Signs of Under-fueling in Female Runners
If you're training for a marathon or half marathon and feel like you're doing everything right — logging miles, eating "healthy" — but still feel exhausted, moody, or plateauing in performance, under-fueling could be the reason. As a sports dietitian who works with female endurance athletes, this is one of the most common (and most missed) issues I see.
Under-fueling happens when you're not eating enough to support both your training load and your body's basic functions. It's not always about restriction on purpose — most of the time it's unintentional, the result of a busy schedule, inconsistent eating, or simply underestimating how much fuel real training demands.
Here are the signs to watch for.
1. Your periods become irregular or stop
This is one of the clearest signals your body is sending. When you're not eating enough to cover your training, your body will deprioritize non-essential functions — and reproductive health is often the first thing to go. A missed period, or several months of irregular cycles, isn't something to just push through. It's a sign of low energy availability, and it's worth addressing directly rather than assuming it's "normal" for athletes.
2. You're constantly fatigued, even with rest
Feeling tired after a hard workout is expected. Feeling tired all the time — during easy runs, at work, on rest days — is not. If you're getting enough sleep but still dragging, chronic underfueling is one of the first things I'd look at with a client.
3. You're getting sick or injured more often
Adequate fueling supports your immune system and your ability to recover and rebuild. Runners who are underfueled tend to pick up more colds, and are at higher risk for bone stress injuries and stress fractures. If you've had more than one stress-related injury in a training cycle, it's worth asking whether your intake is actually matching your output.
4. Your training is plateauing or getting harder, not easier
If your paces are getting slower, your long runs feel disproportionately hard, or workouts that used to feel manageable now feel brutal, it may not be a training problem — it may be a fueling problem. Your body can't adapt and get stronger from a stimulus it doesn't have the energy to recover from.
5. You're dealing with mood swings, anxiety, or trouble concentrating
Low energy availability doesn't just affect your legs — it affects your brain. Irritability, anxiety, low mood, and difficulty focusing at work or in day-to-day life can all be downstream effects of not eating enough to support your training.
6. You're cold all the time
Struggling to stay warm, even in normal temperatures, can be a sign your body is conserving energy by lowering processes like circulation. If you're layering up indoors when everyone else is comfortable, it's worth paying attention to.
7. Your digestion is off
Both constipation and other GI changes can show up when the body isn't getting enough fuel to run digestion normally, since it's prioritizing more immediately essential functions first.
What to do if this sounds like you
If any of this sounds familiar, you don't have to keep guessing on your own. I offer a free discovery call to talk through what's going on with your training and fueling, and figure out together whether 1:1 coaching is the right fit for you. .
Schedule your free discovery call →
