“Sainfoin is harmful to horses because the tannins hinder the absorption of minerals” – is that correct?
Recently, an article has been frequently quoted on the Internet stating that the tannins in sainfoin bind minerals and that feeding sainfoin is therefore harmful to horses.
If you start looking deeper into these unsubstantiated claims, it becomes clear that the topic is more complex and less of a concern then originally thought.
Naumann et al. summarised the current state of research on tannins very well in 2017 (http://dx.doi.org/10.1590/s1806-92902017001200009)
If you analyse the various studies, you will find that it repeatedly mentions that a process of binding iron is taking place (depending on the study design and the tannins used). According to various studies, magnesium (Mg), calcium (Ca), phosphorus (P), potassium (K), sodium (Na), copper (Cu), manganese (Md), aluminium (Al), zinc (Zn) and cobalt (Co) are also taking part in the binding process to a lesser extent.
What is the outcome of the different studies that have been conducted?
The results of the various studies are contradictory: Afsana et al. (DOI: 10.1271/bbb.68.584 https://www.tandfonline.com/doi/abs/10.1271/bbb.68.584) have investigated the effect of tannins on the absorption of minerals (iron, zinc, copper, manganese) in rats. As already confirmed by many studies previously, the finding here also was that tannins reduce the absorption of iron.
The absorption of the other minerals was not or only slightly affected. Some studies show that copper is not binding (Scalbert et al., 2002). Others show that zinc is binding, but to a much lesser extent than copper (McDonald et al., 1996). There are virtually no in vivo studies on most other minerals.
However, the effect of tannins on iron depends, among other things, on the feeding scheme and the type of tannin: the iron reduction effect is hardly detectable with several meals a day.
If the condensed (CT) tannins also contained in sainfoin are fed instead of soluble (HT) tannins, the effect on iron is no longer detectable in many studies, even if only one meal per day is fed (DOI: 10.3945/cdn.116.000042 https://academic.oup.com/cdn/article/1/2/1/4558629).
Differentiate between the different tannins
It is important to distinguish between the different tannins, especially the condensed tannins (CT as found in sainfoin) and the soluble tannins (HT as found in many other plants and animal feeds). The effect is sometimes very different.
However, which minerals are bound and to what extent depends not only on the type of tannin used in the study, but also on many other factors. These include the organism in question.
For example, studies on the binding of calcium, phosphorus, magnesium, sodium, and potassium can be found primarily for chickens when fed tannins from millet (e.g. Hassan et al., 2003). However, chickens have a completely different digestive tract and metabolism than horses and millet contain different tannins than sainfoin.
It is highly questionable to simply transfer such results directly to the feeding of sainfoin to horses. There are practically no studies on the subject of tannins in horses; most studies relate to rats, ruminants or humans and the tannins from tea are usually analysed and not those from sainfoin.
Although the chemical mechanisms of complex formation between tannins and minerals are well understood, the extent and effects of this are either unknown or contradictory.
If the test animals actually developed deficiency symptoms, then this would have been investigated and documented in studies. It is assumed that the mineral binding effect is simply so negligible in relation to the quantities of minerals contained in the feed that there is no biological effect.
In the case of iron in particular, horses have a massive surplus of iron in their basic feed (hay, grass), so that a large proportion of the iron is in the diet regardless and therefore is naturally excreted in the faeces. If the sainfoin forms complexes with a few iron ions, it is irrelevant as it won’t have an impact on the horse’s metabolism.
The binding capacity or binding effect of minerals when feeding tannin-containing feed therefore depends on:
- the amount of minerals in the feed (here our horses are generally over-supplied rather than under-supplied)
- the animal species studied (there are virtually no studies on horses, but horses naturally have a high proportion of plants with sometimes considerable tannin content in their diet)
- the type of tannin studied (condensed tannins behave very differently to soluble tannins) and
- the feeding scheme (single meals in a rat have a different effect than a species-appropriate diet with hay ad libitum in a horse)
To draw the general conclusion that ‘sainfoin is harmful because it binds minerals’ is simply unscientific and wrong. Which brings us to the scientific validity of practically all the statements in the article.
Claims without scientific evidence
Claims are made without being backed up by a single scientific reference. When these claims are analysed in detail, most of them turn out to be just empty statements for which there is simply no evidence.
To say “tannins bind minerals and are toxic” is just as correct as saying “mammals live as solitary animals and are dangerous”. This may be true for a mammal tiger but is not correct for a mammal rabbit.
Just as ‘mammals’ is a collective term for animals with a similar reproductive pattern, ‘tannins’ is used as a term for tanning agents found in plants. Chemically, there is a very high variance between the different tannins and their respective effects.
As is so often the case: The quantity makes the poison. If I consume 2kg of green tea a day, I will certainly develop health problems. However, if I drink a pot of tea made with a tablespoon of green tea every day, it will have a positive effect on my health. The same applies to sainfoin: if I feed my horse 20kg of it every day, it will suffer serious health problems.
If, however, following the manufacturer’s instructions and give a maximum of 3 kg per day for an adult horse (600 kg), a positive health effect due to the astringent and anthelmintic effect of the condensed tannins can be achieved, as well as an increased digestibility of the proteins.
The binding of iron by soluble tannins in food is particularly a problem in human female nutrition (monthly iron loss) who follow a vegetarian or vegan diet (low iron absorption).
Sainfoin: the binding of iron does not play a role in the iron balance
When feeding sainfoin (condensed tannins) to horses (permanent eaters of extremely iron-rich feed, namely grass and hay), the binding of iron plays no role in the regulation of the iron balance.
There is no evidence that other minerals are binding to a greater extent and can therefore lead to deficiency symptoms in horses, there have been no findings to date.
Not even in ruminants, and here sainfoin has been used for many years in feeding, especially in land management due to its deworming effect, without any evidence of negative effects on the mineral balance. A distinction must be made between effects that are provoked under laboratory conditions and those that work in the biological system of everyday feeding.
It seems apparent that horses are easily able to compensate for the feeding of condensed tannins in the quantities recommended by the manufacturers without developing problems with their mineral balance.
You can find out more about sainfoin in our podcast: “Sainfoin for horses – good or toxic?”
