Arthritis: A Short Course
A normal equine joint functions with the aid of synovial fluid (a viscous liquid about the consistency of egg whites) that helps lubricate the joint. Synovial fluid is a nutrient-rich brew that contains proteins, enzymes, water, leukocytes, and the key ingredient sodium hyaluronate, which is responsible for the fluid's elastoviscous qualities. Sodium hyaluronate (formerly known as hyaluronic acid) is a negatively charged glycosaminoglycan (GAG)--more simply put, a sugar chain--that arranges itself in complicated coils, adapting to the pressure changes in the joint capsule as the horse moves. It assures the unhindered passage of metabolites to and from the joint, and serves as a stabilizer and shock absorber for the structures that are undergoing continual mechanical stresses.
Cartilage, the other main shock-absorbing component of a joint, coats the ends of each bone. It's made up of a web of collagen fibers, with cells called chondrocytes scattered along the matrix. Chondrocytes produce giant proteoglycan molecules (that resemble microscopic bottle brushes) that bind the GAGs; the GAGs in turn extract and loosely hold large amounts of positively charged water molecules.
Unfortunately, cartilage is one of the body's most primitive structures. It has no blood or nerve supply of its own, so it has little or no ability to repair itself. As a result, although cartilage performs admirably under normal conditions, it only takes a 5% to 10% overload of work stresses to begin the process of cartilage breakdown. The wear and tear of hard work, uneven or unyielding footing, poor conformation, or any number of other factors might trigger the deterioration of cartilage, which is where osteoarthritis begins. Although the horse notices no pain and continues to work with no observable lameness, havoc is ensuing inside his joints.
As soon as cartilage cells begin to erode, the cells of the synovial membranes respond by cleaning up the debris--a classic inflammatory response. The process of breaking down and removing the foreign bodies from the area changes the chemical makeup of the fluid in the joint, introducing excess fluids and a greater concentration of destructive enzymes and prostaglandins into the closed space. This destroys the lubricating GAGs. The synovial fluid begins to lose viscosity, becoming less like egg whites and more like water.
The chondrocytes eventually suffer from a compromised nutrient supply, can't keep up with repairs, and the cartilage develops pits, holes, and fissures, opening the bone ends to direct trauma. The bone responds with a defense that only causes further destruction--it lays down new bone to strengthen the surface (a process called sclerosis) and extend its margins (bone spurs and osteophytes). In its final stages, if left unchecked, arthritis causes fusing of equine joints.
In the 1960s, our only veterinary defenses against this destructive process were anti-inflammatories (such as Bute and corticosteroids) and rest. Then someone got the bright idea of replacing some of the natural constituents of joint fluid and/or cartilage, in the hope that the body could use those building blocks to restore normal function and halt the destructive cascade. Such substances would act in a chondroprotective fashion, altering the progression of arthritis and perhaps even reversing it.
One of the first such products to be made available was injectible sodium hyaluronate (aka "Hylartin-V," and a host of other brand names), which first went into widespread use around 1970. It was a revolutionary advance, often promoting the production of natural sodium hyaluronate within the joint. But its intra-articular administration was tricky. Eventually, it was found that intravenous injection worked, with the medication somehow finding its way to the injured joint to increase the viscosity of the synovial fluid, inhibit damaging enzymes, and encourage the body to manufacture more of its own sodium hyaluronate.
Polysulfated glycosaminoglycan (PSGAG), or Adequan, was launched in North America in 1984. In many cases it was more effective than sodium hyaluronate in reversing arthritic lameness and inflammation. PSGAGs can be administered by injection directly into the joint, or by intramuscular injection--but it's not cheap, and most horses require a course of injections over a period of weeks or months before they show improvement.
About a decade ago, the manufacturers of joint supplements took the ingredients needed for healthy joint function and put them in a handy jar rather than an intimidating syringe. Suddenly, this was available to the average horse owner, and administering it was as user-friendly as mixing a little powder with your horse's grain. Considering that most performance horses show signs of joint aches and pains, the idea immediately was embraced by the equestrian community.
What's In Them?
The two active ingredients found in oral joint supplements are likely familiar names to you by now, but their modes of action might not be as familiar. Here's some background on the role these naturally occurring chemicals might play.
Sometimes bound to sulfates, sometimes to hydrochloride (there's considerable debate as to which is better), glucosamine is one of the constituents of GAGs and proteoglycans in the joints, as well as an essential ingredient in most of the body's connective tissues, including tendons, ligaments, skin, hooves, heart valves, blood vessels, cell membranes, and so on. It's also a component of sodium hyaluronate. With-out enough glucosamine, the horse has no hope of healing damaged cartilage.
Providing glucosamine to joint cells, in theory, stimulates them to produce more GAGs, including sodium hyaluronate. It's important to note that glucosamine is a small molecule that is water soluble, so it can travel across membranes fairly easily.
This is a type of glycosaminoglycan (GAG) required for the formation of proteoglycans in joint cartilage. Supplementing this molecule might decrease the activity of destructive enzymes by binding with them and inactivating them, thus slowing the process of degradation in a diseased joint. It also might play a role in stimulating the body to produce more sodium hyaluronate and provide additional substrates (most importantly, sulfur) for the formation of a healthy joint matrix.
It's important to note that chondroitin sulfate is a very large and unwieldy molecule when compared to glucosamine, and there are questions about how well it passes through the intestinal cell walls.
In her book, Equine Supplements and Nutraceuticals: A Guide to Peak Health and Performance Through Nutrition, Eleanor M. Kellon VMD, makes this analogy: "A comparison would be how easy it is for you to swallow whole a strand of spaghetti (glucosamine) compared to a hair brush (chondroitin sulfate)."
Chondroitin sulfate can be extracted only from animal cartilage or connective tissue. Most often, it's derived from bovine tracheal tissue, which is a waste product from the meat packing industry. But there are many supplements that, rather than extracting purified chondroitin sulfate, offer the whole unpurified tissue in freeze-dried form in the hope that it might supply other useful GAGs.
Shark cartilage is one of these; extract of perna mussel (a type of sea mollusc) and/or sea cucumber are two others. While it's true that such products might offer some additional fatty acids and trace minerals when compared to purified chondroitin sulfate, you should ask yourself what the environmental im-pact of harvesting mus-sels or sharks purely for an equine joint supplement might be. Extracts of bovine tracheal cartilage likely contain the same trace ingredients at a lower cost to the ecosystem.
In the hope of improving absorption levels or otherwise boosting the performance level of the two key ingredients, manufacturers often add nutrients to oral joint supplements. Among those you're likely to see are methylsulfonylmethane (MSM), a source of organic sulfur (which is important in helping the body form a strong collagen matrix); copper, manganese, and zinc, and vitamin C. Whether these additives have any serious impact on a product's efficacy is unproven.
It's also unclear whether combining glucosamine and chondroitin sulfate in a single supplement provides any synergistic effect. In a paper published in the Alternative Medicine Review in 1998, Gregory S. Kelly, ND, wrote, "Although glucosamine sulfate and chondroitin sulfates are often administered together, there is no information available to demonstrate the combination produces better results than glucosamine sulfate alone."
Keep in mind that some oral joint supplements might contain additives, especially herbal ones like devil's claw. These can act to reduce the pain of arthritis without actually treating the disease--so they can tempt an owner into believing a healing effect is occurring from the glucosamine and/or chondroitin sulfate when in fact the horse might be feeling less discomfort while the destructive degeneration continues!