Vitamins

Vitamins 

The word “vitamin” was coined in the early 1900’s, and comes from 2 Latin words – vita and amine – vita because they’re vital, or essential, to life and amine as it was initially thought all vitamins were amines, but it was soon discovered they’re not all amines after all... but they are definitely essential to life! 

In this article I’m going to explore each vitamin, what it does, where it’s found, and what happens when it’s in deficiency or excess.

Let’s start with the Fat Soluble Vitamins; these are, as the name suggests, vitamins that dissolve in fat, and are stored in the body alongside fat, in the liver and fatty tissue. These can be stored in the body for future use.

Fat Soluble vitamins consist of A, D, E, and K. 

Vitamin A - Retinol 

Herbivores get Vitamin A from precursors of the vitamin which are found in plants in the form of Carotenoids (such as beta carotene). Carotenoids can be converted to Vitamin A in the liver or the intestines, and Vitamin A can be stored in the liver for future use.

Carotenoids are orange or red - so foods that are this colour are good sources of Vitamin A e.g. carrots, rosehips. Vit A is also abundant in grass, but, like most vitamins, it is unstable in sunlight, so there are only very low levels found in hay or haylage. 

Role

• Epithelial tissue integrity and Keratinised cell differentiation - skin, cornea, hooves, gut lining etc.
• Mucous membrane development
• Immune response
• Free radial scavenger - antioxidant
• Bone development
• Night vision (so carrots really do help you to see in the dark!!!)

Requirements

• 500kg horse at maintenance - 15000 IU  
• 500kg horse in hard exercise - 22500 IU 
• 500kg horse in gestation or lactation - 30000 IU 

Whilst those might look like really high numbers, they're really not in the scheme of things - an adult cow at maintenance for example requires 1,500,000 IU of vitamin A per day - 100 times more!!

Sources 

• 100g of rosehips provide 4,345 IU of Vitamin A 
• 100g of carrots provide 16706 IU of Vitamin A 

Deficiency 

• Limited growth   
• Night blindness  
• Excessive lacrimation (tears)  
• Skin hyper-keratinisation  
• Fertility problems

Excess 

• Bone fragility / Hyperostosis  
• Teratogenesis – birth defects, so it is not included in human pregnancy vitamin supplements
• Exfoliated epithelium (flaky skin, or external layers of internal organs)
• DOD (developmental orthopaedic disease) in growing horses
• Liver stress
• Vision problems
• In humans – pruritus, peeling palms and soles, fissuring of fingertips 

So, an important vitamin, where deficiency and excess are equally problematic. It can be easily sourced from many plant sources, and so it is unlikely that you'll need to supplement your horse with Vitamin A.

Vitamin D 

There are many forms of dietary vitamin D. Ergocalciferol is found in plants (D2) and Cholecalciferol is found in animal products (D3). 

Horses mainly get their Vitamin D from sunlight – There are Provitamins (the sterols ergosterol and 7-dehydrocholesterol) which are converted to cholecalciferol by irradiation from the sun. Interestingly, there are some studies that indicated rugging horses has very little effect on synthesis of Vitamin D by the horse!

Dietary sources of vitamin D are absorbed in the small intestines and transported to the liver. 

Role 

• Calcium and Phosphate homeostasis – bone formation     
• Cell growth and differentiation  
• Parathyroid hormone (PTH) interdependency with Vit D
• Regulates the levels of  Calcium circulating in the blood plasma

Requirement 

• 3,300 IU for 500kg horse in average work 
• 7,750 IU for a 18 month growing youngster (to make 500kg)     
• 8,580 IU for a 24 month growing youngster (to make 500kg)   

Sources

• Sunlight!!
• Green leafy plants
• Mushrooms

Deficiency 

Deficiency is very rare now, with no recent documented cases available.

Historically it was seen in pit ponies, equines on  the Northern islands with limited sunlight and those housed inside for long periods of time without access to  sunlight.

• In young animals deficiency presents as Rickets, in older animals it presents as Osteomalacia.
• Decreased bone calcification  
• Stiff gait  
• Swollen joints  
• Serum calcium and phosphate reduced 

Excess 

• Depression    
• Anorexia and weight loss 
• Liver issues, since it is stored in the liver  
• Calcification of soft tissues and organs – lungs, heart, kidney  
• Similar symptoms to deficiency, given the role of Vit D in bone mineral mobilisation  

Another vitamin where an excess is potentially as harmful as a deficiency! Most horses will get enough sunlight and eat enough green leafy plants, so that you will not need to supplement your horse with Vitamin D.

Vitamin E 

Vitamin E is a first line antioxidant. It is necessary for Selenium metabolism – and vice versa.   

Vitamin E also interacts with copper, and zinc to a lesser extent, by modulating the amount of those minerals that are taken up by the cells. In other words, vitamin E can provide protection from copper toxicity.

As an antioxidant, it oxidises quickly – thus it is hard to store or preserve in feeds, and is pretty much absent in hay or haylage.

Role   

• Antioxidant 
• Cell signalling  
• Gene Expression    
• Immune system function

Requirements 

• 500 IU adult 500kg at maintenance – 500 IU  
• 775 IU for a 18 month growing youngster (to make 500kg)   
• 858 IU for a 24 month growing youngster (to make 500kg)   

Requirements for vitamin E increase with higher levels of fat in the diet, therefore when feeding additional fat, it is crucial to ensure the horse is getting enough Vit E to counteract the oxidation occurring through the metabolism of fat.

Sources    

• Green plants - grass, and leaf and stem of any green plant!
• Cereals  
• Rosehips - 5.84mg per 100g

Studies have shown that natural Vitamin E is only marginally more bioavailable than synthetic vitamin E, according to limited animal studies. 

Deficiency 

• Muscle degeneration 
• Reduced immunity  
• Liver damage 
• Reduced phagocytosis  
• Lameness and muscle rigidity - Tying up risk, cell signalling   
• EMND – Equine Motor Neuron disease & EDM – Equine degenerative myeloencephalopathy - Both of these diseases feature damaged myelinated nerve fibres via oxidative injury 

Excess  

Vitamin E excess in of itself is uncommon 

• Blood clotting
• Abnormal bone mineralisation 

There are many interactions with Vit E, with several minerals and enzymes requiring it's presence for uptake by the body and for various biological actions. It is important to consider where your horse is getting Vitamin E from during the winter months when grazing is depleted, or if your horse has high fat levels in the diet, or is on a hay/haylage only diet. For example, horses kept on grass-free track systems will need their vitamin E requirements met from elsewhere. Whether that is from herbs or synthetic supplement is personal choice, but it is a crucial vitamin that is required to be provided in the diet.

Vitamin K 

Phyloquinine 

Essential for blood clotting 

Can be synthesised in the gut from leafy vegetables and forages, thus very unlikely to require supplementation.

 

Let’s now take Water Soluble Vitamins, C, and the B vitamins. These dissolve in water, and are different to Fat Soluble Vitamins because they do not get stored in the body for later use, and so levels need to be 'topped up' frequently. 

All B Vitamins are implicated in metabolism, or converting food into energy. The complete list is as follows:  

B1 (thiamin)

Role 

• Co-enzyme for respiratory pathways 
• Glucose Metabolism, Nervous System Health
• CO2 removal from TCA cycle 
• Carbohydrate metabolism 

Deficiency

• Metabolic crisis - Lack of ATP 
• Cell damage, nerve damage, excess lactic acid 
• Cymbalic et al carried out a Standardbred horse study: bradychardia, ataxia, muscular fasciculations, hypothermia of extremities. blindness and weight loss 
• Bracken poisoning in horses presents as B1 depletion and all the associated problems. "Bracken staggers" can be treated with high dose Vitamin B1.

Sources

Synthesised in the horses' hind guts, thus not usually essential to be provided in the diet

B2 (riboflavin) 

• Protein, Fat and Carbohydrate Metabolism
• Red blood cells formation
• Skin, mucus membranes, eye tissue integrity 

No evidence of deficiency or toxicity of B2 witnessed in horses

Sources

Found in legumes like clover and alfalfa and found in grass and hays, and synthesised in the hindgut at higher rates where microbial activity is higher, thus it is not usually essential to be provided in the diet.

B3 (niacin) 

• Cell differentiation
• DNA processing
• Calcium mobilisation 

Nicotinic acid and Nicotinamide -  Natural sources are hydrolysed in the intestinal mucosa and yield niacin. 

Toxicity and deficiency have not been documented in horses.  

Widely found in plants and synthesised by the gut flora in the horses' hindgut, thus it is not usually essential to be provided in the diet.

B4 (pantothenic acid) 

Role in fat catabolism and carbohydrate metabolism. 

 “Panto” means “common” or “frequent” - Pantothenic - means widely distributed in food sources, as well as synthesised in the gut.

No signs of deficiency were documented when horses were deprived of B5, and no RDA amounts have been established for horses. No toxicity has been documented. Not essential to be provided in the diet.

B6 (pyridoxine) 

Role in Metabolism of fats, proteins and carbohydrates and is a component of many enzymes.

No dietary requirement established, no deficiency or toxicity documented in horses. Not essential to be provided in the diet. 

B7 (biotin) 

Biotin is a co-enzyme of 4 carboxylase enzymes which are involved in metabolism of fatty acids, cholesterol and amino acids. It is also essential in cell proliferation. Biotin is the only water soluble vitamin where scientifically proven improvements have been seen from dietary supplementation.

Sources

• Biotin is manufactured in the hindgut by microbial fermentation of fibre
• Grains such as oats
• Berries (e.g. rosehips)
• Nuts
• Seeds (e.g. sunflower seeds)

Biotin is component of structural proteins e.g. keratin, the primary protein in the equine hoof horn.  

Comben et al found that dietary supplementation with Biotin improved hoof health in the thoroughbred horses studied. Similar studies exist in cows too. 

Comben N, Clark RJ, Sutherland DJ - The Veterinary Record, 01 Dec 1984, 115(25-26):642-64540 horses - 10-30 mg - 6-9 months - Clinical observations on the response of equine hoof defects to dietary supplementation with biotin. "Varying degrees of improvement in the hardness, integrity and conformation of the hoof horn" 

Horse should synthesise enough biotin in their guts through fermenting fibre and so won't normally require biotin supplementaion. However, if you have a significant structural hoof problem, it won't do any harm to try supplementing with biotin to see whether it helps - whether synthetic or natural source is personal choice. Again, the wonderful rosehip is a great natural source of biotin, as are seeds, such as sunflower seeds.

I suspect that if the study was repeated on horses on a higher fibre diet than a standardbred in race training however, we might see a different outcome... since biotin is produced from fermenting fibre, in theory a high enough fibre diet would provide enough biotin.... maybe someone can repeat that study in the future on different fibre level diets!! (If anyone would like to fund a PhD study, I'd happily look into it!)

B9 (folic acid or folate) 

• Cell growth/replication   
• Co enzyme for Carbon metabolism - closely associated with B12 in C metabolism. 
• DNA, Purine and Methionine synthesis 

Source

• Synthesised by gut microbes
• Forage 

Deficiency symptoms have not been documented in the horse to date, but various drugs can impact folate status.

Synthesised folate is not well absorbed by the horse, with natural sources absorbed much more easily. 

B12 (cobalamin)

• Contains the element cobalt (the clue is in the name!)  
• Stored in the liver and blood 
• Needed for cell replication and energy  

Deficiency

• Macrocytic anaemia 
• Reduction in RBCs 
• In humans - 'pernicious anaemia'  

In horses, deficiency has not been documented, however, supplementation has improved anorexia in anaemic horses. 

Long lost B Vitamins

You might notice that there are some B number missing – these are substances which were initially thought to be vitamins, but we now recognise them as other bioactive and important dietary substances, but not true vitamins.

B4 – Adenine - a water-soluble component of DNA and RNA, essential for normal human growth and energy production. Not a vitamin, but deficiency can cause neurological issues, fatigue and depression   

B8 – Inositol – cell membrane component, chemical messenger, implicated in the transport of insulin, serotonin and other hormones  

B10 - para amino benzoic acid – PABA – Used to be used in sunscreen but is now banned! Sold as a supplement in human markets for the treatment of infertility in women, arthritis, "tired blood" (anemia), rheumatic fever, constipation, systemic lupus erythematosus (SLE), to darken hair, improve complexion and treat headaches.

Vitamin B11 - Salicylic acid – Plant hormone – an active compound in willow, meadowsweet, aloe vera etc. It is the principal metabolite of aspirin and thus overdose is a very real risk, which would cause gastrointestinal bleeding and ulceration and potentially death.

Vitamin C

Ascorbic acid    

• Anti-oxidant - along with Vitamin E and Se 
• Horses can synthesis Vitamin C from glucose (like most other mammals) 
• Dietary supplementation is hotly debated in the horse world - studies vary in opinions 

Role   

• Antioxidant  
• Cofactor in synthesis of: 
• Collagen 
• Carnitine 
• Norepinephrine 

Requirements  

Dietary requirements are not yet defined in the horse as it is presumed that their requirements will be met by endogenous synthesis from glucose.

Sources

• Glucose metabolismm
• Rosehips - 5.84mg per 100g 

Deficiency 

• Scurvy - insufficient collage formation - but never witnessed in horses 
• BUT some old evidence of disease susceptibility and lower serum Vitamin C levels 

Excess

• Vitamin C excess has no documented ill effects in horses 
• In humans and laboratory tested animals, excess vitamin C was shown to instigate allergic responses and other issues 

Summary

Hopefully after digesting all that information, you will see that in the main, horses will look after their own vitamin needs when fed a diverse, species appropriate diet. The only exception being Vitamin E, which may need to be supplemented in the diet when preserved forages are used instead of fresh grass, that is, in the winter months, or on grass-free tracks, or where grass is restricted for health reasons.

Supplementing with Vitamins A & D should be avoided as excesses can be highly problematic.

Supplementation with B and C vitamins is unlikely to be necessary, but equally, is unlikely to be harmful. There is some evidence that Vit C can be helpful for older or sick animals, or those in higher exercise, and there is no known detriment to overfeeding it.

Vitamins can be provided to you horse via different plants and herbs, or by a balancer. If using a balancer, bear all of the above in mind, and look for high vitamin E, and no (or very low) vit A or D.