1988; Hartman et al., 1992; Blackman et al., 2002)
  • Some studies showed an improved insulin sensitivity so less insulin is needed making fat stores in the body more accessible to be burned for energy (Heilbronn et al., 2005).  It may also reduce insulin resistance and lower blood sugar levels, which are major risk factors for developing Type 2 Diabetes.
  • May reduce inflammation (Johnson et al., 2007; Aksungar et al., 2007).
  • May improve heart health by supposedly decreasing LDL cholesterol (bad cholesterol), blood triglycerides, blood pressure. inflammatory markers, blood sugars and insulin resistance (Barnosky et al., 2014; Varady et al., 2009; Azevedo et al., 2013)
  • May improve brain health by increasing the brain hormone BNDI, and this may help aid the growth of new nerve cells.  It may also protect against Alzheimer’s disease (Lee et al., 2000; Mattson, 2005; Duan et al., 2003).


  • Has some side effects such as hunger during fasting periods, feeling weak or lightheaded, brain fog.
  • Is thought to be less effective, and maybe even harmful for females (Sharp, 2021).  One study shown that IF improved insulin sensitivity and blood sugar levels of males but worsened them in the females.  A study on rats shown that Intermittent fasting caused the female rats to become emaciated, more masculinized, infertile and to miss cycles (Gunnars, 2020, 101).  Anecdotal evidence has shown that women who follow Intermittent fasting periods stop and when they start eating normal again their periods come back (Gunnars, 2020, 101).  I know that might not sound like a bad thing for some females, but when you stop having periods, unless you are pregnant or premenopausal something isn’t right!
  • Could spiral into disordered eating or an eating disorder if not closely monitored.
  • Interferes with the social aspect of eating (Sharp, 2021).  If you go to a celebration, like a birthday, wedding, graduation etc, you might find it difficult if the timing doesn’t coincide with your ‘eating’ period.  It could also lead to feelings of guilt if you do eat outside your eating period, and feelings of annoyance from friends and family if you become very obsessive over when you can and cannot eat.
  • Some people, especially if you already suffer from IBS, can experience digestive discomfort from having to eat large amounts of food in short periods of time (Sharp, 2021).
  • Fasting for a prolonged period can cause the body to go into ‘starvation mode’ (Sharp, 2021).  During this time metabolism slows down as the body tries to conserve energy for the most essential functions (heart beating, breathing, brain etc).
  • It can also be hard to maintain, especially if you get hungry.  You’re going against human nature which is to find food and eat when hungry.

Be aware that there have been very limited long-term studies on the effects Intermittent fasting has on humans, most of these studies have been carried out on animals, mainly rats.  A lot more long-term research is required before any of these claims can be made for certain. 

Anecdotal evidence (that means evidence not from scientific studies), as claimed that Intermittent fasting can lead to improvements in a variety of illnesses like insulin resistance, asthmas, seasonal allergies, osteoarthritis, neurological conditions, cardiac arrythmia, infectious diseases, autoimmune diseases and hot flushes (Johnson et al., 2006)….seems a bit too good to be true in my opinion, but like I mentioned earlier the actual proven effect Intermittent fasting has on chronic diseases is still unclear and requires a lot more long term human trials to be carried out (Johnstone, 2015).  One reason that these claims are being made could be because most of these chronic illnesses have been shown to improve with weight loss if the person is overweight, and like I said earlier.  Intermittent fasting if done properly and safely should lead to weight loss.  For example, one study looked at the effects of intermittent fasting on overweight adults with asthma.  After 2 weeks of Intermittent fasting their lung function, mood, energy and weight improved with and average weight loss of 19lbs (about 1.4 stone) in 8 weeks.  These improvements could have been due to the weight loss. 

Due to the lack of human studies done there is no solid proof that intermittent fasting will lead to an eating disorder, but it is strongly recommended that if you have a history of disordered eating this diet is not for you.  One study claims that Alternate-day fasting doesn’t cause any harmful psychological effects (such as developing an eating disorder) (Hoddy et al., 2015).  However, the studies that looked at the psychological effects of Intermittent fasting specifically excluded those with a history of disordered eating (Schaumberg et al., 2015).

Usually improved cholesterol levels comes with weight loss, so why wouldn’t the claim that intermittent fasting may improve cholesterol levels not stick?  Well there have been a few human studies that contradict this claim.  One study had 2 groups.  One group followed an Intermittent fasting diet and the other followed a daily calorie restriction diet.  After 1 year the Intermittent fasting group showed significantly increased LDL cholesterol levels (that’s the bad cholesterol that clogs arteries), compared to the other group (Trepanowski et al., 2017).  Another study backed up these results by claiming that after 24 hours with no food LDL cholesterol levels may be increased due to so much fat being released into the system (Horne et al., 2015) (fat stores in the body are being burned for energy due to no carbohydrates being available).  One study did show that the LDL cholesterol levels started to decrease again by 7-33% once weight starts to decrease (Antoni et al., 2016).  Those were all relatively small studies though.  The largest human trial looking at the effects of calorie restriction on overweight adults looked at 100 obese males and females.  They were randomly placed into 1 of 3 groups.  The Alternate-Day fasting group, the daily calorie restriction group and the control group who maintained their normal diets.  The results showed that although the Alternate-Day fasting group and the daily calorie restriction group lost, on average, the same amount of weight, the cholesterol effects were very different.  In the calorie restriction group LDL cholesterol decreased, but in the Alternate-Day fasting group their LDL cholesterol levels didn’t drop and by the end of the year they were actually 10% higher than the calorie restriction group who had almost exactly the same weight loss (Trepanowski et al., 2017).  High LDL cholesterol levels is one of the biggest risk factors for the world’s number one killer.  Heart Disease.

In Conclusion I think that loosing weight by cutting down on the amount of calories you take in on a daily basis, and of course trying to eat a balanced healthy diet is probably the safest and best course of action.  Intermittent fasting just doesn’t have enough full proof evidence backing it up.  Also, any diet that ignores hunger and can possibly lead to disordered eating, infertility, increased LDL cholesterol and digestive issues in my opinion, isn’t a good idea.


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Antoni, R., Johnston, K.L., Collins, A.L. and Robertson, M.D. (2016) Investigation into the acute effects of total and partial energy restriction on postprandial metabolism among overweight/obese participants. The British Journal of Nutrition, 115(6) 951–959.

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Ho, K.Y., Veldhuis, J.D., Johnson, M.L., Furlanetto, R., Evans, W.S., Alberti, K.G. and Thorner, M.O. (1988) Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. Journal of Clinical Investigation, 81(4) 968–975. Available from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC329619/ [accessed 20 January 2021].

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Horne, B.D., Muhlestein, J.B. and Anderson, J.L. (2015) Health effects of intermittent fasting: hormesis or harm? A systematic review. The American Journal of Clinical Nutrition, 102(2) 464–470.

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Johnstone, A. (2015) Fasting for weight loss: an effective strategy or latest dieting trend? International Journal of Obesity (2005), 39(5) 727–733.

Lee, J., Duan, W., Long, J.M., Ingram, D.K. and Mattson, M.P. (2000) Dietary restriction increases the number of newly generated neural cells, and induces BDNF expression, in the dentate gyrus of rats. Journal of molecular neuroscience: MN, 15(2) 99–108.

Mattson, M.P. (2005) Energy intake, meal frequency, and health: a neurobiological perspective. Annual Review of Nutrition, 25 237–260.

Schaumberg, K., Anderson, D.A., Reilly, E.E. and Anderson, L.M. (2015) Does short-term fasting promote pathological eating patterns? Eating Behaviors, 19 168–172.

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Varady, K.A., Bhutani, S., Church, E.C. and Klempel, M.C. (2009) Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. The American Journal of Clinical Nutrition, 90(5) 1138–1143.

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