Women in endurance: menstrual cycle, iron and RED-S

Why generic recommendations fall short for women

The classic sports nutrition recommendations were built on studies with predominantly male subjects. Female athletes have a different physiology across four critical axes: cyclical hormonal variation that alters substrate use and tolerance, iron losses through menstruation, fluid retention modulated by oestrogen and progesterone, and an increased susceptibility to chronic energy deficit. Applying generic recommendations without adjustment can compromise long-term health.

The consensus position from Elliott-Sale et al. published in Sports Medicine in 2021 explicitly acknowledged that previous research has low methodological quality, largely because it does not verify cycle phase with serum hormones and generalises from small samples. The meta-analysis of 78 studies by McNulty et al. (2020) in Sports Medicine concluded that the average effect of the menstrual cycle on performance is trivial, but the individual variability is enormous. In practical terms, this means: you, the recreational athlete, must identify your personal pattern before adjusting training.

On the other hand, there are areas where the differences are not trivial and where blindly applying male recommendations causes harm. Holtzman and Ackerman (2021) proposed a female-specific pyramid of priorities: first energy availability, then hydration, then macronutrients (carbohydrates 6 to 10 g/kg/day in endurance, protein 1.4 to 1.8 g/kg/day), micronutrients (iron, calcium, vitamin D, folate) and timing. Skip the first layer and none of the others work.

The five menstrual cycle phases and what changes

The reference cycle has 28 days. Knowing the five phases helps you anticipate how you are going to feel and adjust nutrition rather than fight your body. Day 1 is the first day of menstrual flow.

The study by Bruinvels et al. (2021) in the British Journal of Sports Medicine, with 6,812 recreational women via Strava, found that 78% report symptoms that affect their training. Do not dismiss menstrual pain or cyclical fatigue: validate your experience and use it to plan.

What RED-S is and how to recognise the signs

RED-S (Relative Energy Deficiency in Sport) is the syndrome described by the IOC that originates in low energy availability (LEA). It manifests in 10 physiological systems (menstrual, bone, haematological, metabolic, gastrointestinal, immune, cardiovascular, endocrine, psychological and performance) and affects 18 to 58% of female endurance athletes, according to the review by Logue et al. (2020) in Nutrients. The operational threshold is energy availability below 30 kcal per kg of lean mass per day. The optimum sits at 45 kcal per kg of lean mass.

The IOC 2023 consensus (Mountjoy et al.) published in the British Journal of Sports Medicine replaced the older Female Athlete Triad. RED-S expands the model to include men and recognises that more than 170 studies published between 2018 and 2023 brought new data specifically on carbohydrate availability. Diagnosis follows a three-step Clinical Assessment Tool: initial screening, traffic-light risk stratification and clinical diagnosis.

Areta et al. (2021) in the European Journal of Applied Physiology synthesised 28 prospective studies and showed that fewer than 5 days of energy deficit are enough to reduce LH pulsatility (the hormone that regulates the cycle) and lower T3, which explains how quickly the female athlete's body starts to deregulate.

Ferritin, iron and sports anaemia: the forgotten problem

Iron is the number one issue for the female athlete and at the same time the most overlooked by general medicine. Sim and colleagues (2019) in the European Journal of Applied Physiology synthesised the literature: 15 to 35% of female athletes are iron deficient, against just 5 to 11% of male athletes. The monthly losses through menstruation add up to the increased consumption driven by exercise and to the absorption blockade caused by training.

The mechanism is mediated by hepcidin, a hormone that blocks intestinal iron absorption. McCormick and colleagues (2019) in Medicine and Science in Sports and Exercise showed that afternoon exercise keeps hepcidin elevated for up to 24 hours, reducing absorption. The practical solution is simple: take iron in the morning, ideally after a morning training session, with vitamin C. Avoid coffee and tea one hour before or after.

The study by Solberg and Reikvam (2023) in Nutrients confirmed that supplementation only has a real effect when baseline ferritin is below 30 ng/mL. Stoffel et al. (reviewed in Sim 2019) showed that alternate-day supplementation absorbs better than daily dosing: the hepcidin peak triggered by the supplement lasts roughly 24 hours and blocks the next dose if given the following day. Every other day, in the morning, is the correct protocol.

In dietary sources, prioritise heme iron: lean meat, turkey, liver, fish. For vegetarians and vegans, lentils, spinach, pumpkin seeds, fortified tofu, always with vitamin C in the same meal. The FuelRace calculator adjusts protein and iron in the personalised notes when you indicate a female profile and age.

Does your hydration change during the cycle?

Sweat rate does not change significantly between cycle phases, according to Hutchins et al. (2021) in Nutrients. What changes is sodium balance. Sims and colleagues (2021) in the European Journal of Sport Science showed that oestrogen lowers the osmotic threshold for thirst and vasopressin release, and progesterone increases aldosterone. In the mid-luteal phase, this translates into greater sodium retention but also greater plasma dilution (roughly an 8% drop in plasma volume).

The practical implication is specific: in the mid-luteal phase, especially in long races in the heat, add 300 to 500 mg of sodium per hour to the base plan. In the late luteal phase, pre-hydrate before training with an isotonic drink to compensate for the reduced plasma volume. Weigh yourself before and after long sessions to calibrate your personal sweat rate; a loss above 2% of body weight means you are under-hydrated.

Protein: 0.32 g/kg per meal, 1.4 to 1.8 g/kg/day

The reference study is Wooding and colleagues (2017) in Medicine and Science in Sports and Exercise, which used the IAAO methodology (indicator amino acid oxidation) to show that active women have protein needs similar to men. The practical recommendation is 1.4 to 1.8 g/kg/day, distributed across 4 to 5 meals, with at least 0.32 g/kg per meal to maximally activate muscle protein synthesis.

For a 60 kg athlete, this means 84 to 108 g of protein per day, in meals of 20 to 25 g each. Above the age of 40, this rises slightly to 0.4 g/kg per meal in the post-training meal, because protein synthesis responds better to larger doses in masters athletes. Always combine with carbohydrates to support glycogen recovery.

In endurance, carbohydrate oxidation during exercise remains essential. Tarnopolsky (2008) and later reviews confirm that women oxidise proportionally more lipids and fewer carbohydrates than men during submaximal exercise, but this does not mean they should eat fewer carbohydrates. In the pre-fuel of long sessions and in races, the recommendation is the same: 6 to 10 g/kg/day in heavy training, 60 to 90 g/h during the race (combined with the gut training protocol that teaches you how to tolerate these doses).

Bone health, contraceptives and bone mineral density

The literature is clear: female athletes in energy deficit have 4.5 times more bone stress injuries over 12 months, according to Heikura et al. (2018) in the International Journal of Sport Nutrition and Exercise Metabolism. The mechanism combines hypo-oestrogenism, hypoleptinaemia and reduced IGF-1, all consequences of chronic low energy availability.

On hormonal contraceptives, Elliott-Sale et al. (2020) in Sports Medicine reviewed the literature and concluded that the combined pill has no significant effect on acute performance or hypertrophy gains. However, in adolescents and athletes with a history of stress fractures, the combined pill can reduce peak bone mass, and in these cases a non-hormonal IUD or progesterone-only method is preferable. Always discuss with your gynaecologist; contraceptive choice is an informed personal decision, not a performance call.

Peri-menopause: what changes between 35 and 50

Peri-menopause can start as early as age 35 and lasts on average 4 to 8 years until the FMP (final menstrual period, 12 months without a period). It is characterised by irregular hormonal fluctuation: oestrogen and progesterone are still present but in chaotic patterns. Peri-menopausal athletes report symptoms as severe as sedentary women and roughly 60% report a direct impact on training, mostly through sleep quality.

The drop in oestrogen accelerates processes that were previously protected. Chidi-Ogbolu and Baar (2019) in Frontiers in Physiology showed that oestrogen is critical for tendon collagen synthesis. Without it, Achilles tendinopathy, supraspinatus tendinopathy and plantar fasciitis rise substantially. Progress volume by less than 10% per week, keep eccentric work in the mix (jumps, descents, slow calf raises) and consider the Shaw/Baar protocol: 15 to 20 g of gelatine or hydrolysed collagen plus 50 mg of vitamin C, 30 to 60 minutes before impact training.

In parallel, insulin sensitivity drops. Cabre et al. (2022) in Menopause showed that peri- and post-menopausal women have reduced fat oxidation during submaximal exercise and more visceral fat, even with the same training volume. Fasted training is no longer advisable. Eat 20 to 30 g of carbohydrates plus 10 to 15 g of protein before morning sessions and prioritise HIIT and interval training, which preserve insulin sensitivity better than long, steady sessions.

Menopause: protein, creatine and the masters protocol

Menopause marks the start of the sustained low-oestrogen state. Nutritional priorities shift radically: protein rises to 1.6 to 2.2 g/kg/day distributed across 4 meals with 0.4 g/kg per meal (anabolic resistance), creatine monohydrate at 3 to 5 g/day becomes the intervention with the best cost-to-benefit evidence for preserving muscle mass, and resistance training 2 to 3 times per week becomes non-negotiable.

The key concept is anabolic resistance: the muscle loses sensitivity to the protein stimulus. Moore (2021) in Sports Medicine synthesised the masters literature: athletes above 35 need 0.40 g/kg per meal (vs 0.24 to 0.30 in younger athletes), with 2.5 to 3 g of leucine per meal to cross the leucine threshold and activate mTOR. For a 70 kg athlete, that is 28 to 35 g of protein per meal, 4 times a day. Typical British snacks such as toast with ham (about 8 g of protein) are insufficient: swap for Greek yoghurt with nuts, cottage cheese with berries or eggs on toast.

Creatine is no longer just a strength supplement. Sims et al. (2023) in the Journal of the International Society of Sports Nutrition wrote it explicitly: "Post-menopausal females benefit from bone health, mental health, and skeletal muscle size and function when consuming higher doses of creatine". The standard protocol is 5 g per day, taken continuously (not cycled), combined with resistance training 2 to 3 times per week.

On menopausal hormone therapy (MHT or HRT), recent literature has normalised its use in healthy masters athletes. Benefits in bone mineral density, tendon biomechanics, sleep and body composition (Smith-Ryan et al. 2022 in Sports Medicine). It does not replace resistance training or proper nutrition, it is an adjunct. The decision is always individual, taken with a gynaecologist, especially when symptoms are moderate to severe.

Post-menopausal bone density: the critical window from 45 to 55

Bone mineral density (BMD) loss accelerates dramatically around the FMP. The SWAN study (Greendale et al., 2012, J Bone Miner Res) showed that Caucasian women lose on average 2.46% of lumbar density per year during the 3 years around the FMP, with a total cumulative loss of 10.6% over 10 years. The loss starts roughly 1 year before the last period.

The most critical intervention window is from 45 to 55 years of age. Strategies that work: add impact to your running (vertical jumps, short sprints), heavy resistance training (3 to 5 reps at 80 to 90% 1RM), ensure 800 to 2,000 IU of vitamin D per day and 1,200 mg of calcium (prioritising diet over supplement), take creatine 5 g/day and consider MHT in discussion with your gynaecologist. A DEXA scan around age 50 gives you a useful baseline.

On hot flushes during exercise, Witkowski et al. (2023) in Menopause concluded that moderate to vigorous aerobic and resistance training reduces the subjective frequency of hot flushes in the long run, via improved central thermoregulatory control. On very hot days they can be acutely triggered, so pre-cool with ice on the neck, reinforce hydration with 500 to 700 mg of sodium per litre and consider training early in the morning or at the end of the day.

Consolidated recommendations table by cycle phase

This is the quick practical reference for the recreational athlete. It applies to the 28-day cycle; in different cycles adjust proportionally. The recommendations for carbohydrates, protein and sodium are per kg of body weight.

Sources: Holtzman and Ackerman 2021, Sims et al. 2021, McNulty et al. 2020.

Five habits to start tomorrow

1. Request full bloodwork. Full blood count, ferritin, haemoglobin, vitamin D, folate and TSH. Twice a year for the recreational athlete, every 3 months if there is a history of RED-S or training above 8 hours per week.
2. Track your cycle for 3 months. A simple app (FitrWoman, Strava). Log day 1 of menstruation, subjective energy and symptoms. After 3 cycles you will know your personal pattern before adjusting training.
3. Protect energy availability. On heavy days, add up dietary calories, subtract exercise calories, divide by lean mass. If it falls below 30 kcal per kg, increase intake, especially carbohydrates.
4. Top up iron in the morning with vitamin C. Post-training breakfast with eggs, spinach, citrus. If ferritin is below 30, supplement on alternate days in the morning with a medical prescription.
5. Distribute protein at 0.3 to 0.4 g/kg every 3 to 4 hours. Always have a post-training meal with 25 to 35 g of high-quality protein. Combine with carbohydrates.