Fueling at Altitude: Does Your Strategy Need to Change When Training or Racing High?

Altitude training camps and high-elevation races are now standard for serious endurance athletes. Whether you’re preparing for an Ironman in Colorado, a trail ultra in the Rockies, or simply chasing gains at a camp in Flagstaff or the Alps, one question comes up again and again: “Do I need to change how I fuel and hydrate when I’m at altitude?”

The short answer is yes — at least initially. The physiology of altitude creates real changes in energy demands, carbohydrate use, and fluid loss that can quietly sabotage your training if you don’t adjust. But with the right strategy, you can turn altitude into a performance advantage instead of a hidden limiter. Most people under-fuel and under-hydrate in the first 1–2 weeks, which slows adaptations and increases fatigue. Here’s exactly what changes at altitude and how to fuel smarter.

What Actually Happens to Your Body at Altitude

The common myth is “there’s less oxygen.” That’s not true — the air is still ~21% oxygen. What drops is air pressure, which reduces the driving force pushing oxygen from your lungs into your blood. This creates a hypoxic (low-oxygen) environment for your tissues.

Key physiological changes (Stellingwerff et al., 2019; Young et al., 2018):

• VO₂max drops ~6% per 1,000 m of elevation gain. At 2,200 m (common camp height), you’re working at a higher percentage of your max just to maintain the same power or pace.

• Increased carbohydrate reliance: Your body burns more carbs at any given intensity because fat oxidation is impaired in hypoxia.

• Resting metabolic rate (RMR) rises: Studies show ~300 extra kcal/day at moderate altitude (Woods et al., 2017 — the Flagstaff camp study). Over 4 weeks, that’s an extra ~8,400 kcal your body needs.

• Blunted exogenous carb oxidation: In the first 1–3 weeks, your gut and muscles are less efficient at using carbs you ingest during exercise (20–50% reduction in some studies). This means you can deplete glycogen faster even if you’re eating the same amount.

• Higher fluid loss: Increased ventilation rate (you breathe more) and drier air = greater respiratory water loss. Sweat rate can also rise initially.

These changes are most pronounced in the first 5–14 days. After 3–4 weeks of acclimatization, many (but not all) of them improve — hemoglobin mass rises, carb burning normalizes, and you adapt.

Does Your Fueling & Hydration Plan Need to Change?

Yes! Especially during the first 1–3 weeks. Here’s what the research shows (Stellingwerff et al., 2019; Karpęcka-Gałka et al., 2024):

1. You need more total energy

   • Aim for an extra 300–500 kcal/day above your sea-level needs, especially on harder training days.

   • Focus on nutrient-dense carbs and protein to avoid the “training camp effect” where athletes under-eat and stall adaptations.

2. Carbohydrate demands go up

   • Daily intake: Increase toward the higher end of your normal range (e.g., 7–10+ g/kg body weight on hard days).

   • During exercise: Add ~10–15 g extra carbs per hour compared to sea level (pros like Victor Campenaerts report doing exactly this).

   • Why? Higher relative intensity + blunted carb oxidation = faster glycogen depletion. After full acclimatization (3–4 weeks), you can often return to your normal race fueling rates.

3. Hydration becomes more critical

   • Fluid needs rise due to increased breathing and potential diuretic effect of altitude.

   • Monitor urine color and morning body weight daily. Many athletes lose 1–2 kg of water weight in the first week if they don’t adjust.

   • Electrolytes matter more — sodium losses can increase. Use your normal sweat-tested strategy but be proactive with extra fluids.

4. Protein needs may increase slightly

   • Support muscle repair and the higher overall training stress. Aim for 1.6–2.2 g/kg body weight.

Project Blue Practical Recommendations

From our athletes who train and race at altitude:

• Week 1–2 at altitude: Increase daily carbs by 20–30% and add 10–15 g/h during sessions. Prioritize easy-to-digest sources (gels, drinks, rice).

• Pre-session: Top up glycogen with an extra 30–60 g carbs 1–2 hours before.

• During long sessions: Use higher carb drinks (60–90+ g/h) and test gut tolerance in training.

• Post-session: Prioritize carbs + protein within 30–60 min to replenish glycogen faster.

• Hydration: Weigh yourself morning and evening. Add 500–750 ml extra fluid per day and monitor sweat rate with our testing protocol.

After 3–4 weeks, most athletes can return closer to sea-level fueling — but staying slightly higher in carbs often continues to pay off.

The Bottom Line for Your Next Altitude Camp or Race

Altitude doesn’t require a complete overhaul of your plan, but it does demand intentional increases in energy, carbohydrates, and fluids — especially early on. Under-fueling is one of the most common (and preventable) reasons athletes feel flat or fail to adapt during camps.

The good news? Once you acclimatize, your well-practiced sea-level strategy often works again — and you come home with more hemoglobin, better mitochondria, and a stronger engine.

If you’re heading to altitude soon, book a quick consult with us — we’ll build you a personalized fueling plan that matches your exact elevation, training load, and sweat profile. Have you trained or raced at altitude? What fueling adjustments worked (or didn’t) for you? Share in the comments — I read every one!