This essay argues that while fasting may provide metabolic benefits for some populations, it has significantly more negative implications for female athletes and the full extent to this is yet to be discovered.
Fasting is the restriction of food intake to specific time windows, often justified by the evolutionary arguments that humans have adapted to intermittent food availability before farming was developed. Some scientists believe that our bodies still operate in this same mode and neuroscientist, Mark Mattson refers to this as ‘metabolic switching’. This is when the body switches to using fat stores for energy instead of the preferred glycogen stores during times of low energy availability.
A major flaw in fasting research is that the majority of studies are based on male subjects, largely because male hormones are more stable making it easier to investigate the role of fasting on the body. The findings are then commonly generalised to females despite emerging evidence demonstrating sex-specific physiological adaptations. Many athletes utilise fasted exercise as a way to enhance fat oxidation/ lipolysis when glycogen levels are depleted. This can improve metabolic health and utilisation of fats as an alternative energy source, especially important for endurance athletes who may exhaust glycogen stores before the end of their race. However, the extent to which these adaptations translate to race day performance remains debated.
One of the most significant benefits to fasting, potentially common to both sexes, is autophagy. This is a process where senescent cells, which persist abnormally but cause inflammation are broken down and damaged components are removed. This is supported by a meta-analysis where eighteen eligible randomised controlled trials were selected, showing that “intermittent fasting regimens significantly reduced C-reactive protein (CRP)”, a marker of inflammation. Therefore, fasting can increase resistance to oxidative stress which may slow aging and reduce risks of diseases such as cancer or diabetes. Fasting also stimulates increased mitophagy, the specific removal of dysfunctional mitochondria resulting in more efficient energy production.
Fasting is often proposed to enhance metabolic health and a study examining this investigated whether “fasting activates AMPK and PGC-1a in mice”. However, a systematic review from 2024 showed that “results do not support activation of AMPK/PGC-1a by fasting in human muscle”. They also “observed inconsistent effects of fasting on AMPK/PGC1a in rodent muscle”. They concluded that data supporting the benefits of fasting on human muscle mitochondria was inconsistent. AMPK and PGC-1a both activate fatty acid oxidation; however, this research questions as to whether fasting is worthwhile for endurance athletes seeking mitochondrial and fat utilisation adaptations.
Alongside the benefit of autophagy, positives from increased ketone production can be experienced with fasting. Ketones are produced by the liver from fatty acids and can supply up to 75% of the brain’s energy. Fasting may therefore help preserve memory function and enhance cognition. While this can positively impact those that are sedentary, athletes may already achieve similar adaptations from exercise alone, reducing the additional need for fasting.
Many athletes, predominantly those in endurance events, use fasting to maximise cellular efficiency. During races such as an Ironman or the Tour de France, glycogen stores are often depleted near the end of the event. In order to prevent this, many professionals are now consuming more than 90g of carbohydrates an hour and the importance of fuelling correctly has skyrocketed recently. For example, Solvieg Losveth reportedly consumed 132g of carbohydrates per hour during her Ironman World Championship win in Kona 2025. She described feeling “really good on the bike, but it started to feel hard on the run”. This ‘bonk’ in energy levels or ‘hitting the wall’ is what athletes desperately try to avoid. This highlights that optimal performance relies heavily on carbohydrate availability and it challenges the necessity for fasting-based adaptations.
Although, secondary fuels like fats are utilised when glycogen stores are low and athletes who improve the efficiency of fat oxidation may gain an advantage if ‘bonking’ does occur. By regularly depriving the body of readily available glucose the cells are forced to undergo fat oxidation more frequently. However, further evidence often fails to support consistent performance benefits, especially for females.
An active voice in female health and exercise is Dr Stacy Sims who regularly advocates that “women are not small men” and she acknowledges the physiological differences between genders. Her view is that fasting in active women often “leads to more harm than good”. Neuropeptides (chemical messengers produced by neurons) play a key role in reproductive health, hormone regulation, blood sugar control, appetite and body composition. Specific ones like Neuropeptide Y are lower in women during rested, parasympathetic states but increase at a greater rate in fasted states compared to men. This stimulates a greater conservation of energy, which is exacerbated further by the additional stimulus of exercise. In the short-term, a physiological drive to increase food intake is more common in women. If this energy demand is not met, long-term thyroid function and the menstrual cycle can become disrupted and body fat percentage often increases. Therefore, fasting can negatively impact female performance and it increases the risk of low energy availability and bone injuries due to a deregulation of bone protective oestrogen. A 2021 meta-analysis also found that exercising in a fed state and having a post-exercise meal resulted in lower overall daily calorie intake. In comparison, exercising fasted and skipping a post-exercise meal led to the lowest energy expenditure, due to reduced movement and the highest hunger later in the day.
A 2024 female-only study also found that exercising in a fed state improved both appetite control and blood glucose regulation. Overweight and sedentary populations often benefit from a modified calorie intake and better blood glucose regulation, whereas for athletes – particularly females – fasting increases resting blood glucose and sympathetic drives (stress response).This can impair recovery and worsen sleep quality which is detrimental to performance. It was also commonly observed that muscle mass was lost as protein can also act as a secondary fuel instead of glycogen. Many athletes already find it difficult to consume the recommended amount of protein for active individuals and this additional stressor will not aid muscle synthesis.
Dr Mindy Pelz also specialises in women’s hormonal health, but she suggests that fasting should be synced along with the phases of the menstrual cycle instead of adopting the more common approach of time-restricted eating. She recommends longer fasts during day 1-10 of the cycle as oestrogen is rising, shorter fasts during ovulation when the body is active hormonally and no fasting during days 20-28 when more energy is needed to support progesterone production and the parasympathetic system. Her underlying message is to listen to the body and honour the monthly flow of hormones rather than work against them. However, much of her evidence is clinical rather than from peer-reviewed research. This means confirmation bias may be a problem, and so the outcomes may not be universally replicable or reliable. In comparison, Dr Stacy Sims who recommends against fasting for female athletes (which she believes to be anyone who exercises intentionally) bases her conclusions on controlled studies that are subject to scientific scrutiny. Dr. Pelz’s advice of fasting from day one of menstruation contradicts the increased energy demands of the female body and ignores common signals such as increased cravings. Such mixed advice represents the lack of evidence-based guidelines and experts may interpret what female athletes require based on male data. There is a necessity for more research to be conducted in this field to confirm whether fasting is a performance enhancer for the female athlete specifically.
A major problem of fasting is the increased risk of REDS (Relative Energy Deficiency in Sport), especially for females due to increased hormonal sensitivity. Symptoms include increased illness, increased bone and soft tissue injuries due to delayed recovery and menstrual dysfunction. In males, testosterone levels may decrease, leading to performance decline. Although short-term results may improve, this is not long-lived. Commonly, athletes suffer mental symptoms like irritability and poor concentration too. The energy and training demands for athletes are exceedingly high and fasting – particularly in the morning – can put the body into a state of low energy availability even if daily caloric intake is met throughout the day. This delays recovery post-sessions, resulting in fatigue and an increased risk for developing REDS. To avoid low energy availability but still maximise the supposed benefits of fasting, Dr Stacy Sims recommends working with the body’s circadian rhythm.
This involves not eating right before sleep to allow for a natural overnight fast and pre-digestion of food, which results in better sleep quality too. Breakfast or a small snack should be eaten within thirty minutes of awakening to blunt the cortisol peak that occurs in the morning. This is especially vital for females to aid returning to a parasympathetic state.
In conclusion, fasting has differing implications for males and females, especially in athletic populations. While some benefits like increased autophagy and improved metabolic efficiency may exist, evidence suggests that these are inconsistent. Female athletes are more vulnerable to negative hormonal, metabolic and performance consequences due to increased sensitivity to energy availability. Therefore, fasting is unlikely to be a performance tool for females, and similarly caution should be applied to male athletes too. Research on fasting in females, especially athletes, remains incredibly limited. Most studies investigate men, animals or post-menopausal women. A meta-analysis on fasting in females noted that “very few studies have been conducted in this topic area” making comparisons difficult. This lack of data represents a critical gap in biological research, limiting the ability to make accurate, sex-specific recommendations. Further research must prioritise female-specific studies to ensure that athletes receive safe, effective and individualised guidance.
H. Sari, Lower Sixth
