What the Research Actually Says About Fueling Female Athletes
If you've ever been handed a nutrition protocol designed for a 75kg male endurance athlete and told to scale it down, you've experienced the central problem this review addresses. Women make up nearly 50% of sports participants. The research base informing how they should eat looks nothing like that split.
A 2021 review by Holtzman and Ackerman, published in Sports Medicine, synthesizes what's known about nutrition and hydration specific to female physiology, identifies where the evidence is thin, and provides a practical framework for filling the gaps. It's a useful document not because it resolves everything, but because it's honest about what remains unresolved — and what that means for practice.
The Research Problem Is Structural
Between 2011 and 2013, women represented 39% of study participants across three leading sports medicine journals — and only 4% of studies were female-only. In studies of athletic performance specifically, 63% used male subjects only, 33% used mixed groups, and 3% focused solely on female athletes.
The stated reason women are harder to study is real: hormonal complexity means no two menstrual cycles are identical, proper characterization of luteinizing hormone levels requires overnight blood sampling at minimum every three hours, and well-designed studies need to stratify by menstrual status, contraceptive use, and pregnancy status. This is genuinely more expensive and logistically complex. It also doesn't make it acceptable to apply male findings to female athletes and call it evidence-based practice.
The consequence is predictable: practitioners are working from recommendations that may not apply to the population they're actually serving.
The Foundation: Energy Availability
Before any conversation about nutrient timing, carbohydrate periodization, or menstrual cycle-based fueling strategies, there's one number that matters most: 45 kcal per kilogram of fat-free mass per day. This is the energy availability (EA) target the review identifies as optimal for health and performance maintenance in female athletes.
Energy availability is calculated as energy intake minus exercise energy expenditure, normalized to fat-free mass. It quantifies what's left over for the body's physiologic functioning after training demands are met. The paper is explicit: optimizing nutrient composition based on menstrual cycle phase is futile without first ensuring adequate total energy. No amount of cycle-synced nutrition strategy compensates for a caloric deficit.
Low EA is more common than most practitioners assume. Estimates of disordered eating or low EA states among female athletes range from 6–45%, and a survey of 1,000 female athletes aged 15–30 across 40+ sports estimated the risk of low EA at 47.3%.
When EA Goes Wrong: RED-S
Relative Energy Deficiency in Sport (RED-S) is the model describing what happens across multiple physiological systems when intake doesn't support training demands. The more familiar female athlete triad — perturbations of energy availability, bone health, and menstrual function — is a subset of RED-S, not the whole picture.
The hormonal consequences of low EA are extensive: decreased estradiol, progesterone, leptin, insulin, T3 and free T3; increased ghrelin, adiponectin, cortisol, and growth hormone resistance. The downstream effects span bone (osteoclast activity predominates when estrogen is chronically suppressed), cardiovascular function (endothelial dysfunction, unfavorable lipid profiles), metabolic rate (resting metabolic rate decreases in energy deficiency), and gastrointestinal function.
The performance consequences are also direct. Low EA is associated with decreased muscle strength, decreased endurance performance, impaired judgment, reduced coordination, increased injury risk, and worse training adaptation. One study of adolescent female swimmers found that the ovarian-suppressed group showed a 9.8% increase in 400m freestyle times over a competitive season while the eumenorrheic group improved by 8.2% — an 18-percentage-point performance differential that traces back to hormonal and nutritional status.
One point the review makes explicitly: missed cycles are not normal for athletes. They are a signal of energy deficiency. This still requires correction in how many athletes and coaches understand the relationship between training stress and menstrual function.
Iron, Calcium, and Vitamin D — Three That Matter Most
Once total energy is addressed, the review turns to micronutrients. Three deficiencies are particularly common in female athletes and have disproportionate consequences.
Iron. Estimates of iron deficiency prevalence in female athletes range from 15–35%, with some studies suggesting rates above 50%. At baseline, 24–47% of women experience iron deficiency without anemia. The distinction between iron deficiency and iron deficiency anemia is operationally important: it's possible to be iron deficient without meeting criteria for anemia, and poor athletic performance is the most common presenting symptom in that state. Ferritin alone is an insufficient screening marker — it's an acute-phase reactant and can be falsely elevated during illness or stress. A full iron panel is necessary for accurate assessment.
For dietary strategies, heme iron (from meat) is better absorbed than non-heme iron (from plant sources), and plant-based food sources contain compounds that further reduce non-heme absorption. Athletes with restrictive diets warrant consultation with a sports dietitian. Where supplementation is needed, the review recommends approximately 100 mg per day in divided doses for 8–12 weeks, with a vitamin C co-administration for absorption, using slow-release ferrous sulfate formulations. Empiric supplementation without physician evaluation isn't appropriate — underlying pathology needs to be ruled out first.
Calcium. Athletes at risk for low calcium should target 1,500 mg per day to protect bone mineral density. The gut can't absorb more than 500 mg at once, so intake should be distributed across the day. Calcium supplementation generally requires adequate vitamin D for optimal absorption.
Vitamin D. Studies have found 21.5–80% of female athletes and dancers with abnormal vitamin D values, depending on the population. Athletes training indoors, those at northern or southern latitudes (above the 35th parallel), and those who consistently cover exposed skin are at higher risk. A daily maintenance dose of 1,000–2,000 IU vitamin D3 is reasonable to maintain 25-OH-vitamin D levels above 50 nM.
Hormonal Physiology and What It Changes About Fueling
This is where the review covers ground that most general nutrition guidance doesn't. Estrogen and progesterone fluctuations across the menstrual cycle create real, measurable changes in substrate metabolism — but the practical implications are often overstated or misunderstood.
During the follicular phase, gluconeogenesis rates are higher. During the luteal phase, when estrogen levels are elevated, women are less reliant on muscle glycogen and oxidize more fat relative to carbohydrate compared to men at the same exercise intensity. Estrogen also impairs gluconeogenesis, which creates a theoretical performance disadvantage during the luteal phase — one that can be mitigated with a high-carbohydrate snack 3–4 hours before exercise.
Protein catabolism is higher in the luteal phase due to elevated progesterone, suggesting protein requirements may be above the already-elevated baseline during this period. Current ACSM recommendations of 1.2–2 g per kilogram per day may underestimate luteal phase needs.
On carbohydrate loading: the data is more complicated than the popular version. Dedicated carbohydrate loading (8.4–9 g per kilogram per day) during the mid-follicular phase shows 17–31% improvements in muscle glycogen but no consistent performance improvement. Mid-luteal phase loading shows smaller or absent glycogen benefits. Oral contraceptive use further complicates the picture because it eliminates the natural hormonal variation that drives these phase differences. For athletes consuming a relatively low-calorie diet — say, 2,000 kcal per day — a 55 kg athlete would need to dedicate 88% of her calories to carbohydrates to hit standard carb-loading targets. In practice, the pre-competition pasta party is probably more useful for team cohesion than glycogen supercompensation.
Regarding hydration: the hormonal complexity carries through here too. Estrogen and progesterone receptors are present in the hypothalamus, cardiovascular system, and kidney. The luteal phase brings increased fluid retention, a 0.5–1.0°C increase in basal body temperature, and paradoxical intravascular volume depletion through extravasation. Despite all of this, current evidence doesn't suggest increased heat illness risk for women compared to men. Exercise-associated hyponatremia is more common in women — but after accounting for BMI and exercise time, the sex difference disappears. The risk factors (overdrinking, slow pace, exercise duration over 4 hours) are the primary drivers, not sex itself.
A Practical Hierarchy
The review proposes a hierarchy for nutrition planning that's worth keeping in mind: start with adequate energy availability and hydration, then macro composition, then nutrient timing, then exercise type and duration, then hormonal profile, then exogenous hormone use, then individual adjustments. The principle is that each level is only meaningful if the foundation below it is solid.
This has direct implications for how sophisticated nutrition conversations should be with any given client. Periodizing carbohydrate intake by menstrual cycle phase is a reasonable topic for an athlete who is well-fueled, iron-replete, has an intact and regular cycle, understands basic macronutrient distribution, and times her fueling around training sessions. It is not the right starting point for an athlete who is under-eating.
Limitations Worth Naming
The review was supported by the Gatorade Sports Science Institute and the lead author received an honorarium for its preparation. The authors declare no relevant conflicts of interest. The funding source is worth knowing, though it doesn't directly affect the recommendations, which are consistent with independent literature and ACSM positions.
More substantively: the evidence base the review synthesizes is itself thin. Many of the specific recommendations — cycle phase-based protein adjustments, optimal carbohydrate loading strategies for female athletes — rest on small studies with limited generalizability. The authors are consistent about naming this. More research is needed isn't a disclaimer here; it's a description of actual gaps with real consequences for practitioners.
What This Means for Programming
Female athletes need individualized nutrition planning that doesn't default to scaled-down male recommendations. The practical starting points from this paper are:
Know the energy availability target (45 kcal/kg FFM/day) and assess whether clients are actually meeting it before engaging in higher-level nutrition optimization. Missed or irregular cycles are a flag, not a baseline.
Prioritize iron screening, particularly for athletes with restrictive diets, high running loads, or heavy menstrual bleeding. Poor performance is often the first symptom, which means it's frequently misattributed.
Calcium and vitamin D status are worth addressing proactively, not just when injury occurs. Bone stress injuries in female athletes often have nutritional antecedents that preceded the injury by months.
Build cycle phase awareness into nutrition conversations at the appropriate level — only once the foundational requirements are met. The follicular-to-luteal shift in substrate preference is real physiology, but its practical significance is modest compared to simply being adequately fueled.
Direct clients to sports dietitians for individualized planning. The complexity described in this paper is a reason to refer, not a reason to improvise.
Source: Holtzman B, Ackerman KE. Recommendations and nutritional considerations for female athletes: health and performance. Sports Medicine. 2021;51(Suppl 1):S43–S57. https://doi.org/10.1007/s40279-021-01508-8
