How Much Should Runners Eat? Understanding RED-S and Energy Availability

Kayleigh Webster
Jul 14
4 min read

I eat a lot. I’ve always had a big appetite and never really resonated with the stereotype of the runner picking at salad leaves. So when I found myself feeling flat, fatigued, and emotionally frayed despite training well and eating plenty, I was shocked to realise what was really going on: I was underfuelled.

Not during the run. After. The deep, dragging tiredness. The grumpiness. The sense that my body was constantly in catch-up mode. It turns out that RED-S (Relative Energy Deficiency in Sport) doesn’t just affect people who consciously restrict food or eat too little—it can sneak up on those of us who eat a lot but still don’t meet our full needs.

If you’re training hard and still feeling depleted, read on. Because understanding how much you really need to eat could change everything—from your mood to your pace, to your long-term health.

Two-image collage: Left, a woman in a teal shirt wipes sweat while jogging; right, she smiles, eating spaghetti at a table with a sandwich.

What Is Energy Availability?

Energy availability is what’s left over for your body’s basic functions after exercise has been accounted for.

Energy Availability = Energy Intake – Exercise Energy Expenditure

So even if you eat 2,500 kcal a day, but your training load is high—say you’re regularly burning 800–1,200 kcal per session—your body might only be left with 1,300–1,700 kcal to run everything else: hormone regulation, immune function, bone health, recovery, mood. And that might not be enough.

Over time, your body starts making trade-offs.

What Is RED-S?

RED-S (Relative Energy Deficiency in Sport) is a condition caused by prolonged low energy availability. It can affect any athlete, regardless of gender, and impacts almost every system in the body:

🔁 Irregular or lost menstrual cycles

🦴 Stress fractures or slow-healing injuries

🛌 Fatigue, poor sleep, and mood swings

❄️ Feeling cold often

🧠 Brain fog, anxiety, or low mood

📉 Declining performance despite consistent training

One of the hardest parts? It doesn’t feel dramatic at first. Just a quiet drift into fatigue, frustration, and inconsistency.

You Can Eat a Lot—and Still Be Underfuelled

This is what catches so many of us off guard. If you’re training hard, especially for ultras or back-to-back long runs, your energy demands are enormous. Even when you're eating full meals, snacking regularly, and enjoying hearty portions, you might still be in a deficit—especially on rest days, during recovery, or after high-volume weeks.

And it’s not just about calories—it’s about timing, consistency, and removing guilt.

A hand takes a slice of veggie pizza with mozzarella on a white marble table, accompanied by a glass of white wine and a dipping sauce.

How to Increase Fuel Intake (Without Feeling Stuffed or Stressed)

Here are simple, sustainable ways to get more energy in—without overhauling your entire day:

1. Add “Extras” to What You Already Eat

Sprinkle seeds, nuts, or olive oil onto salads, soups, oats.
Stir nut butter into porridge or smoothies.
Add cheese or avocado to wraps and toast.
Use milk or full-fat yoghurt instead of water or low-fat versions.

2. Build a Post-Run Fuel Routine

Prioritise recovery within 30–60 minutes of finishing.
Pair carbs + protein (e.g. banana + peanut butter, chocolate milk, toast + eggs).
Don’t wait until your next meal—snack immediately after and again within 2–3 hours.

3. Eat Before Bed

A small snack with carbs and protein can support overnight recovery and sleep.
Try: oatcakes with hummus, Greek yoghurt with honey, or a cereal bowl with milk.

4. Plan for Rest Days

You still need as much energy, if not, MORE, on recovery days to repair and rebuild. Don’t skimp just because you’re not running.

Food Is Not a Reward. It's the Foundation.

One of the most powerful mindset shifts I’ve made as a runner is this:

You don’t earn food by running. You run well because you fuel consistently.

For years, I saw long runs as permission to “treat myself.” A second helping. A pastry. The good chocolate. But now I see it differently—fuel isn’t a prize, it’s a requirement.

This means eating even when I’m not hungry, planning snacks even on short run days, and refusing to link my worth or discipline to how little I can eat.

Your body deserves fuel before, during, and after effort—regardless of your weight, pace, or goals.

Golden battered fish and thick-cut chips on crumpled brown paper, creating a classic, crispy and inviting meal setting.

What Does “Enough” Look Like?

While individual needs vary, most runners need between 2,200–3,500 kcal/day, with ultra runners often exceeding that.

Look for these positive signs of fuelling well:

Stable energy throughout the day
Strong recovery after sessions
Regular menstrual cycles (for women)
Steady mood, sleep, and motivation
Gradual performance gains over time

Still Not Sure If You’re Underfuelled?

Red flags include:

Constant tiredness or irritability
Getting sick frequently
Disrupted periods or low libido
Nagging injuries or slow healing
Thinking about food all the time
Anxiety around rest days or eating more

If any of this sounds familiar, please know: you're not weak. You're not doing it wrong. You're just running on empty—and you don’t have to.

Final Thoughts: Fuel Like You Mean It

I thought I was eating enough—until I wasn’t. Underfuelling isn’t always about restriction. Sometimes, it’s simply not realising just how much your body gives you every day—and how much it needs in return.

So let this be your permission to eat more. To ditch the guilt. To fuel even when you're not “hungry” yet. Because the strongest, most consistent runners I know? They’re the best eaters too.