Fuelling Your Threshold Run | Endurance Kollective
Most runners make the same mistake: they eat — or don't eat — the same way before every session, regardless of intensity. If you're heading out for a Zone 2 base run, that approach is fine. But if you're about to push into threshold territory, underfuelling isn't just inefficient it's actively working against your training adaptations.
Here's why your fuelling strategy needs to shift when your training does.
The Core Principle: Match Your Fuel to Your Workload
Carbohydrates aren't simply 'high' or 'low' — they need to be matched to the metabolic demands of your session. As training intensity increases, so does your reliance on carbohydrates as a fuel source. The harder you run, the more glycogen you burn.
This is why Zone 2 day does not equal VO2 max day — and your nutrition shouldn't treat them as though they do.
Understanding Your Training Zones
Z2 Easy Run At easy, aerobic effort (roughly 65% of threshold pace), your body primarily burns fat for fuel. For shorter durations, you may need no exogenous carbohydrates at all. For longer easy runs, you're looking at 40–60g of carbs per hour depending on duration.
Z3 Tempo Run Tempo pace increases your reliance on glycogen. Fuelling guidelines shift here: no fuel required for shorter tempo sessions, but as duration increases, you may need up to 80g of carbohydrates per hour.
Z4 Threshold Run This is where the difference becomes most significant. At threshold intensity, your carbohydrate demand rises sharply. Expect to need 40–90g of carbohydrates per hour depending on duration and body weight.
Real Numbers: The Same Runner, Different Fuel
Example 1 — 70-Minute Run (60 kg Runner, Threshold Pace: 4:16/km)
| Z2 Base Run — 65% Threshold (5:00/km avg) | Z3 Tempo Run — 100% Threshold (4:16/km avg) |
|---|---|
| No fuel — no exogenous carbs required | 40–60g carbs per hour (160–240 kcal/hr) |
Example 2 — 2-Hour Run (80 kg Runner, Threshold Pace: 4:16/km)
| Z2 Base Run — 65% Threshold (5:00/km avg) | Z3 Tempo Run — 80% Threshold (4:30/km avg) |
|---|---|
| 40–60g carbs/hr (200–240 kcal/hr) | 70–90g carbs/hr (280–360 kcal/hr) — up to +30g/hr more |
What Happens When You Underfuel Your Intervals?
Fuelling below the demands of your target intensity and duration can have real consequences for your training:
- Pace and power output drop sharply during intervals
- Interval quality deteriorates — you fade before the set is done
- Perceived effort increases for the same pace
- Recovery between sessions slows — you arrive at tomorrow's workout already depleted
The bottom line: you cannot build the fitness you're training for if your body doesn't have the fuel to do the work. Underfuelling limits adaptation.
The Takeaway: Fuel for the Session You're Actually Doing
Zone 2 day? Fuel for Zone 2. Threshold day? Fuel for threshold. VO2 max session? Fuel accordingly.
Your fuelling strategy should be as deliberate as your training plan. When training shifts, fuelling should shift too. Right fuel, right intensity, right adaptation.