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The majority of sports nutrition research is in men: what does that mean for female guidelines?



Depending what area of sports science research you are interested in, analyses have placed research including women at between 11%-40% of total publications, female only studies at under 10% of total publications, and the relative number of female : male participants at potentially less than 1:5 ratio (Costello et al 2014; Cowley et al 2021; Nuckols 2022).


An inevitable consequence of this is that the current sport related guidelines, including nutrition guidelines, are based predominantly on research findings in male participants.


Is this a problem? Do women differ in how they respond to nutrient and supplement intake? And so should there be gender specific guidelines?


We can’t answer these questions with 100% clarity for all areas of sports nutrition as yet. This article will look at a little of what has been found to date, and the similarities and potential differences between male and female responses to food.


It is worth noting before we begin that gender is just one aspect of variability between individuals that may impact nutrition. Genetics, age, lifestyle, sport, trained status, muscle mass, fat mass, health status are other variables that impact nutritional needs. So I for one would not expect gender to necessarily have even the leading role in any differences in nutrient requirements between two individuals. But that is also not to say it does not have a role because men and women are different, with different hormonal profiles and base physiology.


Before we look at specific areas of sports nutrition and gender, it is worth considering why research in females lags behind men.


I think there are at least three significant reasons. The first is that historically men participated in sport more than women, particularly at the elite level. So naturally research was concentrated in males. Second is linked to this: if there are more men in sport, then there are more potential male research participants than female participants. Law of averages if nothing else! Third is that men are biologically simpler than women. Women post puberty either have a menstrual cycle, are on one of a multitude of contraceptives, are peri-menopausal or post-menopausal. Each of these comes associated with a different hormone profile, which may differ depending on the week of the cycle, and each varying ‘state’ has the potential to impact nutrient requirements. So when starting research in a topic that we have no idea about, it is more straightforward and may be more likely to yield insight faster, if the ‘simpler’ model (men) are studied first.


As more women participate in sport at all levels (49% of Tokyo 2020 Olympians were female), as we start to understand more about exercise physiology and sports nutrition in males, and as we start to realise there may be health and performance related differences in nutritional needs there is a greater drive for research in females. And we are starting to see that come through in the number of research studies including women.


So now, let us consider a few key areas of sports nutrition and what we do (and don’t) think we know about women vs men.


Protein

The short answer here is that, based on the research to date, current protein intake recommendations are as applicable to women as they are to men.


A 2018 meta-analysis by Morton et al identified no sex difference in fat free mass gains in response to protein intake with resistance training. They reported an optimal protein intake of 1.6g/kg bodyweight for individuals not in a calorie deficit to lose weight. Although the number of females included in the analysis was relatively low.


A similar finding can be intimated from the 2020 meta-analysis by Tagawa et al. They reported an optimal protein intake of 1.7g/kg bodyweight and this was approximately equivalent in the model that adjusted for sex as a co-variate and the unadjusted model.


Considering the fact that - regardless of gender – the proteins in the human body require all essential amino acids and that protein cannot be stored in the body outside of functional tissue, it seems likely that females will have broadly similar needs to males when it comes to quality and timing of protein intake.


Having said all of this, there may be some nuances to protein metabolism in women across the different phases of the menstrual cycle. There is evidence that high oestrogen levels reduce amino acid oxidation during exercise (Phillips et al 1993; Hamadeh et al 2005). This suggests that in the follicular phase, where the oestrogen to progesterone ratio is high, there may be less protein catabolism during exercise; in particular during endurance exercise, or exercise with low glycogen availability where exercise protein catabolism is typically highest. Whereas in the luteal phase, when the oestrogen to progesterone ratio is low, protein catabolism may be higher (perhaps more aligned with males who have naturally low oestrogen levels). This may not significantly impact total protein requirements, but it may increase the benefit from ensuring protein intake before and after exercise is of increasing importance to recovery and – potentially – muscle mass during the follicular phase. A recent review by Moore et al (2022) explores this topic (and carbohydrate needs in females) in more detail and is well worth a read.


Carbohydrates

Based on research to date, the overwhelming driver of carbohydrate needs is the duration and intensity of exercise undertaken by an individual: “fuel for the work required” (Burke et al 2011; Impey et al 2018). And well trained women at least seem to respond similarly to carbohydrate feeding during exercise and refeeding after exercise, and can load muscle glycogen to the same degree if sufficient carbohydrates are consumed (Wallis et al 2006; Thomas et al 2016; Flynn et al 2020).


However, oestrogen does impact the use of carbohydrates versus fats and so within this there may be subtle differences in the amount of carbohydrates required for a given intensity of exercise, depending on the phase of the menstrual cycle a woman is in, whether she is on hormonal contraceptives and / or if she is peri- or post- menopausal.


High oestrogen levels increase fat oxidation, and hence women tend to burn a greater proportion of fat compared to carbohydrate for a given exercise intensity compared to a man, all else being equal. Thus we may expect that a women is going to have a greater dependency on carbohydrates during exercise in the low oestrogen phases of the menstrual cycle, and thus potentially lower muscle glycogen stores if the same amount of carbohydrates are consumed compared to when oestrogen levels are high.


Further research is needed to determine the significance of this to real world carbohydrate intake.


Caffeine

Although only as much as 15% of research investigating caffeine and performance may be in women (Salinero et al 2019), the data indicates women seem to respond to caffeine in an equivalent way to males both in terms of the magnitude of response to supplementation and the effective dose (3-5mg/kg bodyweight).


Just as in males, there is evidence that caffeine may support enhanced anaerobic, aerobic and strength performance in females (Lara et al 2014; Ali et al 2016; Mielgo-Ayuso et al 2019; Skinner et al 2019; Lara et al 2021).


Although, as in most areas, it is still unknown whether there are nuances and differences in the response to caffeine at different stages in the menstrual cycle and in those using hormonal contraceptives.


Creatine

Creatine has multiple roles in sports performance, including explosive and repeated high energy movement, strength, muscle mass, muscle glycogen storage and cognitive function.


An excellent recent review by Smith-Ryan et al (2021) explores the role of creatine and creatine supplementation across the female lifespan and is well worth a read if you are interested in this area.


Women may have up to 70-80% lower endogenous creatine stores than males (Smith-Ryan et al 2021), which suggests women may be highly responsive to creatine supplementation. Indeed, based on the research to date – similar to caffeine – women appear to be responsive to creatine in an equivalent manner and with equivalent doses to males (Smith-Ryan et al 2021).


Similar to protein intake, however, further research is warranted to explore the effects of different stages of the menstrual cycle on creatine metabolism. Creatine kinase, the enzyme that converts creatine to phosphocreatine (the energy store) and back again exhibits fluctuating levels and activity through the menstrual cycle (Smith-Ryan et al 2021). This may have implications for the responsiveness to the supplementation and / or effective dosing to maintain high creatine levels in the muscle and brain. And there may be implications with other aspects of anabolism and catabolism that oestrogen has a role in regulating. Further research is needed as – given the complexity of cellular bioenergetics, anabolism and catabolism – there are many plausible hypotheses that could be drawn in this regard.


In Summary, and the future …

As can be seen, whilst research – at least for the areas discussed in this article – seems to indicate that women respond at least in a broadly similar way to males, there are potential impacts of hormone fluctuations through the menstrual cycle (and so perhaps likely into menopause and beyond also) that may have implications for nutrient timing and dosing. More research is needed! Some of which is already going to be underway in labs across the globe.


References


Ali, A. et al (2016). The influence of caffeine ingestion on strength and power performance in female team-sport players. doi.org/10.1186/s12970-016-0157-4


Burke L.M. et al (2011). Carbohydrates for training and competition. doi: 10.1080/02640414.2011.585473


Costello J.T. et al. (2014). Where are all the female participants in Sports and Exercise Medicine research? doi: 10.1080/17461391.2014.911354


Cowley, E. S. et al (2021). “Invisible Sportswomen”: The Sex Data Gap in Sport and Exercise Science Research. doi:10.1123/wspaj.2021-0028


Flynn, S. et al (2020). Males and females exhibit similar muscle glycogen recovery with varied recovery food sources. doi: 10.1007/s00421-020-04352-


Hamadeh, M. J. et al (2005). Estrogen supplementation reduces whole body leucine and carbohydrate oxidation and increases lipid oxidation in men during endurance exercise. doi: 10.1210/jc.2004-1743


Impey S.G. et al (2018). Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. doi: 10.1007/s40279-018-0867-7


Lara, B. et al (2014). Caffeine-containing energy drink improves physical performance in female soccer players. doi: 10.1007/s00726-014-1709-z


Lara, B. et al (2021). Similar ergogenic effect of caffeine on anaerobic performance in men and women athletes. doi.org/10.1007/s00394-021-02510-6


Mielgo-Ayuso, J. et al (2019) Caffeine supplementation and physical performance, muscle damage and perception of fatigue in soccer players: A systematic review. Doi: 10.3390/nu11020440


Moore, D.R. et al (2022) Fuelling the female athlete: Carbohydrate and protein recommendations. doi:10.1080/17461391.2021.1922508


Morton R.W. et al (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. doi: 10.1136/bjsports-2017-097608

Nuckols, G. (2022). Where Are All The Female Participants In Strength, Hypertrophy, And Supplement Research? https://www.strongerbyscience.com/representation/


Sheridan, H.C. (2022) Dietary supplements for consideration in elite female footballers. doi: 10.1080/17461391.2021.1988149


Phillips, S.M. (1993). Gender differences in leucine kinetics and nitrogen balance in endurance athletes. doi:doi.org/10.1152/jappl.1993.75.5.2134


Salinero, J.J. (2019). More research is necessary to establish the ergogenic effect of caffeine in female athletes. Doi: 10.3390/nu11071600


Skinner T.L. (2019). Women Experience the Same Ergogenic Response to Caffeine as Men. doi: 10.1249/MSS.0000000000001885


Smith-Ryan, A. E. (2021). Creatine Supplementation in Women's Health: A Lifespan Perspective. doi: doi.org/10.3390/nu13030877


Tagawa R. (2020). Dose-response relationship between protein intake and muscle mass increase: a systematic review and meta-analysis of randomized controlled trials. doi: 10.1093/nutrit/nuaa104


Thomas, D.T. (2016). American college of sports medicine joint position statement. doi:doi.org/10.1249/mss.0000000000000852


Wallis, G.A. (2006). Metabolic response to carbohydrate ingestion during exercise in males and females. doi: doi.org/10.1152/ajpendo.00357.2005



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