I always enjoy watching TR content with Kyles input. This one had me asking, why can’t we calculate a pre fueling plan for each workout and display it with the session plan?
I asked Chat GPT to give me a formula for this based on FTP and IF, and this is the response.
‘can you generate a formula for carb consumption of a training session based on FTP and intensity factor for the session?’
ChatGPT said:
Yes – you can create a practical formula to estimate carbohydrate (CHO) consumption in a cycling training session using FTP (Functional Threshold Power) and Intensity Factor (IF).
Here’s a structured approach:
1. Key Concepts
FTP: Rider’s approximate 1-hour maximal sustainable power.
IF: Intensity factor = (Normalized Power ÷ FTP).
kJ of Work: On a bike with power meter, total work (kJ) ≈ calories expended (kcal).
Energy Yield:
1 g CHO = ~4 kcal
Carbohydrate vs fat contribution depends on intensity (higher IF = more carb reliance).
2. Carbohydrate Contribution by Intensity
From exercise physiology research:
At IF ~0.6, CHO contributes ~30–40% of energy.
At IF ~0.7, ~50%.
At IF ~0.8, ~65%.
At IF ~0.9–1.0, ~80–90%.
At IF >1.05, almost exclusively CHO (~90–100%).
We can approximate CHO fraction (
CHO_frac) as:CHO_frac=0.3+(IF−0.6)×1.5CHO\_frac = 0.3 + (IF - 0.6) \times 1.5CHO_frac=0.3+(IF−0.6)×1.5
(limited between 0.3 and 0.95).
This gives ~30% at IF=0.6 and ~95% at IF=1.2.
3. Formula for Carbohydrate Consumption
First, calculate work done in kilojoules:
WorkkJ=Average Power (W)×Duration (h)×3600/1000Work_{kJ} = \text{Average Power (W)} \times \text{Duration (h)} \times 3600 / 1000WorkkJ=Average Power (W)×Duration (h)×3600/1000
But Average Power can be estimated via:
AvgPower=IF×FTPAvgPower = IF \times FTPAvgPower=IF×FTP
So:
WorkkJ=(IF×FTP×Durationh×3600)/1000Work_{kJ} = (IF \times FTP \times Duration_{h} \times 3600) / 1000WorkkJ=(IF×FTP×Durationh×3600)/1000
Since 1 kJ mechanical ≈ 1 kcal metabolic,
TotalEnergykcal≈WorkkJTotalEnergy_{kcal} ≈ Work_{kJ}TotalEnergykcal≈WorkkJ
Then:
CHOg=TotalEnergykcal×CHO_frac4CHO_{g} = \frac{TotalEnergy_{kcal} \times CHO\_frac}{4}CHOg=4TotalEnergykcal×CHO_frac
4. Final Combined Formula
CHOg=(IF×FTP×Durationh×3.6)×CHO_frac4CHO_{g} = \frac{(IF \times FTP \times Duration_{h} \times 3.6) \times CHO\_frac}{4}CHOg=4(IF×FTP×Durationh×3.6)×CHO_frac
Where:
IF= Intensity Factor
FTP= Functional Threshold Power (W)
Duration_h= session duration in hours
CHO_frac= 0.3+(IF−0.6)×1.50.3 + (IF - 0.6)\times 1.50.3+(IF−0.6)×1.5, bounded [0.3, 0.95]
5. Example
Rider with FTP = 250 W, 2 h ride at IF = 0.8:
Avg Power = 0.8 × 250 = 200 W
Work = 200 × 7200 / 1000 = 1440 kJ
CHO_frac = 0.3 + (0.8–0.6)×1.5 = 0.6
CHO_g = (1440 × 0.6) ÷ 4 = 216 g CHO
This gives a session-level carb consumption estimate, useful for fuelling strategies.
So if the above is actually correct can TR implement this calc into our calendar to aid in understanding how we should fuel our work?