I have been trying to determine my electrolyte needs for an IM for a few years now but think I need some experienced help. I have read about pros having testing done to help them determine how much salt (of all sorts) and fluids to take (and how this should be adjusted for conditions, intensities, etc). I just don’t know where to find such a facility. I live near Philadelphia but don’t mind traveling.
For IMLP, on the bike I took in the following and still started cramping at mile 95 (even though my HR, power and perceived exertion were ok): 4 grams Na, 3 grams K, ~1 gram Mg along with ~2100 calories and 8-9 bottles of fluid. Since I am taking in a lot of electrolytes and fluids, this may not be my issue.
However it is easy to do measure your own sweat rate. Weigh yourself naked before and after your workout. The difference in your weight is the pounds (or kg) of sweat lost. You can then convert this to volume per hour. Do this in a variety of conditions for both biking and running, because for me the numbers are way higher for running. If you drink any fluids during your workout, add those to your sweat loss. For example you have the same starting and ending weight, but drank 20 oz, then you sweat 20 oz per the length of your workout.
I too am curious. However, after just finishing the new book The Runner’s Body, I am not sure Electrolyte loss is our problem. Without citing the research they reveal in the book, the bottom line is, studies on both Ultra distance runners and IM athletes are NOT confirming the Electrolyte loss model as the cause of cramping. They are postulating a fatigue based model that has more to do with neuron response, muscle fatigue/weakness etc. etc. No easy solution. But, before you dump a bunch of money in to a laboratory, read the relevant chapter in this book. It’s a great book on all levels, but the cramping problem is addressed more academically than I’ve seen done anywhere else for laymen.
I suffered badly with cramps at IMC this year and blew out the Marathon. I took LOTS of salt. I think the fatigue based model might be worth investigating.
However it is easy to do measure your own sweat rate. Weigh yourself naked before and after your workout. The difference in your weight is the pounds (or kg) of sweat lost. You can then convert this to volume per hour. Do this in a variety of conditions for both biking and running, because for me the numbers are way higher for running. If you drink any fluids during your workout, add those to your sweat loss. For example you have the same starting and ending weight, but drank 20 oz, then you sweat 20 oz per the length of your workout.
Good approach. Then use 1g Na/L (roughly 50mEq/L) as an average concentration of sodium in sweat to get a ballpark idea of the amount of Na you’re losing through sweat. There is individual variation so your actual concentration might be different, but that number is consistent with some papers I’ve seen and what was told to me by a physician/IM triathlete working at Rice University several years ago. Of course, it assumes your sweat rate stayed the same and the concentration of your sweat stayed the same throughout the duration of exercise which might not be accurate.
I was lucky enough to get tested by the local university. If you check maybe some graduate students might be able to use you as part of a research paper.
I think you eat a lot of salt. And for sure a lot of calories. What is your bike time?
I can’t help you for test locations, but I would suggest that you don’t need to test any of these things in lab conditions. you will never race in those conditions again.
x2 on weighing yourself before and after timed training with a measured fluid intake (or none at all). If you want more precision as to your particular sodium concentration, one quick way would be to use a marine aquarium refractometer to measure the sodium concentration in your sweat. All it takes is a couple drops.
interesting way of testing. Is it accurate? In other words, doesn’t salt accumulate on the skin during activity if the water vaporizes and wouldn’t your salt concentration in sweat drops on skin much higher then the concentration you are really losing?
4 grams Na, 3 grams K, ~1 gram Mg along with ~2100 calories and 8-9 bottles of fluid.
How did you feel? How was your stomach? That is a ton of fluids and calories (assuming a 5-6 hr bike). You may have shut down your stomach will all of that, so you didn’t actually absorb it.
Good points on whether the concentration of surface drops is typical of sweat as a whole as it emerges. I’ve never tried the refractometer testing–it just occured to me as I was reading this thread as a way of combining two of my hobbies.
I think you could use the test if you really want to, but you have to catch the first drops of sweat during a overdressed trainer ride. As soon as salt accumulates you are done.
I have done a huge amount of research on this and I could tell you how to solve your little mystery but my advise will probably get mixed in with the guy who said you should drink salty or diluted water based on how you are tasting it. The sweat content testing is optional but not required. Most good universities can do that and it’s similar to the process in the cystic fibrosis sweat test so your doc should be able to hook you up. Your cramping likely came from one of two things and there are a lot of factors that could affect either source. What you took and your pace on the day of the race is not the problem, what you did leading upto the race is probably the source of your woes. You will need a lot more data than a sweat test.
Thanks to all who replied. FWIW, my bike time at IMLP was ~5:44 (2:44 1st lap & ~3:00 2nd lap but this also includes an unexpected bathroom break ~mile 70 and slowing down when cramping at mile 95). I wouldn’t consider myself a very strong cyclist (certainly below average for ST standards) but I did 5:24 at IMAZ last November and averaged just under 22 mph at Timberman a month after IMLP. At IMLP, I felt fine until the cramping. I didn’t experience any GI issues or additional cramping on the run. The run was slow (3:54) but not a complete surprise (my IM marathons have been ~4:00 and my marathon PR is 3:14). Although muscle fatigue is certainly a factor in cramping, salt (Na, K, Mg, ??) and hydration are very important in my anecdotal experience (of course, it doesn’t have the “scientific cred” of studies of N=8).
My swim coach (who is the wife of my now ex-tri coach) attended a tri coaching seminar where one of the speakers had trained several Olympic-level athletes (cycling and tri?). He stated that for endurance events, athletes should take a minimum of 800 mg of Na/hr. Sorry I am light on the specifics but the conversation was a couple months ago. Since then, I have started taking in 1000-1500 mg/hr (depending on total fluid consumption). Seems to be working for me in long rides and half irons. The big test will be IMAZ this year.
Getting lab testing (preferably while following a typical lower-salt and another while following a high-salt approach) would provide a lot of useful info that I could use to determine my electrolyte needs (or tell me it’s not my real problem).