Sweat Tests: HDrop vs. Precision Fuel and Hydration

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A little bit of an FYI post. I’ve been an HDrop v2 user for over a year now, but bit the bullet and also did a Precision Fuel and Hydration Sweat Test this morning:

I don’t use either for sweat rates (I use a scale for that) just sodium concentration.

Here’s the long and short of it - both tests are extremely close for me. If I look at the general averages for the HDrop in terms of mg/L sodium loss, it’s effectively the same as the Precision Fuel test - both between 1100 and 1200 mg/L sodium concentration loss.

I did the PF test at a local place called Human Powered Health, and had an interesting discussion with them based on based on the calculator they have for what that means for hydrating and sodium needs for different length events. They’re supposed to be sending me some of this data and a more in depth plan, but the recommendations were effectively as such.

Short rides or training sessions, or even longer cold rides where you’re not sweating much: Basically no sodium needed or “To Taste”

Working up the spectrum, take a 4h ride, losing ~1L an hour, I think they recommended 200-300mg / L.

Even further, take Leadville at 8-9 hours losing .75L/hr, they recommended ~400 mg/L. Bump that up to 1.25L/hr loss that jumps to a recommended 900mg/L.

This all assumes you come in well hydrated and with a “good” electrolyte balance, but for any of those durations, it didn’t have me ever replacing what I was losing in my sweat. The longer the duration, the higher the sweat rate, the closer the recommendation got to what I was losing.

I think the only change I’m going to make, is that I probably overcook it a little bit and take in more than I need on shorter and colder rides. But, for a longer and or hotter event, I’ll probably stay pretty close to what I’m losing because I haven’t had an issue. But, I’ve wondered if I’m either retaining more water than I need or have my body working to get rid of excess sodium and having to pee more. For anything longer I can almost never get by without a pee break in the first couple hours. Maybe for an event like Leadville where it’s cold in the morning I start out with less sodium in my bottles, and build to full concentration by Columbine.

@grwoolf - FYI, this came up elsewhere recently as I think you had the PF test done too.

@Dr_Alex_Harrison, might be interesting to hear your thoughts, I think your partner may be racing Leadville this year too?

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I got a Precision test too, and am at about 1100 mg/l. Is there a downside to trying to do closer to full ‘recovery’ of the sodium? I’m using Sodium citrate so I don’t taste it in my drink mix, so is there a downside to trying to do 1100 mg/l (or close to it) for a long race like Leadville?

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FYI, here’s the recommendation they sent me:

  • Attached below are the results from your Precision testing. Based on this data and your self-tested sweat rates, the recommendations below outline hydration recommendations for Leadville 100. For that race, your intake needs will likely increase as the day goes on and it gets hotter and more demanding.

  • With these recommendations the goal is to keep our sodium plasma concentration within an equilibrium over the amount of time you are exercising. The hydration recommendations are meant to keep you above a 2% dehydration threshold - falling below this has been shown empirically to decrease cognitive and physical performance. Then based on the volume of water you had to bring in to stay above that threshold + your sodium concentration will determine the amount of sodium you will need as an electrolyte in your mix to keep you within that sodium plasma equilibrium.

  • For the first portion of the race (0 - ~3hr), based on an estimated sweat rate of 1L/hr, you would need to consume 800ml (0.81L) of fluids with 320mg of sodium per hour.

  • For the 2nd portion of the race (3-8.5hr), based on an estimated sweat rate of 1.5L/hr, you would need to up consumption levels to 1.3L of fluids with 900mg of sodium per hour.

  • Make sure you are going into both races and training with hydration levels topped off. Hydration prep starts the day before! Consume 800-1000mg of sodium both the evening before and morning of long efforts and races. Post-workout and race consume a moderate amount of sodium (800-1100mg) through either electrolytes or food.

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I actually think doing very close to your loss rate for the entire race is probably the “safe” approach, especially if that’s how you’ve trained and you know it works for you.

I may not change anything except dialing it back for the shorter or colder stuff.

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Thanks for sharing and I had also done the PF test earlier this year. I am a really salty guy and my sodium number came back at 1658 per liter of water. Been trying to dial in the nutrition/hydration further this year which has taken a lot of testing and time. I think I am finally getting close.

First, I am not sure why but I need to drink a lot more water than last year. I seem to be sweating way more on rides and need at least 1L/hour. I did a three hour ride on Saturday and was short on water and really suffered!

Second, I find it hard to only drink super high carb/sodium bottles only. I have started using a USWE hydration pack that holds 2 liters water and will have around 25 carbs/1500mg sodium that is easy for me to drink and will sip on frequently. I then have high carb/sodium concentrated 750ml bottles that are around 150 carbs/1200 sodium each and sip every 15 minutes. One bottle will last me about 2 hours and is really nice to have the hydration pack to sip after hitting the bottle. I will also supplement with a gel once an hour to bump up the carbs further for big rides.

On Sunday I followed this approach on a 5 hour ride where I road zone 2 the whole ride and did a 10 minute threshold effort every hour. This worked great and still felt really strong on my last threshold effort at 4.5 hours ride time, also having my highest power numbers.

The whole nutrition/hydration/sodium thing is a journey and really need to find what your body needs and what works. Having the sodium test really helps in trying to dial these items in.

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Biggest thing that has helped me - separate your fueling and hydration for any harder rides.

Everything mixed together in one pack or bottle is great, until it isn’t… Separating allows you to drink as much as you need to fulfill your hydration needs without worrying about getting enough fuel.

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Agree on separating nutrition/hydration and has helped me as well. Although there is some overlap between both but each has its purpose.

Other thing I have found is I know I hit my nutrition/hydration well if after a long ride I am not feeling like I want to eat everything in the house or drink a gallon of water.

I also need to wear lighter colors since I have some crazy salt marks on everything and really shows if I wear black.

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Thank you so much for sharing this. I really appreciate it.

I haven’t tested my sodium concentration - I intend to. But I find trying to do 1000mg of sodium citrate an hour for hard 4-6 hour races makes my stomach cramp (0.8 to 1 liter of liquid an hour) . I have also found that around hour 4 of a hard training ride where I don’t take in much salt at the beginging the 1000mg of sodium citrate in a liter bottle is life saving.

I will try keeping it mellow(er) during the first 3 hours on the salt.

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This is wise.

This is a good strategy, and I look forward to getting it built into our app. :wink:

She is! (@michelleihowe) Wish her luck! She lurks here.

Not really. The first downside of too much sodium is gut distress. If no gut distress, no issue.

Reducing low-sodium fluid consumption pre-race (or increasing sodium) can help reduce this need. If you’re going to take the aforesaid down-scaling sodium consumption strategy in the first hours of the race, then it should be because you’re also scaling down fluid consumption in accordance with reduced fluid needs during the colder hours, too. Hope that sentence makes sense!

@PiperPrincess, note the comments hereinabove about GI distress. Tagging you in case it’s helpful info! Otherwise, safely ignore!

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@Dr_Alex_Harrison

I have been told by a doc that this is a poor safety check, because your gastrointestinal tract adapts long before your kidneys complain.

Also, chronically taking more sodium could cause kidney problems. Be careful!

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Sorry, let me clarify context and scope: the first downside of too much sodium during exercise is gut distress. This applies to folks who do not have high blood pressure or other CVD / CV event risks.

I’d be curious to know if your MD friend sustains the claim about kidneys complaining if there is no change in blood pressure or if blood pressure stays in normal/healthy ranges?

As in, is the sodium-dependent kidney danger independent of changes sodium could cause in less fit folks’ blood pressure.

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@Dr_Alex_Harrison sure! I am going for a ride with him today and will ask and report back. I am also testing these devices (HDrop, Nix, and FlowBio) that is how I got into this thread. Really want to nail down my hydration over this summer and going down the rabbit hole.

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Would be good to have you ask him specifically - if you lose 2000-3000g in a workout is it an issue taking in less than you’re losing? eg. lose 3000, take in 2000-2500.

Not a doctor, but I did have a related conversation with a friend who is one and my sense is if you’re trying to replace what you lose (or less) then you’re probably not putting stress on the kidneys. But If you’re chronically taking in too much, that’s where you could start to have issues.

Overdo it here and there (e.g. your A Race) and it’s probably not a big deal if your gut can handle it.

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@Dr_Alex_Harrison Alright, here is his take on the conversation:

Kidney stress can be BP-independent and symptom-free; lab markers react long before you would feel anything. A 2023 systematic review of 29 studies shows that structured “gut-training” protocols cut exercise GI complaints by 26-47 % within 1 - 2 weeks, even as athletes doubled fluid/electrolyte intake. After adaptation, the same salty bottle that once caused cramping can feel fine, while the renal load is unchanged. So, a slow increase on sodium intake over time, can make your gut tolerate it without distress, but your kidneys pay the toll silently.
Chronic overshoot, not an occasional race-day blow-out, is what accumulates the highest renal risk. But here is the thing, you can actually train your gut to overdose on sodium without knowing that you are overdosing and damaging your kidneys long term. Let’s say that I start taking 200mg Na on Day 1, and keep increasing the dose every day until I hit 2000 mg Na. Then let’s say on Day 20, I get gut distress during the activity. By that time, I could have already trained my gut to tolerate higher than the optimal sodium intake (which could be 1500 mg), from incrementing slowly the sodium intake without getting gut distress, until I get the actual distress once I hit a high intake value (E.g: 2000 mg / h). That is how the approach of " slow trial and error sodium intake increase" can be harmful, and just listening to gut distress is tricky. The best approach is to know your sweat sodium concentration across different intensities, weather conditions, activities and environments, and then intake about 70% - 90% (to be safe and if you are not doing ultra racing/running, then 100% could be the best recommendation), doing this will allow you to stay out of hyponatremia curve for longer, without stressing your kidneys. Aim for “replace, don’t exceed.” Occasional high-salt days are unlikely to harm healthy kidneys, but routine doses (that won’t show up to your gut it is trained) well above measured sweat losses add up.

One last example:
-Rider A loses ~900 mg Na/h in hot environment (lab test).
-They “gut-train,” slowly increasing sodium intake and eventually tolerating 1.5 gr/h with no discomfort.
-Over a 4 h ride they ingest an extra 2.4 gr Na beyond need.
-Kidneys excrete that surplus the same afternoon; tubular stress biomarkers (NGAL/KIM-1) tick upward even though BP and gut stay quiet. Repeating this 3 - 4 times a week turns thousands of unneeded milligrams into a chronic exposure.

Gut feelings are trainable, and blood pressure can stay normal, but kidney tubules still register the sodium load.

I reaaaally loved the conversation I had with him about this yesterday, and here is what we sent me on a written note. At least, I was able to understand his point and it does make sense to me that quantifiable data is better than any trial an errorr approach on feelings/distress. Also, why would I have to have gut distress to get my nutrition and hydration dialed in? Is does not make sense to me that I would have to damage myself in order to know and improve my nutrition/hydration, so I prefer this objective data driven approach. This is just my humble opinion tho!

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This is the right approach (from what I understood from the conversation). If you know your sweat sodium concentration, then replenishing 70% to 100% (not more!) would make it. @BCM

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How much do we have to pay your doctor friend to join the forum so I can ask him more questions than anyone cares to read?

He seems knowledgeable enough for us to gain from. Maybe I’ll pull in my ER MD friend who runs ultras and MTBs recreationally and see if I can get him to chime in and poke possible holes in my thinking.

In lieu of either of those things…

My first line of questioning which you are encouraged to pass through to him is around “to what degree and frequency—or at what point—is overdosing sodium consumption, and the resultant spikes in NGAL/KIM-1, chronically harmful?” From what I gather both are relatively “developing” biomarkers relevant in diagnosing acute kidney injury. The reason I ask this targeted question is that in the case of exercise, it is exceedingly common for us to be able to measure dramatic perturbations from homeostasis acutely, post-exercise, often related to nutrition, which almost universally end up having zero long-term effect (desired or unwanted, depending on which ‘thing’ people have tried over the years). I’d absolutely be overjoyed (kid in a candy shop kind of feeling) if he happens to have direct causative or even correlational data (or knowledge of it) relating chronic outcomes to such repeated acute overages in sodium consumption.

I’ll refrain from listing out the multitudes of other acute measurements caused by various nutritional or training stimuli/strategies that look either promising or damaging based on post-exercise biomarkers/hormones etc, but suffice it to say lots has been tried and much is a wash chronically. And lots of things you don’t want to find in blood, urine, sweat under resting conditions or without some “known” cause, are ironically completely harmless (or even optimal and healthy and part of a normal recovery or adaptive process) when found under other circumstances.

It’s important we find out the details here and I’ll do some digging when I have a moment and invite any and all contributors who might speed my/our fact-finding! (might be 2026 before I really get into it, but it’s a very interesting and important line of thinking)

Thank you for engaging so constructively!

PS. A note on sweat tests (and all current in-vivo sweat sodium loss measurements available outside a laboratory): they’re often so disastrously inaccurate that it seems unlikely that they’d improve prescriptions.

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I’m seeing him on a long group ride in about a month, I will try convincing him to join. I was in his ear the entire weekend…

Have you done lab sweat testing or testing with these wearable devices? I ask bc I am doing some and comparing with the same machine precision hydration uses. If you have any tips and tricks would love to hear them!

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Bummer my hDrop konked out 4 days before Leadville. Fortunately the data I had was useful for planning my hydration and electrolytes.

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From the lab tests you’ve seen, how big are possible variations between different conditions and/or different levels of heat adaption/fitness levels?
I am bought in that I should get a sodium concentration test, but wondering if it should be at end or beginning of season, heat adapted or not etc.
I’m thinking it’s easier to fit in a lot of testing in early parts of the season, but not helpful if the values will change with training.

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@Dr_Alex_Harrison before we get too far down the road, are you actually qualified to talk about accuracy of sweat tests? Meaning, do you work in this field or been involved with the technology on any way?

Not trying to dis you or anything, but sometimes doctors, provide advice outside of the area of expertise and its alway not 100% correct. But when people see the “dr” they assume theres expertise.

You may be totally qualified, if so, great.