Optimal dehydration during a race

Yes, title is correct, I’m asking about DE-hydration.

In running races such as the marathon it is expected to be dehydrated by the end, and ultimate performance would be hindered by a 1:1 loss:replacement ratio.

I’m training for a gravel century race, and have been tracking sweat loss. My loss is around 800-900 mL/hr. I would get pretty slushy replacing that much, but moreover can’t easily bring that much liquid on the race, as the front group doesn’t stop at the single aid station.

I am wondering what an optimal dehydration level after 5 hours (or per hour) would be.

I hope to at most carry 4 bottles, with preference of 3 and “cameling up” beforehand. There will be mix and salt involved too, but not getting into salt details here. Even if I drank 4 x 600mL bottles, that’s 2kg of net loss. Is that too much?

I typically lose 700ml an hour and replace 500ml an hour, if its hot I’ll aim to take in more and ensure I’m fully hydrated at the start.

2kg seems like a lot but what % of your weight is that?

From British Cycling

Studies have shown that surprisingly low fluid losses can significantly affect your ability to ride. A 2% drop in body weight due to sweating (1.6 kg for an 80 kg rider) will impair performance noticeably, 4% will decrease your capacity for muscular work and, at 5%, heat exhaustion can become an issue and your capacity for work will drop by up to 30%. Hit 7% and you’ll start experiencing hallucinations and, at 10%, circulatory collapse, heat stroke and even death become possibilities.

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Apologies, wasn’t sure how personally specific to get vs general. 77kg, so 2kg is 2.6%.

Using specific values:

18mph = 5.56 hrs
850 mL/hr loss = 4.73 kg(L) loss
600 mL/hr consumed = 3.36 L consumed
(Avg suggested in other resources such as Here) )
Net loss = 1.37 kg
= 1.78% of bodyweight.

Like all things its down to how you respond personally and importantly the weather on the day.
My event last weekend was very hot and I needed to take in a lot more than normal. In the TdF some stages were so hot riders we taking in 1000ml an hour.

If its warm I try to get a 500ml bottle in to me in the 30mins before a start and I allow myself a 30 min buffer on hydration.

If its a gravel race I imagine the its going to be hilly so the trade off is cutting it fine being self supported, carrying an extra bottle or stopping to take in some liquid.

Finishing slightly dehydrated is fine but I’ve also really run out of steam (literally) not hydrating enough

Of course.

We’ll see how weather shakes out, but compared to the likes of Gravel Worlds and DK etc. I expect my race to be easier, Fall Weather in the midwest is quite temperate, and the course is <3000’ of elevation (Course not released, but confidently less than 3000’).

Some of those sweat rate tests are on similar course conditions and (relatively) similar weather (<10F hotter than expected). The outdoor gravel rides tend to be on the low end of the 800-900 mL/hr range.