As we finished up last time, we touched on some strategies of the body to replenish fluid and elyte losses after exercise. This is referred to as the “acute” stage.
The secondary “adaptive” stage results in an “enhanced ability to cope with future systemic disturbances” - because exercise is in a nutshell a disturbance in fluid and elyte balance. And the changes that occur during this adaptive phase are all about being able to do the same work better (less disturbance) than last time.
The more I research different topics related to exercise physiology, the more I realize that conditioning my endurance horse goes way beyond musculoskeletal and building better muscles, heart, and lungs. I’m conditioning their GENES (epigenetics), and the way the horse handles everything from water to electrolytes to what proteins (and how much protein) they produce as well as a million other small biochemistry and metabolic changes that add up to a horse that can do 50 or a 100 miles. A horse doesn’t just need to build a better musculoskeletal system to do the distance, they need to build a better biochemical environment.
It turns out that the adaptive response to exercise is a lot like the adaptive response to heat training.
The number one adaption that occurs in response to training is......an increase in plasma volume.
You will remember that heat training causes an increase in plasma volume. And that getting old causes a reduction in plasma volume. In short, plasma volume seems to have a great deal to do with thermoregulation ability and performance.
Studies have demonstrated that the increase in plasma volume causes a total body water increase - the plasma volume isn’t increasing at the expense of “stealing” water from another compartment.
How does the plasma volume increase?
60% of the mechanisms are related to stimuli associated with thermoregulation. Even if we aren’t technically “heat conditioning” our horses, you will remember from the heat posts that the TNZ of the horses is significantly lower than most of our competition temperatures, even in mild weather. Thus, exercise in horses is about them being able to thermoregulate and the same factors that stimulate an increase in plasma volume during “heat training” are also playing a role in normal riding and training.
40% of the mechanism is related to the actual exertion of exercise. This includes a stimulation in the intake of water (caveat - mixed results in the literature on this point when it comes to long-term training), and a decrease in the loss of water and electrolytes in the kidneys.
ie - the kidneys are actually retaining water and electrolytes (seems to be ALDO-related) to increase plasma volume so that the next time exercise occurs it will result in less disturbance of body water and elytes.
Just the increased intake of water and kidney retention doesn’t totally explain the entire increase in plasma. So, the thought is that there is also an adaptive response in the gut to increase uptake of water and elytes that will contribute to the plasma volume increase. Not just sodium uptake is increased, other vital elytes are also retained.
Hinchcliff emphasized several times that the ALDO mediated retention of water/elytes in the kidney and the increased uptake in the gut of water/elytes occur BEYOND the acute phase of replenishing water/elytes lost in exercise and that this was truly an ADAPTIVE response of the body.
There is also an increase in plasma proteins. Remember a couple of posts ago when I told you that the plasma and interstitial compartments, which make up the ECC, are almost identical? Most things can freely pass through the barrier between the plasma and interstitium....except plasma proteins. Mostly because of size, they are big enough that they are trapped on the plasma side. They act like a solute and will draw water into the plasma compartment side and trap it there. So, an increase in plasma proteins increases the total solute content of the plasma, which will draw water in and keep it there.
In horses (and humans) an increase in plasma proteins in the plasma is first because of proteins shifting into the plasma from other places (what those other “places” are I have no idea...). Later on during the adaptive phase, the protein increase is because more plasma protein is actually being produced!!!!!! This warrents exclamation marks because.....anytime you have an increase or decrease in something like this, it’s a change at the gene level ==> which genes are being expressed, and to what degree.
How long does this extra plasma stick around if you aren’t continuing to exercise and condition the horse? Ummm...no idea. Just like heat training (which seems to be very much dependent on the increase in plasma volume) the answer is unknown.
We aren’t even sure HOW the body allows the extra plasma to exist. We talked a bit about hypovolemia (low plasma volume) and how the body responds to correct it, and there’s mechanisms for the body to correct increases in volume too. However, somehow extra volume that is created because of conditioning is allowed to exist. Is it the receptors that sense plasma volume adapting so that they accept the increase volume as normal? In humans training decreases ANP (that cardio hormone that has a secondary renal response that causes increased Na/K excretion) as well as other responses - perhaps as a way to support and keep that extra volume.
How long does it take for horses to increase their plasma volume to training? REALLY REALLY FAST. In fact, the increase in plasma volume takes place so much faster than the increase in red blood cells to fill it up, you can end up with “sports anemia”.
Other adaptions that were cited in endurance training was the reduced rate of lactate and hydrogen ion accumulation in contracting muscles and the blood during the first week of training. We will cover the issue of hydrogen ions and lactate (and postassium) in future posts, but as a side note “acidification” of the muscles because of lactate and hydrogen ions are NOT a major reason for fatigue in endurance horses.
Aren’t these adaptations exciting? Gene expression modification, increases in total body water, increases in plasma volume, changes in how electrolytes and water are extracted from the gut - all so the body can prepare itself better for “next time”.
We’ve just scratched the surface of how the body can adapt, and Hinchcliff emphasizes that the impact of training on acid-base balance especially has not been well studied.
There is evidence in the human field that giving iatrogenic substances can be detrimental to the adaptive process - look at the most current research on anti-inflammatories, antioxidant, and vitamin use in the human world. As a general rule, the research seems to suggest that when you provide the body something to solve a problem (like taking high levels of antioxidants, or anti-inflammatories) the body decides it doesn’t need to upregulate it’s own pathways to create those substances - after all, everything produced biologically costs something in the body’s energy currency, so if it’s just going to “magically” be there from an outside source, it’s more efficient to use what’s given. I’m not saying that if you have an injury, you shouldn’t use anti-inflammatories, and if you have a documented deficiency of an antioxidant (selenium and vit E comes to mind) you shouldn’t supplement. The body has to have the raw materials.
But in general, learning about the incredible changes in biochemistry that occur as a result of training, makes me incredibly leery about messing with it. For years the advice to marathoners was to take “vitamin I” (ibrophen) to reduce inflammation, and to even prophylaxily take it before running a marathon. Now, we think that taking anti-inflammatories actually dampens the body’s response to training and mal-adpats the muscle to the work (and doesn’t even significantly relieve DOMS, which was a major reason it was recommended). What about giving my horse electrolytes during races or during conditioning? By increasing the sodium content of the horses gut does that mean that less “transport channels” for sodium uptake will be created because they aren’t needed? No one knows the answer to that question. Might there be value in giving some additional sodium/elytes during a ride because my horse hasn’t fully conditioned to the distance and made all those biochemical pathways/transport channels it needs to get the sodium out of the gut it needs? Perhaps - but as time goes on, and the horse more fully adapts to the exercise biochemically perhaps the need for supplemental elytes decreases? There’s the rule of “nothing new on ride day” which suggests that if you use elytes during ride day, you might want to give it during conditioning rides too. I’m not sure this is the wisest thing. Rarely do I ride as long or hard as I do on ride day, and by providing electrolytes on my shorter conditioning rides, am I shortchanging the adaptive process of sodium retention and uptake? I’m not sure. Will some horses never adapt biochemically to the task of going 50 or 100 miles and always require more sodium/electrolytes than what they can either get into their gut naturally in feed? Maybe.
We are going to talk about this further in the next couple of posts when we talk specifically about acid/base balance, but here’s an interesting thing. During endurance exercise, by the end of a 100 mile ride, horses slowly develop a metabolic alkalosis. Horses that are “properly electrolyted” according to one study, do not. They maintain their acid/base balance. But is this slowly developing metabolic alkalosis a bad thing? When horses eat, they actually become metabolically ACIDIC for a time period post eating. Should we electrolyte to prevent the alkalosis? It could be argued that 100 miles is not natural for a horse and thus require “metabolic support” in the form of elecrolytes. Or, is a perfectly stable acid/base balance NOT as constant as we thought.....because the simple act of eating can change it? I also find it fascinating that while exercise produces alkalosis, eating produces acidosis. A natural adaptation?
In thinking about how best to support a horse through 50 or a 100 miles it is wise to keep the following things in mind:
1. Acid/base balance and other adaptations to exercise have NOT been well studied.
2. Iatrongenic substances are not benign and DO have an impact on response to exercise (good? bad? detrimental? helpful?)
3. Exercise causes changes at the genetic (epigenetics!!!!) level through expression and regulation of genes.
My bottom line for elyte supplementation after going through all my reading? Elyte supplementation may be needed for a horse that is under-conditioned for the task at hand - a hot ride that we didn’t heat condition for, or a bump up in distance that we’ve never gone before (like moving from 50’s to 100’s). A “well-conditioned” horse is more than musculoskeletally ready for the distance, they are biochemically ready too. But just like some horses can adapt better on the cardioresp or musculoskeletal systems for the distance, there are probably some horses that biochemically adapt better to, and some horses may need that elyte support for longer distances......I’ll just be doing my best to make sure that I’m not providing iatrogenic support to the detriment of the adaptive process- which means until proven otherwise, I probably will administer only minimal amounts of iatrogenic "support". Like anything else in this sport, it’s a balance.
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