Intermittent Fasting/Restricted Eating Windows: A Word of Caution for Active Women

Intermittent Fasting/Restricted Eating Windows: A Word of Caution for Active Women

Intermittent fasting (IF), the act of abstaining from eating for a certain period in the day (or for a set amount of days), is another approach to eating that has risen in popularity for the purpose of weight management. When it comes to managing one’s weight, IF has not demonstrated superiority to eating at regular intervals so long as calories consumed are the same [1-2]. With that said, some find IF makes it easier to consume less calories meaning it can still be a helpful strategy for weight management. However, when it comes to the topic of exercise performance and reproductive health, IF may not be so ‘take it or leave it’.

IF’s effect on training has been shown to be neutral, if not detrimental, to performance [3] but low energy availability (not eating enough to support your body and activity levels) has been associated with declines in performance, unfavorable changes in body composition, lowered immune response and also menstrual dysfunction (MD). MD, when escalated, can lead to more serious health complications in later years [4]. One might assume that so long as an athlete finishes up the day with enough calories to support their needs, even if consumed in a condensed eating window, that such unwanted adaptations would not occur. A study by Fahrenholtz et. al. (2018) suggests that within day energy balance may be equally important to protect a female athlete from their reproductive system being affected by negative hormonal adaptations which contradicts that assumption. The data from this study is interesting as it could mean that an active woman going too long without eating could face similar consequences to those if she was under-eating, even when getting in enough calories by the end of the day.

25 competitive female endurance athletes were assessed over a 7 day period where their within-day energy balance was monitored. Of the 25, 15 had menstrual cycle disturbances though all 25 were eating enough to maintain their activity levels. What was the difference with those 15? They spent more time in a catabolic state/negative energy balance over the course of the day. Basically, they went longer periods without adequate calories to fuel their bodies. The authors found ”within-day energy deficit to be associated with higher cortisol and lower estrogen levels and a trend suggesting lower T3 levels, as signs of biological stress. LH pulsatility, T3 and cortisol levels are regulated by brain glucose availability hence extensive periods in an energy deficient state are likely to cause these hormonal alterations.” Interestingly, in another study similar adaptations were also seen in men where ‘’despite similar Energy Availability, athletes identified with suppressed RMR spent more time (21 vs. 11 h) with an energy deficit exceeding 400 kcal across a 24-h period. Within-day energy deficiency was associated with higher blood cortisol level and lower testosterone: cortisol ratio’’ [4].

This is something that needs further research but what can we take from these findings? It is a demonstration of the complexity of the body and how easy it may be to negatively impact our hormones if we do not respect our needs. It also may provide an additional precaution to consider for women trying to regain their menstrual cycles or alleviate menstrual cycle dysfunction brought upon by restriction, over training or a combination of the two. If you are trying to regulate your cycle after dieting, a tough competition prep or years of restriction, this could suggest that your eating window is something that may influence your success. These findings suggest that 24 hour energy intake being adequate may not be enough to ensure optimal reproductive function if there are still long enough stretches in a catabolic state. Though things like fasted training and skipping breakfast may seem harmless, it may be prudent for active women and women wishing to regain/regulate their cycles to reconsider these habits until more is understood on this matter. Please keep in mind that this study shows those negative hormonal changes in competitive endurance athletes which does not necessarily mean that it is applicable to recreational athletes or your casual gym goer. Despite the participants of this study being limited to competitive endurance athletes I’d personally recommend females use caution around IF, due to potential risks, until more information becomes available. This form of meal timing may put unnecessary stress on the body, and may serve as a hindrance to maintaining a healthy menstrual cycle for those whose reproductive systems are less robust.

What could make your reproductive system less robust? One factor could be your age as younger women may have less stable reproductive systems than women who have been menstruating for 14 years or more [5]. But it may ultimately come down to the fact that some women may simply be more sensitive than others to stressors like alterations in energy balance. For such women, smaller eating windows and fasting may have the potential to bring upon hormonal changes that disrupt or impede restoration of proper reproductive function. It is also important to note that a woman may be menstruating regularly but still be experiencing a menstrual dysfunction called luteal phase dysfunction (LPD). LPD is not always detectable via cycle length but would require reliable ovulation predictor kits plus blood tests to ensure that your progesterone levels are rising to adequate levels [6]. This is why I now recommend that the practice of IF be avoided altogether by active women until more research has been done on this topic. This recommendation is to eliminate the chance of unnecessary strain on your body and can also be a measure to protect yourself from unwanted alterations to your menstrual cycle happening without your knowledge.

Since IF isn’t a requirement for either improved health or exercise performance, and may actually impact both negatively, here is a summary of my recommendations for active women whose goals are to be strong, fit, and healthy:

– Eat enough calories to support your needs
– Do not voluntarily skip or delay meals
– Ensure adequate pre and post-workout intake to support optimal performance, recovery, health, and function

REFERENCES

[1] Seimon RV, Roekenes JA, Zibellini J, Zhu B, Gibson AA, Hills AP, Wood RE, King NA, Byrne NM, Sainsbury A. Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Mol Cell Endocrinol. 2015

[2] Stockman MC, Thomas D, Burke J, Apovian CM. Intermittent Fasting: Is the Wait Worth the Weight?. Curr Obes Rep. 2018

[3] Levy E, Chu T. Intermittent Fasting and Its Effects on Athletic Performance: A Review. Curr Sports Med Rep. 2019

[4] Logue DM, Madigan SM, Melin A, et al. Low Energy Availability in Athletes 2020: An Updated Narrative Review of Prevalence, Risk, Within-Day Energy Balance, Knowledge, and Impact on Sports Performance. Nutrients. 2020

[5] Loucks AB. The response of luteinizing hormone pulsatility to 5 days of low energy availability disappears by 14 years of gynecological age. J Clin Endocrinol Metab. 2006

[6] Schliep KC, Mumford SL, Hammoud AO, et al. Luteal phase deficiency in regularly menstruating women: prevalence and overlap in identification based on clinical and biochemical diagnostic criteria. J Clin Endocrinol Metab. 2014