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Intermittent Fasting for Athletes?

By John Corr BSc MSc

It has been quite a few years since I first read about Dr. Michael Mosley and “Intermittent Fasting” (IF). Dr. Mosely utilised the “5 – 2” strategy (5 days normal eating – 2 days fasting). Since that time various forms of IF have been explored and several studies have been conducted. Numerous significant outcomes have been identified in the research including improvements in body composition, improved inflammation levels, heart health and brain health.

Research with Athletes

Less is known regarding the impact of an IF protocol upon the health and performances of athletes. Pons V. et al (2018) evaluated the effects of “every-other-day” feeding calorie restriction (CR) on the body composition and physical performance of well trained athletes. The calorie restriction in this study created a deficit of 33% on CR days. Maximal exercise tests were performed to determine impact upon physical performance. “Well trained” in this study (and in most research) tends to include regular exercisers with a reasonable history of continued training. The participants in this study were males, 18-50 years, training 3-6 times per week with a history of training. 3 days of CR were imposed, every other day over a 6 week period. As mentioned, before and after the CR period, participants undertook a maximal exercise stress test. A blood sample was taken at the beginning and another at end of the IF period.

Calorie Restriction

Each participants daily energy requirement was calculated based upon body-mass, lifestyle, activity levels etc. CR was imposed at 30-40% of normal daily intake for fasting days. In this study the average calorie intake on restricted days was 1500 kcal/day, representing an approximate 800 calorie deficit. (3 days per week restricted, 4 days normal eating). To monitor the overall eating, a detailed food diary was maintained. Researchers confirmed that participants adhered to the protocol throughout. Interestingly, the researchers noted that participants did not overcompensate on “normal” eating days. It has commonly been thought that fasting can lead to over compensation on the following days.


Participants lost a lot of weight. 4.40% of their starting weight, across the trunk (-6.4%), arms (-3.7%) and legs (-4.91%). The main drop in weight comprised bodyfat. A small percentage of lean mass was also lost, exactly how much (2.91%) is a bit unclear because there was a significant drop in body water which results in a shift of water from the lean tissue (muscle) and this creates issues in terms of reading DXA scan data. (This water loss arises as a result of glycogen depletion). Negatives CR days resulted in lower than required intake of certain nutrients including magnesium, potassium, zinc, folic acid and calciferol. These were 60–77% lower than recommended. The CR also significantly limited the ingestion of micronutrients including iron, niacin, riboflavin, pyridoxine and vitamins A and D. Keeping in mind that this was a short term trial, no adverse effects were observed as a result of these nutrient deficiencies however it is possible that a longer term strategy of this nature might lead to a problematic level of nutrient deficiency.


Physical performance was not compromised in fact quite the contrary, from an athletic perspective the IF protocol resulted in improved physical performance parameters. The simple explanation of this is that the drop in bodyweight resulted in a down regulation of metabolism. This resulted in a reduction of heart rates and energy expenditure required for the participants to complete running tests at various speeds (to exhaustion). This is obviously a significant advantage for athletes. The running tests show that the onset of anaerobic phase was pushed out when compared with pre-trial tests. Exercise efficiency was in fact significantly improved (as was cholesterol profile and improvements in vitamin D profile). Bodyfat reductions has a myriad of other benefits (reduced inflammation etc). The concern regarding the nutrient deficiencies observed and the possible concerns over the longer term should easily be addressed with a multi-vitamin supplement. The outcome of this study suggests that there is no reason that individuals training 4 times a week cannot include CR as a means to decrease bodyfat and improve performance

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