It kind of seems it makes a big difference if an activity is continuous or not. By the 1.1 variability test, almost all of my activities seem like ‘interval’ ones. I am not sure this is a good metric since sometimes I have a very easy ride just pimped by an interval at the end. Oviously I didn’t burn too much during that activity just because of those last minutes. Conversely, an activity performed only at threshold (say 30’ at Z4) would have a low variability but probably also a low fat burn percentage. I guess this ‘intervalness’ of activities needs to be revised.
I just checked many of my activities to get a feeling for the algorithm and in most cases, the CHO out is roughly equal to calories burned/4, which would indicate no fat burn at all - and most of those activities are not really high intensity. In some cases, the CHO*4 even exceeds the total calories!
can you maybe implement a user setting for VT2 for anyone with a lab measurement?
I’ve just seen this on my last activity. It seems very high for what was an easy 60m endurance ride . 124g CHO for 428kJ ride at 57%intensity. Xert had it at 37g and Sentiero at 39g
124g/hr CHO for me at 124W NP (My LT1 is about 165W/175W) is very very high. If that was correct I’d be bonking on every long ride I did. A had a metabolic test done some years ago and I was burning 124gCHO at 21OW and with the amount of low intensity training I’ve being doing since then I would be very surprised if it wasn’t less now.
I’ve just noticed this and, as others have noted, the CHO requirement for my latest activity (I haven’t checked any others) is way too high – CHO 1173g (= 4692 kcal) for a ride with an estimated calorie burn (based on power meter data) of 2502 kcal. Teething problems from carbohydrates – a dentist wouldn’t be surprised
Hmm. I don’t know enough about this topic. I played around with the data for a 150km 5h ride I did recently just to see what things the formula is sensitive about. I have unit tests derived from the worked examples in the video so I think I have the calculation right.
Hi David – as a rough rule of thumb, the kJ of work done on a ride is equal to the calorie requirement in kcal. One gram of CHO supplies about 4 kcal of energy.
It’s much more difficult to come up with a rule for how much of your energy is obtained from burning fat and how much from CHO, and it will vary from person to person. Somewhere around mid Z2 you’re likely to be getting half your energy from CHO, and this proportion increases as intensity goes up. Above threhhold you’ll be burning very little fat. The relationship between %fat and intensity is not linear.
For me, riding at an average of 133W, I’m doing about 500 kJ/hr of work and so need 500 kcal/hr of fuel. 133W is somewhere around the middle of my Z2, so I reckon to need about 250 kcal/hr or 60g of CHO per hour. This calculation doesn’t take into account periods of higher intensity, but it’s enabled me to avoid bonking so far.
I must read the paper you’ve reference for any new insights.
PS – for your ride, the calculation of 817g for a 3325 kJ ride means you got nearly all your energy from CHO, which is unlikely for such a long ride. I think the model reflects a relatively narrow range of activities where the relationship between TSS and CHO use is approximately linear. As duration rises and intensity falls, I guess it would increasingly overestimate the CHO used.
Sorry if you’ve got enough datapoints by now, but I think mine’s even more extreme than some others above I think? (With 1g CHO being 4kcal it’s 2.35× kcal number, however reliable that one may be)
I have been reviewing several athletes, including my own activities and my opinion is that you are overestimating your carbohydrate intake, in 2h26m with an intensity of 0.73 you should have eaten 352 grams according to the paper, which is approximately 142 grams per hour. The algorithm I did gives me an estimate of 122 grams, which may not be correct either, but it is more logical.
I’ve been thinking about this. It’s an attractive idea to have a simple formula that you can just plug TSS into and get an estimate for CHO used, but as we’ve seen it works over a relatively narrow range and to get a decent estimate for longer rides would require another model, or more.
Another possible issue is that the variables in the model aren’t independent. TSS is dependent on duration and FTP, which appear elsewhere in the formula. I’ve no idea if this matters or not, but it makes me feel uncomfortable mathematically.
As we have computers to do the donkey work, maybe a brute force approach would work over the full range of rides. If you have a file of power output over the ride, and a table or formula relating %CHO usage to intensity (available from the literature, I’m sure), then it’s a trivial matter to calculate CHO usage on a minute by minute basis and add it up.
This loses the elegance and simplicity of the model, and would suffer inaccuracies from individual variation (a bit like 220 - age for max HR), but maybe it would do a better job for an app like intervals.icu?
The TSS in the formula is actually TSS squared. So it allows curvilinear relationships. But as noted elsewhere, carb dependence really changes past 90 min so these formulas will need to be adjusted on longer rides
Again, I reiterate that the power of this community will make the product (Intervals.icu) better. We’ve got programmers, researchers, coaches all out in the field doing the work. We can work together to further improve and refine the data set to help everyone!
So many great insights into this topic.
I have been using a spreadsheet from Knowledge of Watts and Jeff Rothschild, There are Inscyd, Aerotune, and Xert platforms to converge values and data sets on. I think the basic point is that this nutrition fuelling game is way too beneficial for us NOT to try and get it right along the way.
Using work (kJ) allows measuring volume and intensity at the same time. If you know your burn rate per hour or half-hour, you can work out what you need to consume by using about 40-50% of that value. Let’s say you’re riding at 150W (average power for 6 hours) that equates to:
150W average = 540kJ per hour (watts * total seconds / 1000), or 3240kJ total duration.
3240 * 40% = 1296 / 6 (hours) = 216kJ/hour
3240 * 50% = 1620 / 6 (hours) = 270kJ/hour
The 216-270kJ per hour is what you would need, which equates to 54-60g of carbs; divide the answer by 4.
Edit: Garmin allows this to be displayed as a metric, so it’s easy enough to track.There are charts showing kJ per hour or like me, every half hour. Not sure about other head units.
Yeah, there is a squared term in the equation to capture some non-linearity, but what’s missing (and can be implemented) is an adjustment down to account for decreasing carb dependence as the duration gets longer.
