joddly wrote:
Thanks so much for posting this. If it's valid to extrapolate down to a 50kg female pushing 140W (in my dreams) in an IM, it gives 50g CHO (200kCal) or less which sounds wonderful!
What's the source, or are they just common values used in the sports physiology world?
Not Tilbury or nearly as smart as him, but calorie expenditure in cycling is very well represented by power output, which makes sense since a calorie is just a unit of energy and power is a unit or energy/time. The mass of the rider actually doesn't matter, only the power output (larger riders usually have more power output, but a 100kg rider at 100w is still burning ~half the kcals as a 50kg rider at 200w) The biggest source of error is the metabolic efficiency of the body, it's usually represented between 20-25%.
The math works out as 1w=1j/s and 1cal=4.18j. So for easy numbers say 100w for 1 hour = 100w*3600s=360,000j=360kj, 360kj/4.18=86kcal. Here's the fuzzy bit, human cycling efficiency is 20-25% so your actual 100watthour burn is 344-430kcal. That's a pretty wide margin. It's safe to say if you've been cycling a lot you're nearer the more efficient end, but even things like temperature and position can have an effect. I just use 350kcal/100watthours as an easy number. 2h at 200w is 1400kcal, 4h at 250w is 3500kcal.
Awaiting Tilbury's answer about fat oxidation as that's a more mysterious measure.