The Science Behind Your Plan
Every recommendation Race Day Tech generates is grounded in peer-reviewed sports science. Here's what powers the engine.
1. Calorie Expenditure
We estimate energy cost per segment using the ACSM running metabolic equation:
Calories/min = VO₂ × body_mass_kg × 5 ÷ 1000
Speed is derived from your target pace for each segment, and grade is computed from the elevation profile of your GPX file. This gives a per-segment calorie burn that reflects how hard your body is actually working on uphills, flats, and downhills.
ACSM's Guidelines for Exercise Testing and Prescription, 11th ed. (2021)2. Carbohydrate Fueling
Carbohydrate recommendations scale with race duration based on gut transporter physiology:
- Races ≤ 2.5 hours: 60 g/hr — the single-transporter (SGLT1) absorption ceiling for glucose.
- 2.5 – 5 hours: 90 g/hr — using multiple transporters (glucose + fructose in a 2:1 ratio) to increase absorption.
- Ultras (> 5 hours): 68 g/hr — a sustainable rate accounting for GI fatigue over very long efforts.
We then apply a ±10% grade adjustment: steep climbs (>6%) increase carb demand, while steep descents (<-5%) reduce it slightly due to the lower metabolic cost of eccentric loading.
Burke, L.M. et al. (2011). “Carbohydrates for training and competition.” Journal of Sports Sciences, 29(sup1), S17–S27.Jeukendrup, A.E. (2014). “A step towards personalized sports nutrition.” Sports Medicine, 44(Suppl 1), S25–S33.3. Grade-Adjusted Pace (GAP)
Flat-ground pace doesn't tell the whole story on hilly courses. We use a simplified model based on the Minetti cost of transport curves to compute how much each segment's grade will speed you up or slow you down:
- Uphill: roughly +12% per 1% grade (up to 15%), with diminishing returns beyond.
- Moderate downhill (-1% to -5%): a small speed benefit — gravity helps.
- Steep downhill (< -5%): benefit plateaus and reverses as braking forces increase.
This grade-adjusted pace drives your estimated segment times and, in turn, the per-segment nutrition targets.
Minetti, A.E. et al. (2002). “Energy cost of walking and running at extreme uphill and downhill slopes.” Journal of Applied Physiology, 93(3), 1039–1046.4. Hydration Model
Fluid needs are estimated from a sweat rate model incorporating pace, temperature, humidity, and sex:
- Base sweat rate: 700 mL/hr (male) or 500 mL/hr (female).
- Speed factor: faster pace increases sweat output proportionally.
- Heat factor: +4% per degree above 20 °C.
- Female adjustment: ×0.88 based on average sweat rate differences.
Final output is clamped to 400–1,000 mL/hr to stay within safe physiological bounds and prevent both dehydration and hyponatremia.
Sawka, M.N. et al. (2007). “ACSM Position Stand: Exercise and Fluid Replacement.” Medicine & Science in Sports & Exercise, 39(2), 377–390.5. Electrolytes
Electrolyte baselines are set from consensus guidelines and scaled by a heat index that combines temperature and humidity:
- Sodium: 500 mg/hr base × heat index
- Potassium: 150 mg/hr base
- Magnesium: 15 mg/hr base
Female athletes receive a ×0.88 multiplier reflecting lower average sweat electrolyte concentration.
Hew-Butler, T. et al. (2015). “Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference.” Clinical Journal of Sport Medicine, 25(4), 303–320.Sims, S.T. & Heather, A.K. (2018). “Myths and Methodologies: Reducing scientific design ambiguity in studies comparing sexes.” Experimental Physiology, 103(10), 1309–1322.6. Limitations
While grounded in published research, our models carry inherent limitations:
- Population averages: Formulas are based on group-level data. Individual variation in sweat rate, gut absorption, and metabolic efficiency can be significant.
- Environmental simplification: We use a single temperature and humidity for the entire race, not segment-level microclimate data.
- GI tolerance: The model suggests carb targets your body may need time to train toward. If you haven't practiced race-level fueling in training, start conservatively.
- No altitude adjustment: Elevation above sea level affects metabolic rate and appetite, but is not currently modeled.
- Pace assumptions: If you run significantly faster or slower than your target, actual needs will differ from the plan.
7. Disclaimer
Important: Read before using your plan
Running long distances is an inherently demanding and potentially dangerous activity. Pushing your body to its limits carries real risks including but not limited to dehydration, hyponatremia, heat illness, cardiac events, and musculoskeletal injury.
Race Day Tech provides informational guidance only. Our nutrition plans are generated from population-level sports science models and are not a substitute for professional medical advice. Every runner is different. Consult a physician or registered sports dietitian before making significant changes to your race-day nutrition strategy, especially if you have any underlying health conditions.
Race Day Tech assumes no responsibility for the outcome of your race or any adverse health effects that may result from following the plans generated by this application. You use this tool entirely at your own risk.