Magnesium vs Chelated Calcium diets for PSSM 1 & 2 horses

By Malcolm Green – Technical director, EquiFeast


Horses with PSSM 1 or any of the variants called PSSM 2 are suffering from one or more genetic defects that affect how their muscles function. Since we can’t correct the defects, the responsibility falls to owners to manage the condition as best they can. This management may be achieved by various ways such as carefully controlled nutrition, or it may require the use of drugs.

I like to think of these two this like this:

  • Nutritional solutions enable the animal to function as normally as possible
  • Drug solutions force the body to work in a certain way.


Neither of these approaches is right or wrong. Owners need to identify for themselves what best suits the particular variant or combination of variants they are dealing with.

Managing PSSM horses can be split into a number of objectives:

  • Dealing with the by-products of muscle damage caused by tying up. This primarily means anti-oxidants and there seems to me to be something of an over emphasis on vitamin E in current approaches.
  • Helping to rebuild and repair damaged muscles. Mostly with limiting amino acids and complete animal protein.
  • Dealing with pain using either veterinary drugs or herbal options. These all have the potential to interfere with some of the nutrients in the diet.
  • Preventing the tying up and subsequent muscle damage. This is the subject of this article.


Nerve and muscle cells both use a process called “Calcium Signalling” to switch on and switch off many of their functions. To try to understand this, imagine the inside of the muscle cells as being a calcium free zone. In order to switch it on (contract the muscle) a little burst of calcium ions is pumped into the cell. These calcium ions attach themselves to calcium receptors and those receptors tell the muscle proteins to shorten (contract). Rather elegantly there are also calcium receptors on the pumps that remove the calcium ions and when that happens the muscle relaxes.

The first thing to notice is that magnesium is NOT involved in this process. That doesn’t mean that magnesium doesn’t have jobs to do in muscle cells – it does. But what it means is that the often-quoted idea that calcium contracts muscles and magnesium relax them is simply not true. It was a great theory in the 1970s but like many theories in science they are demonstrated to be wrong as our knowledge progresses.

For calcium signalling to work properly there must be enough calcium ions around and, in all the blood trials we have done we have never yet found a horse with inadequate calcium to function properly.

So now to one of those theories that will probably be disputed or modified in the future – my theory that chelated calcium has an important role in calcium signalling. For more on that please read The Missing Link in Calcium Signalling.

If you read that article you will see that we believe that chelated calcium is involved in calcium signalling and, in particular, in the removal of calcium ions from the cell cytoplasm into internal stores like mitochondria and Endoplasmic reticulum via channels called VDACs.

Further we believe (simply because so many healthy and sick animals of all sorts respond positively to chelated calcium supplementation) that chelated calcium deficiency is common.

So if we put all these issues together in the PSSM context we draw the conclusion that many PSSM horses (just like healthy ones) are chelated calcium deficient. That means their calcium signalling won’t be working properly and that in turn explains why these horses tie up so easily.

With the Px variant being an error in a calcium ion channel there is an obvious link to chelated calcium either helping that variant channel work better or help alternative calcium channels (there are at least 17 different ones) to compensate for the problem.

With the other variants in muscle protein it isn’t obvious how or how much these horses are helped. But in most cases (not all) they seem to benefit from more chelated calcium in their diet.

At this point it is probably worth emphasising that chelated calcium is a perfectly natural material to have in all animals. It is originally manufactured in the xylem of the plants our horses eat. The plant chemists have identified that chelated calcium is moved around the plant through completely different mechanisms to ionised calcium. These plants also use calcium signalling.

Nobody has studied the management of chelated calcium in animals. All we really know is that roughly 5% of the calcium in animal blood is chelated. We have no idea how much there is in the extracellular fluids or inside the cells themselves. Just send me a few million dollars and I can invest in the research (or more likely retire to the Caribbean).

So as best we can tell chelated calcium helps PSSM horses by enabling their bodies to functioning as normally as their issues will allow. This rather explains why we tend to start these horses on very similar diets to those of healthy animals.

As I wrote the bit about magnesium not being involved in muscle relaxation I heard the groans of disagreement. All those people who have helped their PSSM horses with magnesium and all of you who bathe in Epsom salts to relax your muscles were probably outraged. But let’s see how magnesium does act in the calcium signalling process. Of the seventeen or so calcium channels just, one uses magnesium. That is the NMDA channel. Magnesium has a passive role in blocking the channel. That magnesium ion is expelled by an electrical stimulation in the cell membrane surrounding the channel. That enables calcium (and sodium and potassium) to be moved through the channel. The more magnesium there is around the faster the channel is reblocked. So it has been hypothesised that this is one mechanism by which magnesium used in calcium channel blocking drugs produces an anaesthetic effect. If the NMDA channel is in a muscle then this mechanism will prevent proper contraction of the muscle.

The other way magnesium prevents proper contraction is by sitting on and blocking the calcium receptors inside the cell. Magnesium is notoriously difficult to control (despite what you read about a tight homeostatic system). So the more you eat the more builds up in the body (Hintz & Schryver 1972 and 1973) and the more gets inside the cells. The more in the cells the more calcium receptors are blocked. This prevents proper nerve function and prevents proper muscle contraction and relaxation.

So magnesium has found an important role in the management of PSSM horses because it can help prevent tying up and can help recovery. But it does it by interfering with normal function. By my definition that makes it a drug.

So those with PSSM horses have a choice between a drug based approach (magnesium) and an “enabling approach” (chelated calcium). So far we have very good results using chelated calcium. But we haven’t yet tested all variants and we have no expectation that we will help them all. But for the most common ones there seem to be advantages with chelated calcium compared to the more traditional, magnesium approach.