By Duncan McLaughlin

Electrolytes are minerals that occur in your horse's body in a charged form and are essential to the functioning of many physiological systems. In endurance riding we are concerned with the five particular minerals essential to neuro-muscular function that are lost in sweat: Sodium (Na+), Potassium (K+), Chloride (Cl-), Calcium (Ca+) and Magnesium (Mg+). The activity and movement of these important minerals are well understood making it is easy to formulate an electrolyte replacement strategy for training and competition.

Sodium: Sodium makes up 35-40% of common table salt: That is, 40g of Sodium for every 100g of salt - the rest is Chloride. Sodium is nearly always deficient even in well balanced diets. Your resting horse requires at least 25g/day of Sodium and these needs rise with work: 30g/day for light work up to 50g/day for heavy work. The level of Sodium in blood is maintained at a constant level, around 9000mg/dl.

Potassium: Although your horse's needs more Potassium than Sodium, around 45g/day at rest, this will is easily met in a balanced diet with adequate grass and/or hay. The only time your horse may experience Potassium deficit is during competition or a heavy training session, when he does not have time to consume adequate roughage. At these times Potassium is usually delivered in the form of Potassium Chloride - roughly 50% of Potassium and Chloride.

Chloride: You will meet your horse's Chloride(Cl-) requirements if his Sodium needs are being met. Many other minerals also combine with Chloride to make up

Calcium: Your horse's Calcium needs, 25g/day, will be met by a balanced diet with adequate grass and/or hay.Diets with high levels of daily Calcium can cause problems during strenuous training and competition - this has been discussed many times on the list previously BUT if you feed diets with a lot of lucerne (ie more than 500g/day) you may well have trouble with your horse during competition or heavy training with tie-up syndrome, thumps, or shivering . Although his Calcium requirements rise with work due to loss in sweat, these losses are relatively small and can be met by using including a calcium salt in your electrolyte mix.

Magnesium: Your horse requires 8g/day at rest with a small additional amount when working due to losses of small amounts in sweat. Magnesium is not individually regulated but follows calcium. However, when Calcium levels are low the kidneys conserve it whereas Magnesium is not so regulated. Also when excess potassium is excreted it takes magnesium with it. For theses reasons Magnesium levels can be deficient.

Your horse's hydration is directly related to his blood Sodium level and regulated by his kidneys. Blood sodium is maintained at around 9000mg/l. Excess sodium causes your horse to drink. Once maximum hydration levels are reached he excretes any excess sodium in urine until blood sodium levels are normal. However, your horse's diet rarely includes excess Sodium, it is more often deficient. In the absence of adequate salt the kidneys excrete water until the appropriate blood sodium level is reached - initiating a low level of dehydration (up to 3-4%). This low level of dehydration is not apparent using skin pinch or capillary refill tests. As deficiency increases Sodium moves from his extra-cellular tissues to maintain blood Sodium levels, taking fluid with it (ie skin pinch will now pick up dehydration but your horse is now over 5% dehydrated). Increasing Sodium is clearly an important part of maintaining hydration, particularly triggering your horse's thirst mechanism. Other triggers to drink include fast/heavy work and eating of fibrous foods (hay).

Your horse's sweat is hypertonic - the concentration of electrolytes in his sweat is higher than in his blood (in humans sweat is isotonic - the electrolyte concentration in sweat and blood is the same). The electrolyte content of his sweat is nearly half Chloride and nearly quarter each of Sodium and Potassium with small amounts of Calcium and Magnesium (1-3% of total salts). Some trace minerals are also excreted in sweat but in amounts so small as to irrelevant to electrolyte replacement for any healthy horse. A lightly sweating horse excretes around 5l of sweat each hour and a heavily sweating horse looses up to15(or more) each hour. Your horse is loosing between 2 and 6(or more) grams of Sodium for each hour of work.

The amounts of Calcium and Magnesium lost through sweat in an hour of work are likely to be less than 1g. But as a proportion of the amounts of these elements circulating in the blood this can be a significant amount. For example, Blood Sodium is 9000mg/l compared to blood Calcium at 120mg/l ie a Sodium/Calcium ratio of 75:1. The initial ratio of Sodium to Calcium in sweat however is 20:1. Although amounts are small, Calcium is being lost at 4 times the rate of Sodium. However over prolonged periods of exercise the proportion of Calcium lost through sweat reflects its concentration in the blood. The same applies to Magnesium. In a healthy horse these can be met by making these salts available from tissues (such as bone) but as mentioned above - horses fed diets high in Calcium (ie lucerne) loose this ability and should be supplemented during competition. Any imbalance in any of these salts can cause problems (gut sounds, dehydration, ty-up, thumps) but it is always necessary to first correct Sodium as until Sodium is restored the other salts can not be managed.

Replacing electrolytes is easy. First obtain an electrolyte mix that mirrors the electrolyte composition of sweat. Make your own mix using Sodium Chloride, Potassium Chloride, Calcium (carbonate/acetate) and Magnesium(oxide) in proportions of 60:30:5:5 The higher amount of normal salt is necessary as Sodium Chloride is only 35-40% Sodium whereas Potassium Chloride is half Potassium. Slightly higher amounts of Calcium and Magnesium ensure availability of these salts particularly early in a ride when they are being lost in proportionally higher amounts. Dolomite is a source of Calcium (Calcium Carbonate) but other sources such as Calcium Acetate/Chloride etc are better as they are more available. Magnesium in the form of Magnesium Oxide is cheap and available at your produce store. These are the same ingredients in Endura-Max (which also has some sugar, preservative and anti-caking mixture). Note also that many other commercial electrolyte mixes don't include Magnesium. It is ok to have an electrolyte mix with higher levels of Calcium and Magnesium than ordinarily lost as we know that Calcium and Magnesium are initially lost in sweat at a much higher ratios when compared to blood levels than are Sodium, Potassium or Chloride.

Once you have your mix made up then supply enough mix so that Sodium levels are being replenished. However, research has shown you can actually worsen dehydration if you over-supplement electrolytes. The optimal replacement is 1-3 grams of Sodium for every litre of water he drinks. Now remember if he is sweating heavily he could be losing 6 or more grams of sodium each hour but you can only replace this if he has also drunk around 6 litres of water each hour. To estimate how much he is drinking, water your horse from a bucket while training at home. Count the number of swallows until he is finished drinking then measure
how much he drank. Do this several times, particularly when the horse is thirsty. Then average out the amount of water/swallow and use this as guide for when you are out on track.

So the practical plan is:
1. Your well fed, paddocked horse will be likely be deficient in Sodium. Add plain table salt to his daily feed. If his diet is unbalanced for other minerals (ie excess Calcium from Lucerne) there is a chance he will also need some additional Magnesium in his diet.
2. Your healthy, well fed horse should be able to mobilize enough of the other salts from his body stores and diet to make supplementation unnecessary, except during serious competition, very heavy training sessions or hot/humid conditions.
3. Excess electrolytes are not stored but eliminated in urine.
4. Leading up to the competition you want to ensure maximum hydration by feeding 30g of plain table salt for the two days prior to competition.
5. For the competition you want to replace total electrolytes lost in sweat during travel, nervousness or work by using an electrolyte mix that reflects the loss of each mineral in sweat.
6. During the beginning of a ride your horse loses it's desire to drink because his blood sodium level has dropped (ie he has sweated and lost Sodium and Sodium has not moved from the intracellular tissue to replace it). If your horse has drunk well in camp prior to starting the competition you can give electrolytes in the hour prior to the start. This will raise blood Sodium levels initiating the thirst response and assisting the desire to drink early in the ride. Only do this if your horse drinks well in camp prior to the start.
7. During the ride replace electrolytes on the basis of 1-3grams of Sodium for every litre he drinks. It is generally not possible to replace all (or even much) of the electrolytes he loses in sweat during competition as he will not consume enough water.
8. Post competition it cant take between one and three days before electrolyte levels are replenished in the

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