Many people use zwift to lose weight. When I’m picking a workout, it’s difficult to figure out how many calories that I’ll burn in a workout. This is essential for those of us counting calories.
If you have the expected wattage profile, it’s just a unit conversion with the same efficiency that is used after the workout.
If that’s too challenging because of the efficiency determination then reporting average wattage for each workout is fine. For example, I know that at 166W for 60 minutes, I burn on the order of 600kcal. That should be fairly straight forward to implement.
While you’re waiting for Zwift to implement this , if you can download the workout and then upload it to intervals.icu it’ll show you the average power and total kJ
They already have a working algorithm that they use for runners, it’s about 10% low of what my treadmill estimates for the same distance. Both Zwift and the treadmill have the same weight values and get the same HR data, but the treadmill knows the incline while Zwift thinks I’m running flat
If you own a motorbike and you put fuel in it and then don’t use it, it still has the fuel in it. If you keep putting fuel in it it eventually spills over. If you go for a ride and then, depending how far and how fast you go it’ll use the fuel. At which point you realise that you can decide how much fuel to put in your motorbike based on how far you rode it and how fast.
That’s not a good analogy for how people work. People use fuel all the time whether they are riding or not. Awake or asleep. Sure the amount varies based on activity but not in a direct relationship like that (because you need energy to do fitness adaptations) - and it’s way too complicated to measure to any degree of accuracy.
So knowing how far or fast you rode your bike is not of that much use in deciding how much food you need to eat to maintain or lose weight. And, outside of a lab it’s very difficult to measure it accurately in any case.
Much better to keep track of your exercise, keep track of your weight, keep track of what you eat and adjust based on the long term trend.
At best we get calorie figures by assuming people are around 4.something:1 efficient and using the kj figure from the work you put into the trainer - which is on the zwift hud now anyway you can have kj or cal displayed and it should be in the fit file too.
But the idea “I cycled for an hour so I need to eat 600 calories” doesn’t make much sense as an idea.
It’s actually a perfect analogy… you’re misunderstanding the concept of “work” in an energy balance. The physics is simple, the science has been done.
The car travels a distance with its weight against friction and gravity. That’s work. The engine converts chemical energy over time to do the work. You can absolutely estimate the fuel to purchase knowing the energy efficiency of the vehicle under different driving conditions (you’ve seen this as mpg). If you know the course that you’re going to drive, you can back calculate the fuel and you see this happening every day. Your car estimates fuel remaining by assuming that your current driving conditions will remain constant until you run out of fuel.
The body is the same. We aren’t doing watts of work. Watts is power, which is energy expenditure over time. I’m asking Zwift to integrate the watts (the workout wattage profile) over time for the workout and apply the same body efficiency factor that is used after the workout. The wattage profile is like knowing the driving conditions and the course for the car. Remember, Zwift adjusts the resistance such that my pedaling speed creates the same power as the course.
If you’re talking about a race or a free ride then I agree with you. That wouldn’t be as simple to estimate.
From what I understand the estimate from kJ to kCals using power in cycling is not that far off. I don’t personally track calories at all, but I can understand how someone would want to.
Yeah, it’s probably the best of all the methods that won’t accurately measure or guess your calorie usage - at least outside of a lab.
The question is how efficient you are. if you take kj=calories then you’ve made the assumption it’s the same as the conversion factor between kilojoules and kilocalories. There’s a fair amount of variance between different people though.
Just as most of the calorie information on food products isn’t really telling you how many calories you will absorb if you cook and eat the food.
If it did and if the calorie figure on strava was correct, sure it feels like you should be able to get the scales out and weigh your meals and take the figures from strava and calculate an exact 500 calorie deficit or whatever you want.
I see most of the apps don’t seem to take exactly kj=kcal, but the difference is at a level of precision that would make no difference anyway - like strava has for my last ride 360kj and 343 calories - the 17 calorie difference from just taking the kj figure would make no real difference.
As for being told physics is simple, aside from the fact it isn’t, it’s human physiology that is more important. A good example of this is that my dad is eating about 1.5x the amount of food you’d expect, he does no exercise but is still losing weight. Of course there’s an explanation for this weight loss - he’s not breaking the laws of physics - but physics isn’t really your friend to explain what is happening.
Michael, I have been doing what you’re suggesting. I track every gram of food I eat and I log all of my active calories. I can predict my rate of weight loss very accurately and the scale agrees.
This is not high level physics. This is a basic energy balance. Yes, there are assumptions based on empirical studies such as the efficiency but you shouldn’t conflate not being perfect with not being useful.
At the end of the day, I just want to know what the estimated active calories for a given workout is rather than trying to look at every graph trying to figure out if I’ll need to skimp on dinner or pack an extra sandwich for lunch.
For folks that don’t think counting calories is accurate, you will get much better results with apps that use lab measured food standards and then weighing your food. The labels on food are allowed to be off by 20% or something of that magnitude.
Your scales are not that accurate, you say yourself that the calorie figures for food are not that accurate, you have no accurate way of measuring how many calories you use in zwift or anything else - and you say “this is not high level physics” - you couldn’t have understated it more
And, of course, as anyone who exercises regularly and considers their diet discovers : you’ve lost weight at about the rate it’s reasonable for a healthy person to lose weight. I could have predicted that too. Without any data.
Good idea. Would be simple to add the theoretical calorie burn value. They do list the TSS value (aka Stress Points). You might be able to determine your own approximate personal conversion of that value to calories by looking back at past activities. Eg. 8x TSS = Calories or whatnot
, if you can download the workout and then upload it to intervals.icu it’ll show you the average power and total kJ 
