Bioavailable is a term you may regularly see on marketing material, product labels and websites -often in the context of ‘contains a highly bioavailable form of X’. It implies that being bioavailable is a good thing. But what does this term really mean, and are some nutrients more bioavailable than others?
Bioavailability in horse nutrition
Bioavailability, in the specific context of nutrition, refers to how much of a substance that is fed is taken up from the gut into the bloodstream, and how much reaches the place where we want it to go.
For example, in the context of vitamin E bioavailability, if we are feeding to help support muscle problems or the muscles of horses in hard work, we want to know how much vitamin E that we feed enters the bloodstream (step 1) and how much enters the muscle from the bloodstream (step 2).
With respect to vitamin E, there are now several good studies which show that natural forms of vitamin E have a higher bioavailability (result in higher blood values) than synthetic forms of vitamin E. The bloodstream (circulation) is key, as that is how all tissues (like muscle, skin, etc.) or organs (like the brain, kidney, liver, etc.) in the body receive their nutrition; via the blood.
The gut is slightly different of course. It receives nutrition from the blood circulation, but the cells lining the gut can also take up nutrients directly without them having to pass through the blood circulation.
So, in order to be bioavailable, a substance must be first taken up from the gut and secondly reach the target tissue via the blood stream. A good example of a substance with very low bioavailability, but which is often fed to horses, is creatine.
Creatine supplements have a significant effect on muscle power and athletic performance in sprinting events for human athletes. Creatine is very high in meat. If that creatine is concentrated and taken over several months as a supplement to athletes, then it is both taken up AND it reaches the muscle.
In horses, only a tiny fraction of any creatine fed as a supplement is taken up – far too little to have any effect on muscle creatine. Therefore, we say that creatine has very low bioavailability in horses. In light of this you might be surprised to see a larger number of creatine supplements on the shelves specifically for horses! Should we be surprised that horses don’t absorb creatine? Not really. Creatine is high in meat and very low in plants. Horses don’t eat meat.
Bioavailability of some nutrients can also vary with other components in the diet. Here, calcium is an interesting case. Many supplements contain limestone as a source of calcium. Calcium bioavailability can vary dramatically between horses fed the same amount of ground limestone. For example, one horse might absorb 40% of the calcium fed whilst another could absorb 80% – twice as much.
When we are faced with horses that show low calcium bioavailability on limestone, we may feed a more bioavailable source of calcium, such as calcium gluconate. Other components in feeds may also interfere with calcium bioavailability. For example, bran contains high amounts of compounds called phytates which bind to calcium and reduce its bioavailability.
Finally, bioavailability of calcium can actually be enhanced by magnesium and vice versa. So, whilst some companies are promoting a low magnesium diet, this should be undertaken with caution as this may impact on calcium bioavailability. For horses at pasture or in light work this may not be a major issue. But for growing animals and those in hard work, this could lead to decreased bone calcium content and increase the risk of fractures if the dietary magnesium intake is low combined with a borderline calcium intake.
Bioavailability is often referenced in relation to chondroitin sulphate (CS) used in joint supplements. CS is a relatively large compound and its bioavailability is linked to its molecule size. Low molecular weight (smaller) CS is not surprisingly better absorbed and many products will claim to use ‘low molecular weight’ CS. However, low molecular weight CS is more expensive, so if you see cheap joint supplements with a high amount of CS (more than 4g per day) then ‘buyer beware’.
Interactions between different minerals can lead to some potentially serious health issues in horses. Iron intake is a particular concern. Iron deficiency is extremely rare in horses in the UK. Most companies have reduced or eliminated iron from their feeds and supplements. Iron supplementation is dangerous for three main reasons:
- Firstly, once absorbed into the body, horses have no mechanism to excrete iron and it remains in the body for life. The only way iron is lost is through haemorrhage as red blood cells contain iron.
- Secondly, iron accumulates in tissues and leads to oxidative stress through free radical production, which in turn causes inflammation.
- Finally, iron can interfere with the absorption of other essential minerals such as zinc. Iron supplements DO NOT have any effect on immunity, red blood cell function, anaemia or performance in horses.
The bioavailability of vitamin C
Vitamin C (L-ascorbic acid or just ascorbic acid) is another example of a nutrient for which the bioavailability depends on the form that is fed. Vitamin C is essential for healthy cartilage, joint function, immunity and in horses is the main antioxidant in the airways and lungs, helping to control inflammation and infection.
Vitamin C as we all know is high in citrus fruits and certain vegetables such as kale, broccoli and red peppers. Horses, unlike people, can make their own vitamin C and hence we might imagine they have no need for vitamin C supplements. For healthy, young horses not in work this is usually true. However, horses in regular exercise, older horses, horses with Cushing’s, young foals, mares in foal and horses with respiratory disease usually have an increased use of vitamin C in the body and require supplementation.
One might imagine horses would be able to absorb the natural form of vitamin C found in plants, (ascorbic acid) but surprisingly the bioavailability of L-ascorbic acid in horses is very low. Despite this, most equine supplements use this form of vitamin C in feeds and supplements. Studies have clearly shown that other forms of vitamin C such as calcium-ascorbyl monophosphate have much greater bioavailability in horses, leading to much greater increases in plasma vitamin C.
In summary, just because a nutrient is listed on the side of a bag or a supplement tub does not mean that it will be 100% absorbed by your horse. The ability to absorb nutrients or the bioavailability of nutrients varies between horses, the form in which it is fed and other compounds in the diet which may enhance or decrease bioavailability.