A question of metabolics/energy/weight

[ColinOldnCranky]

Every year I have to do my age in kms at least once. It's a right-of-passage sort of thing that a lot of unicyclists do from about 55years onwards.

So, today I made an effort to get that one out of the way nice and early. Finished on 65.55km for the day. I think that is my second highest one day ever.

Some numbers: I rode for around 6.5 hours in an 8 hr period during which time I consumed around 3 litres/3kg of fluid and came home 2.2kg lighter than I started.

So, the question:

I had not been to the toilet in that time. So I guess that the 5.2kg is just from CO2 loss expired thru breathing, water expired thru breathing and water sweated out.

So of that 5.2kg, how much "real" weight loss does it represent ie How much weight have I used to generate the energy to do the ride?

Bear in mind that I will be somewhat dehydrated and that my body/blood/cells will now take on an amount of water to get the body/blood/cells back to correct state.

[gabrielle260]

Colin, I haven't looked up the formula to calculate it but there is some data missing before anyone could do so. Did you eat anything immediately before or during the ride? What was the temperature during the ride? How much wind was there? What was your starting weight?
More importantly, are you feeling ok after the ride as that is a lot of fluid to lose in one ride!!!
Andrew

[Addictr3]

there is a huge difference between weight loss and body fat loss, you have simply lost water weight.

[CKinnard]

The 5.2kg is mainly water, with a small contribution from fat and carbohydrate.
How small?

If we presume you were exercising at a low moderate rate of VO2max, then a reasonable assumption is that your energy burn was 60% fat and 40% carbs (glucose and glycogen).

It's also reasonable to presume you were expending ~250 Calories per hour (includes BMR). This is a realistic rate for the duration you indicated.

So, 250*6.5 = 1625 Calories. Converting to fat and carb

fat = 1625 * .6/9 = 108 grams
carb = 1625 * .4/4 = 162 grams

Thus, total energy reserve weight loss = 270 grams = 5% of total weight loss.

However, being down 5kg of water, and not peeing in that time, is a sure sign you were severely dehydrated.

[ColinOldnCranky]

However, being down 5kg of water, and not peeing in that time, is a sure sign you were severely dehydrated.

Not down 5kg. I have to factor in the 3kg I drank, some of which would have gone to replenishing those depleted cells.

It's not unusual for me to finish a long ride without going to the toot. It is not that it isn't there waiting, just that I am ok with holding it in until convenient. I offloaded after I weighted in.

I would expect some dehydration as a function of the heat and duration.

But I do keep an eye on basic signs during and after significant effort. (Why I weigh before and after.) Colour of urine colour was fairly ordinary afterwards.

[CKinnard]

yeah sorry Col. I forgot you had the 3 liters.
A common rule of thumb for endurance activities is to consume a water bottle (600-750mls) an hour, which would have been appropriate in your case.