When we talk about nutrition, often we don’t distinguish between men and women. Yet there are distinct differences in male and female physiology and hormone profile that could reasonably be thought to impact nutritional needs.
Most sports nutrition research has historically been performed with men. Until relatively recently, elite and professional sport was male dominated – so it would make sense that the research was likewise. In addition, as hormone cycles and hormonal contraceptives could have the potential to impact nutritional responses conducting research in women can be more time consuming and require more participants … because the goal is to either take all possible hormone stages into consideration and / or isolate studies to women at a certain hormone cycle stage. In short, it can be simpler to study males!!
This does not mean studies in males are not applicable to women. Gender is just one of many variables that impact nutritional needs. We see positive responses in women using nutritional strategies developed from male research. This might indicate that duration, intensity, nature and frequency of exercise may be primary determinants of nutritional needs in athletes. However, we cannot rule out that there may be more effective nutrition strategies that women (or certain women) could benefit from … unless it is studied!
With this in mind, what do we understand of the impact (if any) of female physiology and hormone profiles during the reproductive years on nutritional needs from the research that has been undertaken? And what is still unclear?
This is a VERY long article as – despite how little we really know – there is a lot to talk about! It is broken into subsections so you can focus on the areas you are most interested in and / or dip in and out over time.
Female physiology, energetics and nutrition
Women typically have less muscle mass and more fat mass than men. Recommendations on how much protein to consume are described in terms of grams (g) of protein per kilogram (kg) bodyweight per day, ranging from 1.2g/kg to over 2.0g/kg dependent on the sport and total calorie intake (American Dietetic Association, 2009; Aragon et al., 2017; Kerksick et al., 2018). If a smaller proportion of a woman’s bodyweight is muscle, given that muscle protein synthesis is a driver of these recommendations, it might be expected that women would need to eat less protein per kg bodyweight than men. Although this has not been tested directly, male and female muscle actually appears to respond comparably to the same g/kg protein intake (Aragon et al., 2017; Campbell et al., 2018). It is possible that, compared to the variety of factors that affect an individual’s protein requirements (male or female!), differences in the amount of muscle mass stimulated by any one training session is minimal. So, for now at least, it seems protein intake recommendations apply equivalently to men and women!
Interestingly, women may be less susceptible to muscle loss during dieting than men (assuming high protein intake and resistance training is maintained). It is hypothesized this may be because men are more reliant on testosterone to maintain their muscle mass, and this falls during low calorie diets (Roberts et al., 2020).
Women seem to use a higher proportion of energy from fat and a lower proportion of energy from carbohydrates in sub-maximal exercise, compared to equivalent men (Phillips et al., 1993; Sims, 2015). This is perhaps not surprising, considering that women typically have less muscle mass than men (and so less muscle to store carbohydrate energy in), and more fat mass (Sims, 2015). Does this mean that we expect women to need to drink /eat fewer carbs during endurance exercise than men to maintain their performance? Interestingly, maybe not (Wallis et al., 2006)! This could be because, although women do not store as many carbs and can use fats effectively, the muscle will go ahead and use them if they are available in the blood after eating … as carbs can produce energy more rapidly than fats!! On this note, maximal exercise is primarily powered by carbohydrates and so we may not expect to see a gender difference here!
Perhaps consistent with an ability to access their fat stores more efficiently, women may be less likely to resort to using protein for energy in endurance exercise than men when energy levels get low … we typically turn to protein when we can’t access carb or fat energy stores fast enough, or are running out of them (Phillips et al., 1993).
It is worth noting that physiological differences between men and women can be minimized with training. A well-trained and muscular female will likely require more protein to support muscle gain, and have the capacity to use more carbohydrates for energy, than an equivalent untrained male. We are talking about like for like comparisons here, e.g. a male versus female rower!
Female hormones, performance, adaptation and nutrition
During the reproductive years a healthy female who is not on hormonal contraceptives can expect to experience a regular hormone cycle as pictured below (image courtesy of Wikipedia). A typical cycle lasts between 21 – 35 days.
From Wikipedia (https://en.wikipedia.org/wiki/Menstrual_cycle)
The question is whether these hormones impact a women’s nutritional needs and, if so, does this mean her nutritional needs differ across the course of a menstrual cycle? The short answer is that we don’t really know, as the research appears somewhat conflicting (Elliott-Sale, 2014; Desbrow et al., 2019). There are many plausible reasons for these apparently contradictory findings. First, women may respond differently to different levels and cycling of the same hormones and so it is not possible to liken studies looking at natural ‘high’ hormone levels, versus hormonal contraceptives, versus IVF. Second, particularly in older studies, hormone levels may have been incorrectly assumed by researchers and so what was thought to be a response in a ‘high’ or ‘low’ hormone phase may not actually have been the case. And third, different measures of strength and energetic performance were used in different studies and these are not necessarily comparable as they require different ‘types’ of strength, coordination and energetic capacity. And finally, we just need more studies in ‘real world’ situations and athletes!
It does seem that the female sex hormones may have a role in the differential carbohydrate usage noted in the section above. Oestrogen appears to have a role in driving use of fats for fuel. As a result, it may be that in the low hormone phase through menses and the start of the follicular phase, women are less efficient at utilizing fat stores and are more reliant on stored carbohydrates i.e. muscle glycogen (Phillips et al., 1993; Devries et al., 2006; Sims, 2015). In other words, have a more similar metabolism to men in terms of how they produce energy. This might mean that in this phase carbohydrate intake is of particular importance to ensure a woman can effectively fuel and recover from training … research is needed!
Progesterone may increase core body temperature by up to 0.5°c (Stachenfeld, Silva and Keefe, 2000; Sims, 2015). Therefore as progesterone peaks in the latter stages of the luteal phase of the menstrual cycle women may be more susceptible to fatigue as a result of elevated core body temperature. As such, pre-cooling, hydration and post-cooling may be of increased importance in training and competition.
The cumulative impact of high oestrogen and progesterone means women retain more water in their tissue and find it harder to sweat as blood volume drops, which can compound the impact of the increased core temperature noted above (Sims, 2015). The more lean body mass a women has, the more fluid may be retained (Desbrow et al., 2019). Although sweat rates are lower, progesterone associated changes in sodium balance mean that women may be at higher risk of hyponatremia during endurance exercise bouts, and therefore rehydrating through intake of both water and salts should be a focus. Interestingly though, women appear to be equally as effective at rehydrating at any stage of their cycle (provided water and salts are provided), at least after mild (2%) dehydration induced by an overnight period without fluids and heat stress (Rodriguez-Giustiniani and Galloway, 2019).
During menstruation, prostaglandins are released to stimulate contraction of the uterus so the lining can be shed. However, these prostaglandins can also increase gut contractions causing some women can experience diarrhoea. This has the potential to impact hydration, as well as the digestion and absorption of nutrients. If a woman experiences these symptoms, she might need to increase focus on hydration strategies as well as altering her diet to account for changes in digestion and absorption (I am not including specific thoughts on that here, as it will be impacted by multiple individual specific factors).
We cannot move on from consideration of the sex hormones without considering hormonal contraceptives. These alter the normal hormone cycle by providing exogenous combined (oestrogen plus progestin) or progestin only to the body. A women’s own hormone production is suppressed as a result. Any differences between natural hormone levels and hormonal contraceptives (and the different doses of these that exist) on the body’s nutritional needs are, unsurprisingly, unclear. However, hormonal contraceptive use is prevalent across athletes and therefore it is an area that should be of focus. One recent study surveyed 430 national, international and elite female athletes across 24 sports in the UK and found 69.8% had used hormonal contraceptive at some point, above the UK average of 30% (Martin et al., 2018). Interestingly, 43.5% of users said that they had used, or intended to use, hormonal contraceptives to manipulate their menstrual cycle around competition as they felt they performed differentially at different stages of their cycle (Martin et al., 2018).
Low Energy Availability (LEA) and Relative Energy Deficiency Syndrome (RED-S)
LEA and the resulting RED-S are often considered synonymous with female athletic nutrition. This is possibly because it was one of the first areas of sports nutrition that focused on women and because it was, originally, thought to be an issue largely isolated to women. Indeed, the original name of the disorder was the female athletic triad.
Low Energy Availability (LEA) describes a situation where an athlete consumes too few calories for their needs such that, after the calories burned in exercise have been taken away, they don’t have ‘enough’ calories (energy) left to support all the normal functions of the body (Mountjoy et al., 2014; Mountjoy et al., 2018). As a result, some bodily functions are impaired resulting in Relative Energy Deficiency Syndrome (RED-S). The issues can be compounded as low energy intake can be associated with low micronutrient intake, as the athlete is simply not consuming enough food and enough food variety to obtain all the micronutrients they need; studies have shown that female physique athletes in the pre-competition phase of dieting may consume less than two thirds of the daily recommended intake of calcium, iron, zinc and sodium (Alwan et al., 2019). LEA and RED-S may be associated with disordered eating or an eating disorder, but may not (Mountjoy et al., 2014; Mountjoy et al., 2018).
Sports most at risk are those where athletes must maintain a low weight or very lean physique, such as jockeys and physique athletes, as well as endurance athletes who expend very large amounts of energy in training and who benefit from being lighter (i.e. having less ‘deadweight’ to have to carry on a run or on a bike etc) (Sundgot-Borgen et al., 2013; Halliday et al., 2016; Rohrig et al., 2017; Fagerberg, 2017).
The potential health consequences of RED-S include immune dysfunction, gastrointestinal disorders, cardiovascular function, metabolic function, psychology, endocrine disruption, bone health, and menstrual dysfunction (Mountjoy et al., 2018; Melin et al., 2019). I am going to focus on those elements linked most closely to the disruption seen to female sex hormones, namely menstrual dysfunction and poor bone health.
LEA can result in reduced production and cyclical nature of the female sex hormones, resulting in cessation of the menstrual cycle and amenorrhea (Mountjoy et al., 2014; Mountjoy et al., 2018; Melin et al., 2019). It is not clear how quickly LEA results in menstrual dysfunction, and it is perhaps likely it varies from individual to individual. In one study of female physique athletes in the pre-competition dieting phase, which typically lasts 3-6 months, 63% of athletes reported menstrual irregularities (Hulmi et al., 2016).
In females, oestrogen is important in the regulation of bone mineral density. Oestrogen supports accumulation of bone mineral density through teenage and early adult life, and then the maintenance of bone mineral density through adulthood to the menopause. Bone mineral density is important for bone strength, reduced risk of stress fractures, and long term reduced risk of osteoporosis. As noted above, oestrogen levels fall because of LEA, and this is associated with reducing bone mineral density and increased risk of stress fractures (Mountjoy et al., 2014; Papgeorgiou et al., 2018; Melin et al., 2019).
If energy intake is increased, the sex hormones and other aspects of RED-S can normalize over time but this can take many months. And there are some consequences that may never be fully restored, for example bone mineral density if it was not able to reach its peak in early adulthood because of LEA (Mountjoy et al., 2014; Alwan et al., 2019; Melin et al., 2019). It is therefore important that LEA is identified and addressed early. In this regard, it is worth noting that menstrual dysfunction can serve as an early warning sign of LEA and should not be ignored. For females of hormonal contraceptives, this sign will obviously not occur and so it is important to be aware of other indicators (see the referenced review articles by Melin et al and Mountjoy et al for more detail).
The challenge is that optimal performance some sports – as noted at the beginning – require a lean physique and high energy expenditure, and this can leave athletes and their team walking a narrow tightrope between sufficient energy intake to maintain health and performance over the long term, and short term physique or performance goals. One approach, as documented in a case study across the career of an elite female middle distance runner, could be to periodise nutrition – prioritizing energy intake to best support health in the off season, and competition performance in season (Stellingwerff, 2018). Knowing what these levels are brings its own challenges, but it is an approach that may provide the necessary balance between priorities.
This is the final area I want to draw attention to is iron, as female athletes are at higher risk of iron deficiency than the general population. Some studies report iron deficiency in 15-35% of female athletes tested, and reduced iron stores in as many as 50% or more of female athletes (Sim et al., 2019).
Iron has important roles in health and performance. It carries oxygen in the blood, it supports energy production in the cells of the body, and it has a role in cognitive function and in fighting infection as part of the immune system. Consistent with a role in performance, one very interesting case study showed an elite female runner running a personal best following correction of an iron deficiency (Garvican et al., 2011). She could just have been ‘having a good day’ or her training programme across that period was particularly effective … but it is certainly plausible that iron had a causal role.
Athletes, male or female, are generally at higher risk of iron deficiency than the general population because they have a greater need for iron, whilst at the same time losing more iron. They have a greater need for iron because they have more energy producing machinery in their cells and more red blood cells to enable them to carry more oxygen. And they lose more iron, as they sweat more (and iron is lost in sweat) and ‘wear out’ or damage red blood cells and their energy producing machinery at a higher rate (and although the body recycles the iron when this happens, the process is not 100% efficient). And then, on top of this, eumenorrheic females lose iron in their monthly menstrual blood losses.
There are currently no guidelines on recommended iron intake for female athletes. The current recommendation for healthy women of reproductive age range between 14.3mg and 18mg per day between the UK and the US, and studies have shown that many female athletes currently fall short even of these recommendations (Alaunyte, Stojceska and Plunkett, 2015; Sim et al., 2019). And those athletes with low energy intake, for example those with LEA and RED-S, are at highest risk of low intake. This is not just because they are eating fewer calories and therefore have fewer opportunities to eat iron containing foods, but also because training with LEA can increase the expression of factors that prevent iron being absorbed effectively from food and recycled effectively when damaged red blood cells are broken down (Sim et al., 2019).
Iron is found in food in two forms, haem iron and non-haem iron. Haem iron is the form we can absorb most effectively from our food, and use most effectively in the body. It is found in animal products, with the richest sources typically being red meat followed by sources in foods such as turkey and sardines. Non-haem iron is found in plants; in legumes, dark leafy greens and fortified foods. As it is typically not absorbed or used as effectively, individuals who obtain their iron purely from non-haem (plant) sources are also at higher risk of deficiency and may need to eat more than the recommended daily intake to ensure they obtain sufficient amounts.
Despite how important it is, it is not recommended to take iron supplements unless under the guidance of a medical practitioner. This is because too much iron is toxic and can be very dangerous … and it is easier to get to such levels with supplements than with food. If you are worried about an iron deficiency, a doctor can provide a blood test.
In Summary …
It is important to remember that gender is just one of multiple variables that impact nutritional needs. However, aspects of female hormonal profiles and physiology do have the potential to impact nutritional requirements. In most cases we still have much to research in order to understand what the impact is, whether it is significant and, so, whether specific dietary recommendations are required.
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