Beta Fuel + Dehydration = A race gone bad

Hi all, I’ll try to be brief.

I had my A race (100km mtb event) last weekend and as a good kid I had everything prepared in terms of nutrition and hydration (or so I thought…).

Last year I somewhat semi-bonked towards the end due to a rookie error of having only beta fuel bottles with me and getting water at the food stations, which clearly was not sufficient.

This year I thought I had everything figured it out by mixing 160g of Beta in a 500ml bottle and having another 500ml bottle of electrolytes. I also had with me some Beta gels to hit approx 100-120g per hour as tested in training. The plan also envisaged having the same mix of bottles (160g of beta + electrolytes) halfway through the race (approx after 2 hours).

Just as last year though it was very hot and even before reaching my girlfriend with the other bottles midway through the course I had emptied the electrolyte one and was very reluctantly reaching for the beta fuel bottle. In the meantime I had a gel but was starting to regret that too…

After stopping to get the other bottles and after 1 hour of climbing I could not hold 200w (barely above z1) and I was really dragging myself even though it was certainly different from a feeling of bonking for lack of fuel…in fact I had no desire of having another gel or drinking from the beta fuel bottle.

My questions are then:

When very hot and humid should I just bring electrolyte bottles and fuel with gels (being MTB I use no solid food)?

Should I then also bring down the fuel target from say 100g per hour to 60-80 so that it does not sit on my stomach?

Any other useful tip or recommendation?

When it’s hot and/or humid generally food/fuel does not process as well in the stomach. So if in moderate weather you can handle 100g an hour, you may need to back that down a bit to 75-90g. It doesn’t matter as much for events lasting sub 3-4 hours, but as the hours continue the stomach can get more and more fussy. Experiment with this and find what works for you.

This year specifically but also in the past, I’ve struggled with MTB ultras (100mi). I’ve felt I’ve underperformed and have been frustrated with my body. Having optimized everything else, I looked into my sweat rate and electrolyte loss (Precision hydration sweat test). I found I was taking in less than half the electrolytes I needed.

For MTB 100s, I was consuming 90g of carbs and 480mg of sodium an hour. However, I needed 1100mg of sodium an hour. So in these races I felt good for the first 3-4 hours, but as I got further and further behind on sodium my efforts fell off a cliff even though I continued consuming 90g of carbs.

In my last MTB 100k I took in 980mg if sodium and I felt much better. In addition, I preloaded 1500mg of sodium the night before and the morning of the race. I have a MTB 100mi this weekend and I’m going to up my sodium to 1230mg an hour to see if I get an even better improvement.

Though this is a n=1 example, low sodium may be a cause of your troubles.

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How much water and electrolytes are you consuming per hour? For me it’s probably 1L (minimum) per hour in hot conditions (plus electrolytes and carbs on top of that), initial guess is you’re not taking in enough base fluids to stay hydrated and allow you to absorb the carbs.

Thanks very helpful.

Whilst I have decent knowledge on the carb side, I am a bit less expert on the hydration side of things (mostly because I do not usually struggle this much).

What products do you use to preload on sodium or also during the race?
I’ve used the SIS electrolytes tabs during the race (not the powder with carbs in). Should I drink a few bottles the day before with electrolytes to be ready?

shouldn’t be necessary with SIS products, the gels and such are isotonic and don’t require additional water

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Yep, at some point I wanted to have two bottles of only water or electrolytes instead of only one.

On the other hand this would mean relying exclusively on gels for nutrition which is something I might consider in any case since carbs absorption was severely impaired.

It’s very rare that I do not want to touch sugar bottles or gels :joy:

True but those are the “basic” gels. I was using beta fuel.

I’m not sure whether the SIS electrolyte powder which has some carbs might be more beneficial in these cases.

Every time its super hot I fall apart also. My gut feels like it wont empty when its hot so I cant take on more fuel. So what everyone is saying it get more fluid so that your body will be able to absorb it all?

Sounds like hydration, first thing to do is to work out how much hydration you need before you start to see performance impacts. this is a simple method to work out a rough sweat rate that gives you a guide. For me I lose about 1l an hour, which means that I need to keep an uptake of 700-800ml an hour on long events to avoid a performance hit through dehydration. I’m slowly dehydrating at that rate but unless I’m riding for many hours not enough to matter.

Second challenge might be eating when it’s really hot. I can’t keep my carb ingestion up when the temps climb so dip down to around 60-70g an hour. I still try to get the bulk of that through liquid but I move away from the punchy options like Beta or Maurten and on to something a bit more paletable like SIS Go Electrolyte. That and gels normally keeps me on the borderline of enough food and sodium intake. But all my liquids definitely have sodium in and ideally some carbs, even if I’ve had to moderate that due to the heat.

If I’m understanding your post correctly, this means you had about 500mL of non-Beta drink mix for the first two hours of a race on a very hot day. Everyone is different, I suppose, but that sounds firmly in the “not nearly enough” category. My inclination would be to have significantly more fluid on hand (something like Skratch, that has some carbs but nothing like Beta).

I use Precision Hydration tabs
(1500), which are 750mg per tab. So I use 2 tabs the night before in a 16-20oz bottle as well as the morning of. It’s recommended to finish the bottle over the course of an hour and 45 minutes before the start. I add 1 tab (750) to my drink mix (GU Roctane) which already has 480mg of sodium. I only do liquid fuel in 24oz/710ml bottles, then take in additional water at aid stations if I feel like I need a drink.

I lose around 1L an hour also, so you noticed if you knock down the carbs in the heat you can still process calories?

I will have to try that this weekend and see how I do.

Not really what I’m talking about. Basically it sounds like to me he’s not taking in enough base fluids and sweat rate is outpacing what he’s taking in.

@jz91 - maybe I missed it - how many ml of water are you taking in per hour? (In total - water content of beta mix, electrolyte mix, etc?) Basically, I start around 1L of water an hour to balance how much I sweat and add my electrolytes and carbs on top of that at the appropriate ratios (usually electrolytes added directly to the water, and then fuel separate bottle or gels).

I did the precision hydration sweat testing also and was at 1200mg/hour sodium loss. Precision Hydration has the tabs which are fizzy and expensive.

So I have been using sodium citrate instead is way cheaper and no fizz:

I also struggle majorly in the heat/humidity, have had heat exposure several times in the past might be part of why, takes a lot of riding in it to get more used to it for me.

I don’t know how much is in my head and how much is actually real but for sure I struggle to eat when the temps climb. Liquid carbs help but I need to dilute it down from something like a beta fuel to get it down and feel as if I’m getting any hydration.

I am a prime candidate for doing some heat adoption though, I struggle when it’s hot. This is probably trainable

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You are right and that’s exactly the feeling I had, not enough of “plain” fluids.

For the first two hours I had one 500ml with 160g of Beta fuel and 500ml of SiS electrolyte (no carbs) and some water from the food stations.

I would have gladly dropped the beta for just another electrolyte bottle after 90 minutes but it was too late of course for that.

However for practical and race purposes this will mean having to start the race with two electrolyte bottles or maybe one with moderate carbs.

So yeah, considering the restriction of being able to carry only two bottles these should be electrolytes only (I can only carry two bottles) and then gels or food to hit the carb target.

Yep, this I feel is what I have in mind now. Also the Beta fuel liquid was very hard to digest and definitely felt like I was absorbing very little of it after a while.

Maybe an obvious question, but are you preparing for racing in high heat/humidity in training? I guess it might be difficult if you’re travelling to race, but you can still train without fans indoors with the shower on, or wear extra layers outdoors or so on.

Having said that, I know that even when heat adapted a few extra degrees coupled with the intensity of racing can change how I deal with nutrition quite a lot.

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OP was mixing the beta fuel at 2x concentration. that isn’t really close to isotonic. They then exacerbated the issue by drinking all their water before, leaving only the concentrated beta fuel. That is a problem.


For me personally - I’d start with this.

Per Hour: 1L of Water, 1000mg of sodium / Electrolyte, ~100g Carb. (Adjust as needed, maybe you need more/less water, more less / sodium, and maybe you can handle more carbs, maybe not in the heat and long events)

Logistically that requires carrying everything with you, or relying on aid stations. For MTB - look into an USWE 2L Hydration bladder, plus carry one bottle of just fuel, and you have room for another bottle on most bikes.

I just did a 6hr MTB race recently and relied on one bottle only, so used gels for fuel. It was a pain and I had to use all the aid stations so am going to the setup I recommended above.