There has been such an explosion in oral supplements for joint support with various new ingredients that it's hard to keep up with them all. One ingredient that shows up in more products all the time is hyaluronic acid (also called HA).
Hyaluronic acid is a member of a group of compounds called glycosaminoglycans. These substances are what give skin its elasticity, cartilage its "give," and fluids their lubricating properties. Hyaluronic acid is found both in joint fluid and the cartilage itself. It is a major factor contributing to the slippery feel of joint fluid. In inflamed joints, breakdown of HA makes the joint fluid more watery and less able to keep the joint "greased." Inside the cartilage itself, hyaluronic acid combines with another glycosaminoglycan called aggrecan to form a complex that helps trap fluid in the cartilage and keeps it flexible and resistant to being overly compressed.
In the joint, hyaluronic acid is produced both by the chondrocytes, which are specialized cells inside the cartilage, and also by the synovial membrane. In an inflamed joint, enzymes called the hyaluronidases cause the breakdown of hyaluronic acid. High activity of these enzymes is associated with arthritis. Interestingly enough, short chains of hyaluronic acid, produced in the process of breaking down HA, actually contribute to inflammation in the joint by triggering other pathways of inflammation. This is why chronic arthritic problems can become self-perpetuating cycles.
Interest in hyaluronic acid for treating arthritis in horses dates back to the early 1970s, when "black market" hyaluronic acid from Europe started showing up on racetracks, first in the Standardbred world, then among Thoroughbreds. At that time, all hyaluronic acid used was injected directly into joints. It didn't take long for the amazing effects of this treatment to become widely known. Before hyaluronic acid, joint problems were treated primarily with corticosteroids. This worked for a while, but when done frequently and in high doses, the steroids themselves started to cause problems by inhibiting the metabolism of joint cells. Infections of the joint and secondary metabolic effects also were risks with corticosteroids.
How Do They Work?
How joint supplements work is still a bit of a mystery. The effects they have on cell cultures in a laboratory may not be what they are actually doing inside the body. One common effect appears to be to "tie up" the enzymes that break down joint fluid and cartilage. If the supplement can get the attention of these destructive enzymes and occupy them, the balance inside the joint has a chance to get on top of the inflammation. Hyaluronic acid may also have important "cell signaling" effects, meaning it could help stimulate the production of other glycosaminoglycans
It wasn't long before American companies jumped on board the HA bandwagon and FDA-approved hyaluronic acid products became available. The high-molecular-weight products worked better than the less expensive, low-molecular-weight ones that had shorter chain lengths and were more likely to make joints painfully swell for a few days afterward. In those days, it was thought that this was because they weren't as purified as the more expensive products. We now know it was probably because low-molecular-weight hyaluronic acid actually causes an inflammatory response in the joint.
The 1980s and most of the 1990s represented the heyday of hyaluronic acid joint injections. The next big development was when the product Legend came on the scene in 1991. Legend is an intravenous hyaluronic acid that is carried by the blood stream to the joints. While some were skeptical, the experiments done to satisfy the FDA's requirements showed very clearly that it did indeed work. In those studies, joint inflammation was experimentally induced, and the equine test subjects were split into two groups-one treated with intravenous hyaluronic acid and the other was an untreated control group. The treated horses did significantly better in terms of pain, freedom of movement, and even the actual condition of their cartilage. You can read the details of those studies online at http://www.fda.gov/FOI/1392.htm.
