Horses and vitamins

Riboflavin-Vitamin B₂

What it is
At the end of the nineteenth century practical research associated beriberi with a dietary factor related to polished rice. Simultaneously the inclusion of rice polishings improved appetite and growth, reduced the extent of lesions in the corners of children’s mouths and prevented polyneuritis. For a while these improvements were associated with thiamine under the description ”unidentified dietary factor water-soluble B”.
Later it was shown that this water-soluble B factor consisted of two separate ingredients, one which was unstable, and one which was stable, when heated. The less stable factor was re-named vitamin F (thiamine) while the heat-stable product was labelled vitamin G. At a later date they were re-named again, vitamins B₁ and B₂ respectively.
Riboflavin was first isolated from egg albumen in 1933 and was chemically prepared in 1934. The molecule is in two distinct parts: a ribose sugar unit and a three-ring flavin structure known as lumichrome.

What it does
Riboflavin functions in several different enzyme systems. Two derivatives, riboflavin 5’-phosphate (flavin mononucleotide -- FMN) and riboflavin 5’-adenosine diphosphate (flavin adenine dinucleotide -- FAD), are the coenzymes which unite with specific apoenzyme proteins to form flavoprotein enzymes. One of the most important functions of these flavoproteins is the nonenzymic oxidation-reduction reaction between flavins and pyridine nucleotides. The leuco-forms of free flavins are readily reoxidised by a wide variety of oxidising agents. At the same time, in enzymic processes, flavoproteins dehydrogenate various substrates including reduced pyridine nucleotides, aldehydes and purines. Flavin enzymes also serve as oxidases. Most of the flavin coenzyme systems help to regulate cellular metabolism while others are specifically involved in the carbohydrate or the amino acid metabolism systems. Riboflavin also appears to have a role in fat metabolism.
Dietary riboflavin is absorbed from the food in a site-specific area of the small intestine. Here it is bound to a carrier protein (riboflavin-binding protein -- RBP) which transports it to the liver, the adrenals, the ovary and other sites where it is built into functional enzymes. Excess riboflavin is withdrawn from the blood by the kidneys and various flavin compounds excreted via the urine. Riboflavin, as such, is not stored in the body.

If too much is given
There is no evidence that the intake of excessive amounts of riboflavin leads to any form of toxic reaction. Very large amounts have been supplied to horses and many other species without any apparent effect. Since riboflavin is one of the water-soluble vitamins any excess is rapidly excreted via the kidneys and urine.

Biosynthesis
Like other B-vitamins riboflavin is synthesised in the caecum by microorganisms. The site of production is probably beyond the specific absorption site in the small intestine so it is difficult to understand how horses obtain any benefit from this synthesis unless they are able to browse amongst their droppings.

How it is measured
Vitamin B₂ contents of feeds and tissues are generally expressed as mg riboflavin per kg. No International Unit of riboflavin has been defined. Some very early papers referred to rat units which were equivalent to 4mg crystalline riboflavin while others used Sherman-Bourquin units of 20mg riboflavin. The lumichrome portion of the riboflavin molecule emits a characteristic yellow-green fluorescence. Direct fluorimetry can be used for the quantitative determination of riboflavin in feeds or tissues.

Assessment of status
Tissue concentration of riboflavin can be used to assess status but the most sensitive indicator is its concentration in erythrocytes (red blood cells) which should be greater than 20mg /100 ml.

Antagonists
Certain mycotoxins (such as aflatoxin) have been found to reduce the effectiveness of the dietary riboflavin supply.

Requirements and allowances
Research work has suggested that feed riboflavin levels of 2-4 mg/kg are normally sufficient for horses. However, these experimental results were obtained under perfect, or very good, research conditions. Under normal field and stable conditions riboflavin allowances may need to be considerably above scientific requirements to meet specific feed problems.

The following riboflavin supplements are suggested for optimum enzyme activity:

mg / kg mg / day

Adult performance horses in training
3
30

Adult performance horses in light work 2,5 15

Ponies, hacks & hunters 3 9

Mares & stallions 3 12

Young horses 1-2 years 3 9

Foals & yearlings less than 1 year 4 4-10

The increased use of high energy diets also has a direct effect on riboflavin requirements. The ingredients used for high energy rations are naturally low in riboflavin and feed intake is generally lower as well. Under such circumstances daily intakes are liable to be considerably reduced unless the riboflavin supplement is increased. Therefore as the energy level of the diet is increased the riboflavin level should be increased proportionally.

It has also been shown that low levels of some mycotoxins interfere with riboflavin metabolism. Since it is often difficult to determine the presence of these mycotoxins at low levels increased supplementation is sometimes given to combat possible mycotoxin effects.

Vegetable feedstuffs usually contain less riboflavin than feed ingredients of animal origin. Cereal and milling by-products and cassava (tapioca) meals are particularly poor sources. Some oil seeds and dried yeast/brewery/distillery products have higher contents. Liver and milk products are also good sources.

Stability
In the pure crystalline form riboflavin is extremely resistant to oxidation even when heated in air for long periods of time. Although it is fairly resistant to strong acids decomposition is fairly rapid in alkaline solutions. Although stable to heat, riboflavin is rapidly decomposed on exposure to light and should always be kept in the dark. Formulations do not require any extra riboflavin to allow for losses unless it is known that a feed is likely to be exposed to light for long periods of time.