Written By Dr. Kris Hiney
This month we wrap up our discussion of minerals required by horses with a random mix of macro and micro-minerals. We will focus on sulfur, manganese, cobalt and chromium. What these minerals share in common is that they are typically in adequate supply in the average horse’s diet. However, they still warrant attention and understanding as they are vital for the health of the horse. Next month, we will start a new series on protein.
Sulfur is technically a macro-mineral, as it is required in larger amounts in the diet. Sulfur is most commonly found incorporated into two amino acids, cysteine and methionine. In the body, sulfur would be found in proteins (as cysteine and methionine) as well as some key vitamins: thiamin, biotin, and lipoic acid. Proteins containing sulfur include most structural proteins, as well as insulin, heparin and key enzymes: gluthathione peroxidase (an anti-oxidant enzyme) and as part of coenzyme A (an essential part of energy metabolism). Horses ingest sulfur in the diet mainly through consumption of protein. Horses receiving adequate protein that is of sufficiently high quality typically do not need additional sulfur in the diet. The total amount of sulfur in the diet should be about 0.15% on a dry matter basis. Therefore, unless the horse is suffering from protein malnutrition, or is deficient in these specific amino acids (most likely methionine), the horse should be adequate in sulfur. To this date, no one has been able to confirm a sulfur deficiency in horses provided that they are not deficient in protein. Some inorganic sulfur (in the form of a sulfur salt) can be incorporated into chondroitin sulfate (an important component of joint health) (Figure 1) and insulin in the body. More benefits are seen when feeding inorganic sulfur to ruminant animals, due to the ability of the rumen microbes to synthesize the sulfur-containing amino acids de novo (or from scratch). While this can indeed occur in the horse, the site of microbial synthesis is past the primary location of protein digestion and absorption and therefore does not yield the same benefits as seen in the ruminant animal. So bottom line for sulfur, if you are not deficient in cysteine and methionine, you are likely to be adequate in sulfur.
Cobalt is a trace mineral which is found in the body solely as cobalamin or vitamin B 12. Cobalamin’s function, and thus cobalt’s, is rather exclusive, limited strictly to serving as a cofactor for only two enzymes. While only two, they are vital for RNA and DNA synthesis and integral in carbohydrate and lipid metabolism. Those familiar with human nutrition know that cobalamin is found only in animal products, and thus can be deficient in vegans unless they supplement their diet. So how would a horse, who clearly is an herbivore, meet their cobalamin requirement? Bacteria in the hindgut of the horse are able to incorporate cobalt into the complex structure of cobalamin (See Figure 2). Vitamin B12 is then absorbed from the gut of the horse, thus supplying their needs. Currently no cobalt or B12 deficiencies have been reported or induced in horses. They actually seem to be more efficient at extracting cobalt from their environment or conserving cobalamin in their tissues than are ruminants. Horses have remained healthy on pastures that induced deficiency symptoms in cattle. Suggested toxicity limits for cobalt are 25 ppm in the diet with recommended dietary intakes set at 0.5 ppm.
Manganese is a trace mineral involved in both carbohydrate and lipid metabolism and is needed for the synthesis of chondroitin sulfate, whose structure is seen above in Figure 1. Manganese is also part of two key gluconeogenic enzymes as well as arginase, an enzyme in the urea cycle. It also functions as an anti-oxidant incorporated into a form of super-oxide dismutase. Remember that these enzymes function to eliminate the superoxide free radical, converting it to hydrogen peroxide in the body. Hydrogen peroxide is further reduced by other anti-oxidant enzymes in the body. Manganese has even been researched as its potential use an anti-oxidant drug for horses through infusions of inorganic manganese salts.
Most commonly manganese deficiencies are reported to impair cartilage metabolism and may result in growth abnormalities such as contracted tendons, epiphysitis etc. This is due to its role in the synthesis of chondroitin sulfate and thus in articular cartilage In Figure 3. You can see the bottle brush appearance of the proteoglycans attached to hylaronic acid. This provides the resistance, or cushioning effect of cartilage during use of the joints. When cartilage is not allowed to form properly, abnormal growth usually results.
The current recommendation for manganese is 40 ppm in the diet, with typical horse feeds ranging between 15-140 mg/kg. Forages are typically higher in manganese (40-140 ppm) than concentrates. Therefore, the average horse should be adequate in manganese. The source or form in which manganese is fed does not seem to matter. In the current literature, there does not appear to be any difference in bioavailability between organic and inorganic sources of manganese. Yearling horses fed below the recommended value did exhibit a slower growth rate compared to horses fed a diet at twice the recommended intake of manganese. However, in this example it would be difficult to determine if a faster growth rate was achieved due to managnese supplementation, or a slower growth rate resulted from manganese deficiency. Manganese toxicity is a concern in many species, and excess manganese can interfere with P absorption. The safe level of intake of manganese in the horse is suggested to be at 400 ppm. While never researched in horses, in humans Fe supplementation can inhibit manganese absorption (another reason to not over-supplement minerals!)
Finally, chromium is an important mineral for both carbohydrate and lipid metabolism. Chromium enhances or prolongs the time that insulin is bound to its receptor, thus amplifying its effects. The exact mechanism by which it does so is not yet known. Thus it has created much interest in those individuals looking to enhance insulin sensitivity both in humans and in horses. In humans with insulin resistance or type II diabetes, supplemental chromium has been reported to improve glucose dynamics, but the results have not been consistent. In general, the majority of human studies have yielded very mixed results, with more benefits in humans with altered glucose metabolism, but not in diabetics. In addition, insulin resistance increases chromium excretion, which may be leading to a chromium deficiency. As of yet, chromium supplementation in a population of insulin resistant horses has not been performed. However, there has been some equine work that suggests a positive benefit to supplementation. Exercising horses supplemented with 5 mg of chromium from a yeast source had lower glucose values during exercise, as well as lower insulin levels following a carbohydrate meal. This effect was not seen when horses were unfit and sedentary. Other groups have shown either little, or no effect from supplementation of chromium. One study even suggested deleterious effects on the exercise capabilities of horses when supplemented with chromium. At this point in time, the exact amount of chromium horses should receive is unknown. There may be a benefit to feeding chromium to a select group of horses that have demonstrated insulin resistance, but widespread supplementation may be unwarranted, or even harmful to the general population.
On a final note, remember that many minerals interact in very complex ways in the body. Indiscriminate supplementation of one mineral may upset a very delicate balance in the ability to absorb and utilize other minerals. While data in other species may indicated positive benefits of supplementation, it is best to address supplementation in horses thoughtfully, with the goal of providing a proper diet to the animal, which only enhances, and does not detract, from their health.