NRC - Nutrient Requirements of Horses
Excerpt, posted by Eleanor Kellon
Chloride - The Neglected Electrolyte
Chloride has always been a bit of a stepchild in nutrition circles. Until the 2007 NRC Nutrient Requirements of Horses was released last year, we didn't even have a recommended feeding level for this element. It was assumed that horses receiving supplemental sodium chloride to meet their sodium needs would also get enough chloride. Thanks to the meticulous work of Dr. Manfred Coenen in Germany, we now how reasonable feeding guidelines to go along with the previously documented losses of chloride in sweat.
Hypochloremia (low blood chloride) and the metabolic alkalosis that results is the prevalent electrolyte disturbance found in exercise lasting 2 hours or longer. Hypochloremia inevitably results in metabolic alkalosis. This is explained by the anion gap equation:
[Sodium + Potassium] - [Chloride + Bicarbonate]
Alkalosis occurs when bicarbonate is present in the blood in higher than normal amounts. The horse's body works very hard to maintain the anion gap in a narrow range (11 to 16 mEq/L by most laboratories) to keep the blood and extracellular fluid at electrical neutrality. Chloride ions represent 56.9% of the electrolytes lost in sweat. As chloride drops because of sweat losses, bicarbonate inevitably rises, making the horse alkalotic.
The horse is more susceptible to significant chloride losses during endurance exercise for several reasons. As mentioned, more chloride is lost in sweat than sodium and potassium combined. At the same time, the total amount of chloride contained in the body and the intestinal contents is less than sodium and potassium. Sodium is efficiently conserved by both decreased urine production during exercise and increased reabsorption in the kidney. More potassium is excreted but the reduced rate of urine production helps to counterbalance this, and potassium can be released from exercising muscle, which is where the bulk of the body's potassium resides.
Most discussions of chloride revolve around the alkalinizing effect of hypochloremia, but this ion does much more than participate in acid base balance. Every cell of the body is equipped with chloride channels. Movement of chloride through these channels facilitates key functions such as maintaining normal fluid balance in the cell, maintaining electrolyte balance, movement of nutrients and wastes across cell membranes. Chloride channels also play a role in the stability of "excitable" tissues - skeletal muscle, cardiac muscle, smooth muscle (including the intestinal tract) and nerves.
The alkalosis that accompanies hypochloremia can cause it's own secondary problems. Both ionized calcium and ionized magnesium may be bound and inactivated when bicarbonate concentrations are high. When combining the potential direct effects of hypochloremia with the secondary problems related to low ionized calcium and magnesium, it becomes very easy to see why a hypochloremic, alkalotic horse could show symptoms of thumps, muscular cramping or poor gut function.
To fully understand the impact of hypochloremia is going to take much more formal research under a variety of exercise conditions, fitness levels, feeding conditions and ionized calcium and magnesium levels. For now, it is becoming very clear that simply “assuming” a horse is getting adequate chloride is probably not wise – and supplementing to match sweat losses alone might not be enough.
Previous editions of the NRC nutrient requirement guidelines did not pay any attention to chloride levels in the diet. The 2007 version now lists them for common hays and feeds. Maintenance requirements for chloride have now been set at 0.08 grams per kg of bodyweight per day, so an 800 pound (364 kg) horse requires 29 grams of chloride a day at baseline. The baseline sodium requirement is 0.02 grams/kg, only 7.28 grams or 18.2 grams of salt. If you supplement with the required 18.2 grams of salt, you will also be providing 10.7 grams of chloride, or about 37% of the actual requirement. That's a pretty hefty deficit. Grains, beet pulp, brans and seed meals are very poor sources of chloride. Hays range from 0.45% for early cuttings to 0.9% for very late stages of growth. If we use a middle ground figure of 0.75% for good quality hay, every kg (2.2 pound) of hay provides 7.5 grams of chloride. Our 800 pound horse would need to consume about 2.5 kg, or 5.5 pounds of hay to meet his chloride needs just loafing around. If this same horse is not getting the required amount of salt, he would need to eat 8.5 pounds of hay. This is certainly doable for maintenance
On race day, the total electrolyte requirements are a sum of maintenance needs plus sweat and urine losses. The kidneys are working against the horse when chloride is lost because priority will be given to attempting to lower the rising bicarbonate, leading to higher chloride losses in urine. Considering the horse will be losing anywhere from 11.2 to over 44 grams of chloride per hour in sweat, and may not be consuming sufficient high chloride forage to even meet maintenance chloride requirements on a race day, paying more careful attention to this pivotal ion may soon provide a lot of insight into problems frequently encountered by endurance horses.