Heat Training Week 2

Looks like we are continuing to make strides towards better heat tolerance during exercise and I've decided that for week 2 I won't bore you with more tables of data.  I'm headed to Disneyland on Saturday and will be gone for a couple of days, and by the time I get back, it will be mid-week 3 of the experiment and very close to Tevis time and I'm not going to want to go back and write this up - and really, it's just more of what I saw in week 1.  

So, instead, I'm going to share some more "heat stuff" that I've learned and more tidbits for my Readers considering the heat and its effects.

Farley’s sweat DEFINITELY has a different feel to it compared to week 1.  It’s a lot more like water and less like......snot.  The sweat “lies” against the skin and in the hair coat instead of “sitting on top”.  It doesn’t glisten as much.  And when I reach down to pat and stroke her neck in the middle of a ride, there’s actually friction between my fingers and her hair - my hands don’t glide over her hair like they did in week 1. 

Being at home with no internet and a wee android screen Funder graciously looked up whether the difference I was seeing could relate directly to a difference in sweat composition.  I knew that heat conditioning can change sweat composition of horses (http://jap.physiology.org/content/87/5/1843.long) but I didn’t know if you could actually SEE that change in composition to the naked eye.

Of course, Funder got distracted by things like latherin, which is a surfactant protein completely unique to horses that is in their sweat during her quest to dig me up an answer. 

I have to admit - I rolled my eyes at Funder and graciously allowed her to go on and on about the stupid protein.  I had come across it before, filed it away as a “gee whiz” factoid and gone on with my life.  I was much more interested in that the amount of sodium decreases in sweat with heat conditioning and I was trying to figure out whether a decrease in sodium would cause a decrease in viscosity.......

And then I actually re-reviewed the latherin molecule and realized that perhaps it was worth more than just a gee whiz.....

Latherin is a surfactant protein, which is interesting because the only other biological surfactant that I’m really familiar with is a surfactant protein that is produced in the lungs that reduces the surface tension in the alveoli (tiny air spaces in the lungs) that makes it possible for these tiny “lung bubbles” to stay open and thus let us do gas exchange......in fact, pulmonary surfactant is only manufactured by the body late in gestation and the lack of surfactant is a significant issue in premature births. 

What the heck was a protein that reduces surface tension doing on the hide of a horse???????

There’s a good review article on the internet here: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005726

Turns out that its surface tension reducing properties make it a “wetting” agent for hair.  Think about spraying hose of water at a horse at low velocity when the horse is dry (ie - not sweaty), or dumping water or sponging a dry horse.  At least on my horse the water that isn’t directly under pressure of the nozzle, my hand or sponge tends to bead and roll off.  Unless friction, pressure, or some other force is added to “rub” the water into the coat, the hair seems to have a natural “water repellent” nature.  The basis of this water repellence by the hair is surface tension! Adding latherin/surfactant to the liquid reduces the surface tension and allows the hair to get wet/soaked with sweat which helps it move from the skin to the coat and to the air where it cools the horse by evaporating.  Brilliant!!!!!!!

Or in fancy science talk: acts as a “wetting agent to facilitate evaporative cooling through a waterproofed pelt”.  

The structure of the sweat gland is more simple than humans - it’s more of a direct dumping line outside of the body that doesn’t conserve sodium. The extra salt, as well as the latherin alters the evaporative point of the sweat and may lead to better evaporative cooling. Lucky lucky horsey.  So, THAT’S one way horses compensate for their increased size and reduced surface area from us puny humans (puny = advantage of relatively more surface area compared to our body volume). BTW the information in this paragraph courtesy of Hinchcliff’s Equine Exercise Physiology Text.  I’m on page 336 for those of you following along......within a chapter helpfully titled “Body fluids and electrolytes: responses to exercise and training”.

So....apparently this latherin thing is more important than “making the horse lather where something rubs when it sweats”.  I kid you not - that was the level of my understanding before this week.

So......what happens to latherin as you heat condition?  I’ve found resources that say that sodium levels decrease (and of course some resources that say there is not change, because science is a messy thing, but I’m going with sodium levels in sweat decrease with heat training) but a change in surfactant levels of sweat is a much better explanation for why the sweat feels different than the sodium content.

This is what I found: The sweat of an unfit horse contains more latherin - which is why they “foam up”.

And....that’s about all I found.  Which is a bit frustrating, because a week ago Farley wasn't "foaming up" like an unfit horse - she hasn't done that since last fall. Yet, I'm sure there was a higher concentration of latherin in her sweat last week than this week.  Not enough latherin to foam last week......but enough to give her coat more "slip" under my fingers when she sweated. Apparently no one has looked at whether there is some sequential decrease in latherin as the horse gets fitter? Either in general or just heat? Bummer.

mmm....Well.....that was a lead up to a big NOTHING. 

To make it up to you, I’m going to review some concepts from my new *favorite* book by Hinchcliff (Equine Exercise Physiology), in a section promisingly titled “Heat acclimatization” and “Recommendations for preparation for exercise or competition in hot conditions”.

I really appreciate text books that cite the actual primary references that are used for their commentary, and it’s even better when they occasionally give a short summary of the study’s findings.  Often it’s a paper I was trying to find, and to have a short synoposis of it is nice (although, must be weighed against me finding and reading the paper for myself).  

OK, time for cool tidbits in no particular order, with my random commentary
- horses that exercise and train in cool temperatures still have improved physiological responses when exercised in hot conditions (so something is better than nothing.....), but the greatest acclimatization results from training in hot conditions. 

- In human studies heat acclimation starts to occur within 3-5 days of regular exposure to and exercise in the heat (I noticed the same thing).  Most adaptations are complete within a 14 day period.  Thank goodness since most of my sessions will be concentrated in the first 14 days of the 21 day period before Tevis because of going to Disneyland this weekend and beginning of next week (YES!!!!!).  So gives me an excuse to let both of us take it easy that week before Tevis after I come back.  We will do 1 or 2 heat sessions, but for the most part I will be relaxing and resting, knowing we did the hard part of heat conditioning in the previous 2 weeks.

- The most notable changes of heat acclimation are increase in plasma volume (oooohhhhh.....because an older horse has a decrease in plasma volume and this is one reason cited for their higher HR and core temps in the heat.......I’m actually combating Farley’s older age by doing heat training!!!!!!!), a decrease in heart rate and core temps during exercise, an increase in sweating rate, and initiation of sweating at a lower body temperature (other references I found put this at ~1*C), and an increase in blood flow to capillary beds of the skin.  In general the cardio adaptations are complete within the first week of acclimation, whereas alterations in sweating responses require 10-14 days of repeated heat exposure (I did not go back to the original reference to find out what their definition of “repeated” was). Even though all this was in reference to human conditioning, Hinchcliff goes on to say that in horses there is evidence that similar changes occur.

- Sweat dripping from the horse’s body is “wasted” sweat.  It means that the amount of sweat being produced is in excess of what can be used for evaporative cooling, and evaporative cooling is how sweat cools. Sweating rates increase during exercise after acclimization.....however it abates FASTER after exercise is done, so the overall sweat losses were LOWER in acclimated horses. In this particular study that is cited, calculated sweat ion losses were 26% lower after acclimation, which was mostly due to the 10% decrease in mean sweat sodium concentration.

Here is a really interesting table from Hinchcliff that summarizes the concept (click to make larger): 

- how long does heat conditioning stick? So far everything for horses and runners I’ve found refers to conditioning for the heat “21 days prior to the event” without any reference that you could say....get that heat training out of the way a few weeks earlier. However, Hinchcliff cites some human studies that report the “rate of decay” (LOL) as one to “several” weeks.  In physically fit people, there is a slower rate of decay of the heat adaptations.  HOWEVER (and this is why I probably couldn’t find anything) no one has looked at the time course of decay in horses.  *sad eyes*.

Recommendations?  I can’t possibly list them all, but here are the highlights
1.  Horse should have a high level of “event-specific” fitness and be given adequate time to acclimate to exercise in the hotter conditions.

2. Clip

3. During the acclimatization process (14-21 days): minimal exercise during week 1, with initially only light exercise during the heat of the day with harder workouts performed during cooler periods.  Gradual increase in duration and intensity of exercise performed in the heat., including some exercise performed at the intensity required of the horse during competition (which in endurance isn’t as hard to do, but the duration is something that is sometimes hard to mimic in training)

4. Start monitoring the horse's “clinical data” 1-2 weeks prior to travel to a hotter climate so you have a good baseline during the initial days of training in the hot conditions (most of these recommendations assume that you will be taking your horse to the hotter competition area, not trying to acclimate your horse at home with blankets etc). Measure water and feed intake. Record heart rate, respiratory, rectal temps before and after training sessions.  Intensity of work effort can be estimated by heart rate monitor use.  Daily weighing is useful for estimation of fluid loss. 

5. Rectal temps were cited as being particularly important, especially post-exercise, because this is the best way to detect heat illness.  Apparently there is a lag of in rectal temperatures post exercise, especially after heavy exercise in hot and humid conditions, so measuring temps 5-10 minutes post exercise is important (by sheer luck I got this one right!).

6. The normal lecture on hydration and elyte/salt supplementation.

7. A high fat diet (8-10% on total diet basis) may “reduce the heat generated by colonic fermentation when compared to a more traditional diet that is higher in roughage”. Mmmm......no specific studies that actually quantify changes from manipulating the diet ==> heat work.  So very theoretical at this point.

Let's end with a few thoughts from my data.  

I'm finally getting temps in the 101 range post session (up to 25 minutes of work, saddled with a rump rug even in temps over 100*F). At the 10 minute mark post exercise, body temps are declining. After some more research on rectal temps ....apparently a horse’s body temperature can reach 105-106*F during endurance rides performed under moderate climate conditions.....so my work in the 101 range is probably OK.  If I take temps during Tevis, I’ll probably be completely freaked out by how high the temps are in competition and laugh at myself ever being worried about 101-102.  From what I’m reading, severe hyperthermia is considered temps above 42*C, which is about 108*F.  Temps 101.5*F and below are considered normal in the horse, according to the AAEP.

I finally have enough data that I’m looking at averages - average temperature increase from start to end of session, from start to 10 min post session, and from the end of the session to 10min post session (after cooling).  Because the horse’s temperature isn’t as closely regulated to a very narrow range (aka humans), assuming my start temp is in the normal range, I think looking at the degree drop and rise will be more informational than the absolute temperatures.  I’m still looking at pulses as absolute numbers, because I care more about the cut off of 60 and 50 than I do the “average”, and the pulses change much faster than temperatures.  I would rather know that she recovered to 56 in 5min and to 48 in 10min than that she dropped an average of 20 beats in 5 minutes (which is meaningless since getting an accurate pulse right after the work out that represents her HR during the end of the work out is impossible - it immediately starts dropping, even as I try to count it, and it varies during the work out depending on whether she’s spooking at the scary side of the arena etc.).

This will probably be my last significant post on heat training and it's effects!  I'll do one more post either right before or after Tevis on the whole 3 weeks of heat conditioning data I have on Farley and any interesting trends, but I think I've "exhausted" (as in heat exhausted....hahaha I crack myself up....) what I can find easily on this subject during my lunch breaks. 

If anyone finds out any heat related horsey stuff that I've missed, I would LOVE to hear from you in the comments!  Especially about latherin and why it decreases in fit horses?  Maybe it's biochemically expensive and since fit horses don't need it as much it decreases?