Why Women Keep Winning Ultras Outright¶
At the 2026 Cocodona 250, Rachel Entrekin crossed the finish line first overall — beating the top male finisher by more than an hour, on 19 minutes of total sleep across a 250-mile course.
That number is not a feel-good story. It is a data point that demands a mechanistic explanation. And the explanation exists: there are three specific physiological reasons why ultra distance is the zone where the sex performance gap narrows fastest, and in some cases reverses. None of them are motivational. All of them are documented.
This post names the mechanisms precisely, shows the split data that proves the effect is real, and explains why the trend will continue.
The Gap Shrinks With Distance — and the Numbers Show It¶
Before explaining why the gap shrinks, it helps to establish the shape of it. The performance difference between elite male and elite female competitors is not constant across race distances. It changes systematically, and it changes in a predictable direction as duration increases.
At the marathon, the elite sex performance gap runs approximately 10–11%. The marathon world records illustrate this cleanly, and large-scale finishing time analyses across road race fields confirm a similar spread throughout the competitive tier. At the 100K, the gap narrows. A 2025 analysis published in the International Journal of Sports Physiology and Performance, drawing on finishing time data from ultra events with comparable male and female participation rates, found the gap in that tier running approximately 8–9%. At 100 miles, published analyses across large finisher datasets put the gap at roughly 7–8%. At timed events of 24 hours and longer, and at point-to-point events above 200 miles, the gap approaches parity — and occasionally, as Cocodona demonstrates, flips entirely.
| Distance | Typical Elite M/F Gap | Evidence tier |
|---|---|---|
| Marathon | ~10–11% | Road records + large finisher data |
| 100K | ~8–9% | SAGE 2025; ResearchGate sex-difference analysis |
| 100 miles | ~7–8% | PMC/ResearchGate multi-race analyses |
| Timed / 200+ miles | Approaching parity | Cocodona 2026 overall; timed ultra demographics |
The participation skew at extreme distances complicates the signal. RunSignUp's 2025 demographics data shows women at 44–46% of timed ultra events — formats where the historical performance data supports near-parity — but only around 24% of 100-miler entries. Fewer women in 100-mile fields means elite-to-elite comparison is noisier at that distance: you are comparing a larger male pool to a smaller female pool. The timed-event figure is the cleaner signal precisely because field sizes are more balanced. The gap at 100 miles may be even smaller than current data suggests once participation rates equalize.
What matters for this post is not the exact percentage at each distance — the figures vary across studies and field compositions. What matters is the direction of the curve: consistent, measurable narrowing as duration increases. That consistency requires an explanation, and it has one.
Three Mechanisms Behind the Pattern¶
The performance gap at ultra distance does not shrink because the women who enter long ultras are self-selected for unusual ability, though field composition does matter. It shrinks because the physiological variables that determine ultra performance weight differently by sex, and those weights favor female athletes more with every additional hour of racing. Three mechanisms are driving this, and they are not independent — they compound.
Fat Oxidation at Low Aerobic Intensity¶
At intensities below approximately 65% of VO₂max — which is precisely the range where a 250-mile event or a 24-hour race is run — the body's primary fuel shifts toward fat. The critical finding, documented across multiple peer-reviewed studies including a 2023 PMC analysis of sex differences in fat oxidation during sustained exercise (PMC10190831), is that women oxidize fat at a meaningfully higher rate than men at matched relative intensities in this low-to-moderate aerobic range. The difference is not trivial. The PMC analysis found peak fat oxidation rates approximately 20–30% higher in women than men at comparable submaximal exercise intensities, with the gap most pronounced at intensities below 60% VO₂max — the intensities that describe the bulk of any multi-day event.
The hormonal driver is estrogen. Estrogen upregulates hormone-sensitive lipase, the enzyme that mobilizes stored fat for oxidation, and it enhances fatty acid transport into working muscle. This is structural, not a product of training history. A woman at peak estrogen levels doesn't need decades of aerobic base to access this advantage — it is active at the physiological level from the first hour of low-intensity sustained effort.
The duration threshold is the key to why this matters for ultras and not for marathons. In a road marathon at 3–4 hours of effort near lactate threshold, glycogen depletion is partially addressed through aggressive carbohydrate fueling, and the race is short enough that even significant glycogen depletion doesn't fully express itself in performance. At hour 24, hour 40, or hour 60 of a multi-day event, glycogen availability becomes existentially limiting. A runner who can derive a higher fraction of their energy needs from fat oxidation throughout those hours is not burning through carbohydrate reserves at the same rate as a runner running on a lower-efficiency fat oxidation baseline. The advantage compounds with every additional hour.
This is also why masters male athletes narrow the gap with younger men at ultra distance — cumulative aerobic training raises fat oxidation capacity over time, and years of base building produce some of the same metabolic adaptations. But as Ultra Running After 40 explains in the masters context, the training-age mechanism is additive to what female athletes have structurally. For women, the hormonal driver is not a product of training years — it is upstream of them.
Muscle Fiber Recruitment at Extreme Duration¶
The second mechanism operates at the cellular level. Women have, on average, a higher proportion of Type I (slow-twitch, oxidative) muscle fibers than men. The review-level evidence for this is well-established in sports physiology literature, summarized in the Sports Medicine/Springer meta-analysis on sex differences in ultra-endurance performance (Tiller et al., 2021, Sports Medicine). Type I fibers are fatigue-resistant and primarily aerobic. Type II fibers (fast-twitch, glycolytic) generate more power per contraction but fatigue significantly faster and accumulate metabolic debt that is costly to clear.
In a road marathon or a 50K, fiber-type composition matters, but the event duration is not long enough for progressive Type II fatigue to differentiate the field. At 20, 30, or 50 hours of effort, the story changes. As duration extends, Type II fibers accumulate damage. Runners with a higher proportion of Type I fibers maintain output more efficiently in the final stages of a multi-day race because they have less accumulated fiber fatigue to overcome. The performance advantage of a Type I-dominant fiber composition is roughly proportional to the duration of the event — negligible at one hour, substantial at twenty.
This mechanism interacts with the fat oxidation advantage: Type I fibers preferentially oxidize fat as fuel, so a runner with both higher fat oxidation capacity and more Type I fiber composition sees those advantages reinforce each other at extreme duration. The metabolic and structural advantages are not independent. They run in the same direction.
Pacing Discipline as a Physiological Output¶
This third mechanism is shorter, and it bridges from the mechanism section to the split data section that follows. The consistent observation in ultra race data is that women pace more conservatively — they run the second half closer to the first half than male competitors of comparable finishing position. The conventional interpretation is behavioral: women make smarter tactical decisions.
That is partly true. But pacing conservatism at ultra distance is also a downstream output of the two mechanisms above. A runner with higher fat oxidation capacity and more fatigue-resistant muscle fiber composition does not experience the same glycogen pressure or fiber fatigue at the 30-hour mark that a runner with the opposite physiology does. The physiological state of a female runner at hour 25 of a timed event is genuinely different from that of a male runner at the same elapsed time and the same current pace — not because of a better tactical choice, but because the metabolic and structural cost of those 25 hours was lower. Pacing conservatism is not just a decision. It is, in part, a reflection of a different physiological state.
The PMC study on pacing and body weight changes during mountain ultramarathons (PMC8607762) found that female finishers demonstrated a lower positive split coefficient — meaning their second-half pace degraded less relative to first-half pace — across distances from 50K to 100K in mountain events. That finding connects directly to the mechanism argument here, and it is what the next section explores in detail.
What the Split Data Actually Shows¶
Mechanism arguments are necessary but not sufficient. If the three mechanisms above are real and material, they should show up in race data as a measurable pacing advantage at ultra distance. They do.
The PMC study on pacing and body weight changes in mountain ultramarathon (PMC8607762) analyzed sex differences in pacing strategy across a mountain ultra course. The finding on pacing strategy was that female finishers ran more evenly — the positive split (the degree to which second-half performance is slower than first-half) was lower for women than for men at matched finishing positions. Male runners, on average, slowed more in the back half of the course relative to their early pace than women did. The effect was present even when controlling for overall finishing time, which means it reflects something about pacing behavior and physiology independent of fitness level.
A broader analysis of sex differences in ultramarathon performance published in ResearchGate — covering events where male and female participant counts were comparable — found that the performance gap was smaller not just in finishing time but in the structure of the race: women's pace distributions across the event showed less variability than men's, consistent with more sustainable early effort and better late-race execution.
At the extreme end of the duration curve, Entrekin's Cocodona result illustrates the same pattern in individual form. She slept 19 minutes across 250 miles. Male competitors in the finishing tier around her accumulated more sleep time — meaning she was maintaining performance for a higher fraction of the total elapsed time. The pace she needed to hold to beat the top male finisher by over an hour on a 250-mile course required sustained output across a multi-day period without the degradation pattern that defined the male competition ahead of her.
The 2025 SAGE Journals analysis (International Journal of Sports Physiology and Performance) is the most recent peer-reviewed work on this topic. It examined whether women are closing the ultramarathon performance gap over time, drawing on multi-year datasets of ultra finishers. The finding: the gap is narrowest in timed events (24-hour and 48-hour formats) and in events above 100 miles. This is consistent with the fat oxidation and fiber composition mechanisms being duration-dependent — the longer the event, the more hours those advantages have to compound.
The split data pattern is consistent across mountain ultras, timed events, and multi-day formats. The mechanism explains the pattern. The pattern confirms the mechanism.
Where the Data Predicts the Gap Will Close Next¶
Three mechanisms with duration-dependent effects, combined with a participation trend showing women concentrating in exactly the formats where the mechanisms are strongest: this produces a testable prediction. The gap will continue to close — and will most likely produce more overall wins for women — in timed events (24-hour, 48-hour, multi-day) and in point-to-point events above 200 miles.
RunSignUp's 2025 data makes this concrete. Women are at 44–46% of timed ultra entrants — events where duration is capped only by performance, not by a fixed course, and where the fat oxidation and fiber endurance advantages have maximum time to express. They are at approximately 24% of 100-miler entrants. The formats where women are showing up in near-equal numbers are precisely the formats where the physiology arguments predict the performance gap is smallest. That alignment is not coincidence.
The 250-mile format is the current frontier. Cocodona is the first major datapoint. The mechanisms predict that multi-day point-to-point events are the structural extreme where the sex performance gap is most likely to close or reverse — not because 250-mile races attract self-selected elite women, but because 250 miles is enough duration for the fat oxidation and fiber composition advantages to accumulate approximately six times the compounding effect of a 100-miler.
What the data does not yet support is extrapolating this to 100-mile races as a class, or to trail distances below 50K, or to road ultras at marathon effort. The three mechanisms are duration-dependent. They require low aerobic intensity sustained for long periods — the fat oxidation threshold, the fiber fatigue accumulation window, and the pacing advantage all become material at roughly 24+ hours of sustained effort. Below that window, the gap behaves the way it does at marathon: real, stable, and not closed by physiology.
The honest constraint on all of this is sample size. Outright overall wins by women in major ultras remain rare enough that the statistical signal at the extreme tail is still small. Cocodona 2026 is a data point, not a trend line by itself. What makes it interpretable is that the mechanism argument precedes it — the physiology literature explaining why it should happen was published before Entrekin ran Cocodona, not after. The result confirms what the mechanisms predict. That is a stronger signal than an observed trend without explanation.
The next overall wins by women in major ultras will most likely come from timed events — where the format perfectly matches the physiological advantage window — and from multi-day point-to-point events above 200 miles. The 100-mile category remains the noisiest, both because the duration is at the lower bound of where the mechanisms fully express and because female participation rates in that format are still lower, reducing the probability of the right athlete entering the right race at the right moment.
NavRun's training analytics can show you the aerobic zones where your fat oxidation rate is highest — the same intensity window this physiology turns on. Connect your Strava data.