Trail Notes | Female Athlete Physiology

the most important nutrition concept in female endurance

Energy Availability

For Women Who Train.

You cannot adapt to training you are not fuelled for. If there is one concept that quietly decides whether a season builds you or breaks you, this is it.

Energy availability sits underneath everything in female endurance training. It influences how your body responds to long runs, how you recover between sessions, how steadily your cycle moves, how well you sleep, how your bones hold up under repeated load, how you think and feel, and how much joy you can find in the training in front of you.

It is also the concept most often missed in conversations about female athletes. We talk about pace, kilometres, vert, intensity, threshold, lactate. We talk less about whether your body has enough fuel left, after training, to run the rest of you.

This Trail Note is a slow read on what energy availability is, why it matters more for us than for our male training partners, how to recognise when you are running low on it, and how to build a fuelling culture that supports the body you are asking to race.

Trail Note  ·  01

What energy availability actually is

Energy availability is the energy left for your body to run itself once training has taken what training needs. The simple version: total energy in, minus the energy used in exercise, divided across your lean body mass.

That leftover energy is not a buffer. It is the operating budget for your heart, your brain, your hormones, your immune system, your bones, your skin, your gut, your reproductive system and every small repair your body is making in the background. When the budget is full, all of that work happens quietly. When the budget is thin, the body starts choosing.

Researchers commonly describe an adequate range for female athletes as around 45 kcal per kilogram of lean body mass per day. A clinical low energy availability threshold often sits near 30 kcal per kilogram of lean body mass per day. You do not need to count to that number. What matters is the principle: there is a level below which your body stops adapting and starts conserving.

Trail Note  ·  02

Why this matters more for us

Female physiology is exquisitely sensitive to energy availability. The reproductive system is one of the first systems the body de-prioritises when energy runs short. The downstream consequences are not only about cycles. Estrogen and progesterone shape bone turnover, cardiovascular function, brain chemistry, immune response, sleep architecture and connective tissue repair. When the hormonal axis quietens, those systems quieten too.

This is why the medical framework now used to describe under-fuelling in sport, Relative Energy Deficiency in Sport, intentionally reaches further than the older Female Athlete Triad. It names what we have always felt in the body: that fuelling, hormones, bones, mood, immunity, gut, and performance are one connected web. Pull on one strand and the others move.

It also means cycle changes are valuable information. A lighter period, a later period, a missing period is not a bonus of training hard. It is a signal that something deeper is being asked to give way.

Trail Note  ·  03

How low energy availability usually starts

Most of us do not arrive in low energy availability through dramatic restriction. We arrive there quietly. A skipped breakfast on a busy morning. A missed snack between meetings. A long run done on a coffee. A weekend big day fuelled with half of what it asked for. None of these on their own change the picture. Stacked across weeks, they do.

It can also start as a small, well-intentioned tweak. Cutting back to lean down for a race. Reducing snacks because life feels chaotic. Trying a new way of eating that lowers carbohydrate without adjusting around training. The intention is often health. The outcome is often a body that has less to work with than the training plan assumes.

Add a busy week, a poor sleep stretch, family stress, travel, work pressure or a heatwave, and the gap between energy in and energy demanded widens further. The body does not differentiate between training stress and life stress. It only knows whether there is enough.

Trail Note  ·  04

The signals to read

No single symptom is diagnostic on its own. The skill is in noticing a pattern of small shifts that line up. We tend to talk ourselves out of these one at a time. Together, they tell a more honest story.

One alone is not a verdict. A cluster, especially across cycle plus mood plus performance, is worth listening to.

Trail Note  ·  05

Why under-fuelled training does not work

There is a common assumption that under-fuelling will accelerate adaptation, sharpen the body or unlock leanness. In short windows, with very specific protocols and very specific athletes, certain low-carbohydrate strategies have a place. In the everyday training of most female endurance athletes, chronic under-fuelling does the opposite of what we hope.

The body adapts to a stimulus through repair and remodelling. Repair costs energy. So does building mitochondria, capillaries, tendons, bone density, red blood cells and the enzymatic capacity that turns training into fitness. Without enough fuel, the training session still happens, but the adaptation it should have produced is muted.

In the short term you may feel light and capable. In the medium term, training quality drops, recovery stretches, soreness lingers, and the body holds tighter to what it has rather than building more.

Trail Note  ·  06

The body composition trap

Trail and ultra culture can quietly suggest that lighter is faster. Sometimes that is true in a narrow physical sense. Often it is not, especially over long durations on rough ground where strength, durability, recoverability, gut tolerance, focus and decision-making decide the day.

The cost of chasing a body composition change mid-training block is rarely worth what it returns. Lean tissue is fuelled by available energy. Drop available energy and the body protects fat stores while quietly losing muscle, slowing thyroid output, dampening hormonal signalling and reducing the very tissues that make you durable.

If body composition is a goal for you, it is a project for off-season, planned carefully with a sports dietitian who works with female athletes. It is not a project for the middle of a build or the lead-in to a race.

Trail Note  ·  07

What enough usually looks like

Enough rarely looks dramatic. It looks like three meals plus one to three snacks across the day, with carbohydrate, protein and fat at each meal, and easy access to extra food on harder training days. It looks like breakfast, not just coffee. It looks like the same food on rest days, not less.

Carbohydrate is not the enemy of a female endurance athlete. It is the most important fuel you have. Across heavy training weeks, most of us need more carbohydrate than we instinctively eat, especially around sessions. Underneath that, adequate protein supports repair, and adequate fat supports hormonal function. Each meal should leave you settled, not bracing for the next one.

For most of us, the work is not to eat more in one big push. The work is to close the small gaps, especially around training and across the day, so that the body never spends long stretches in deficit.

Trail Note  ·  08

Fuelling around training

Around-training nutrition is where most of us can lift energy availability the fastest, because this is where most of the gap lives. The body does not just need fuel for the session. It needs fuel for what the session has asked it to repair and rebuild.

These are general ranges. Your specifics depend on your size, your training load, your phase, your tolerance and your goals. A sports dietitian can refine them.

Trail Note  ·  09

Recovery weeks are feeding weeks

One of the quietest ways energy availability slips is during absorption weeks. The schedule reduces, and the instinct is to eat less because the body is doing less. But absorption is when the body is doing the most.

Tissue repair, capillary growth, hormonal rebalancing, gut adaptation and central nervous system recovery all step up when training steps down. That work is energetically expensive. Cutting food in a recovery week is like turning off the power during the renovation.

Eat similarly to a heavy training week. Sleep more. Resist the urge to add training. Trust that what looks like less work on paper is significant work inside the body.

Trail Note  ·  10

Sleep, stress and the rest of life

The body counts everything. A demanding workday, a poor night of sleep, ongoing caregiving, an unwell child, a tense relationship, a recent illness or even prolonged heat all draw from the same energy pool training draws from. None of this is moral. It is just physiology.

This is why fuelling needs do not lower in life-heavy weeks. They often need to lift. Stress raises baseline metabolic demand. Poor sleep raises cortisol and disrupts appetite signals so you may not feel hungry even when the body is asking for more. Heat increases sweat losses and reduces appetite. None of these mean you should eat less. They usually mean you should eat with more intention.

When life is heavy, fuelling is one of the most reliable ways to take pressure off the system rather than add to it.

Trail Note  ·  11

Cycle tracking as performance data

Of all the data points female endurance athletes can collect, the menstrual cycle is one of the most under-used and most informative. We are taught to track watts and kilometres but not the most honest signal of our internal energy status.

A regular cycle, with relatively stable timing and bleeding, is a strong sign that energy availability is in a workable range. Cycle changes can show up before performance changes, before bone changes, and before mood changes. Tracking does not have to be detailed. The simple variables are enough: start date, length, flow, any symptoms you noticed across the month.

If you are on hormonal contraception, your bleed is a withdrawal bleed and does not give the same energy availability signal. In that case, performance trends, sleep, mood, training response, soreness, illness frequency and injury history become the bigger signals to read together.

Any new pattern of lighter, late or missing periods deserves a conversation with a clinician. It is not a victory to lose your cycle to training. It is a flag worth listening to early.

Trail Note  ·  12

When to ask for help

Some signals are worth more than a self-audit. They need a professional eye, calmly and early, not after the season has unravelled.

A sports dietitian, sports physician, GP with sports experience, endocrinologist or psychologist, depending on the picture, is the right place to start. None of this is weakness. It is good racecraft, off the trail.

Trail Note  ·  13

A simple weekly check

You do not need detailed tracking to stay alert to energy availability. A short, honest, weekly check is often enough to catch a slow drift before it becomes a real shift.

If one answer is off, sit with it. If three are off, take it seriously.

Trail Note  ·  14

Rebuilding from low energy availability

Coming back from a period of low energy availability is not as simple as adding a snack. The body needs a sustained, generous, kind period of refeeding. Cycles often take months to return. Bone density takes longer. Mood and sleep can shift more quickly, which sometimes leads athletes to assume the work is done before it is.

Training load often needs to come down while fuelling comes up. This is rarely what an athlete wants to hear in the middle of a season. It is almost always what the body has been asking for. Patience here pays back across years, not weeks.

Work with a sports dietitian and ideally a sports physician who understands female athletes. The goal is not just to restore weight or restore a cycle. The goal is to restore the whole system, so that future training lands the way training is supposed to land.

Trail Note  ·  15

Build a fuelling culture, not a fuelling phase

The athletes who train and race well across years are not the ones who occasionally remember to eat. They are the ones who have built a fuelling culture inside their everyday life. It looks unremarkable from the outside. Breakfast happens. Snacks live in the bag. Long runs are fuelled the same way every time. Recovery starts within the first hour, not the first day. Food is treated as part of training, not as a reward for it or a punishment after it.

For most of us, the work is not learning anything radically new. The work is closing the small daily gaps that quietly add up to a body that is asked to do more than it has been given to do it.

Energy availability is not a number to chase. It is a relationship to maintain. The body you race with in five years will be built by the way you feed it across this season, not by any heroic week.

Trail Note  ·  16

This applies to every kind of athlete

Whether you are training for your first half marathon on the road, your first 50 kilometre trail race, your tenth ultra, a return after a long break, or you are simply running because you love it, energy availability still applies. The body does not know the entry list. It only knows the load it is being asked to carry.

The intensity of the work changes. The principle does not. Fuel enough that adaptation can happen. Fuel enough that your hormones, bones, mood, immunity and joy can keep up with the running you love. The runner you are becoming is being built underneath all of that.

Trail Note  ·  17

The deeper rhythm

At Her Trails we want you to learn your body well enough that it stops being a problem to manage and starts being a partner to work with. Energy availability is one of the most useful lenses we have for that work, because it pulls together so much of what otherwise looks unrelated. Cycle, mood, sleep, training response, recovery, soreness, illness, hormonal health and bone health are not separate stories. They are one story, told in many languages.

You do not need to be perfect. You need to be present. Notice the small drifts. Close the small gaps. Let your body have the fuel it needs to adapt to what you are asking of it.

The invitation

When the next big training week is in front of you, when an absorption week opens up, when life gets heavier than you expected, or when you are not sure what your body is asking for, return to the same questions.

Have I fuelled the work I am asking of my body. Am I closing the small gaps. Is my cycle behaving the way it usually does. Have I eaten today the way an athlete in training should eat.

Because strong training is not about training harder than your body. It is about training in partnership with it, fuelled, rested and respected enough that it can keep saying yes for the long road ahead.

fuel the body that is doing the work

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