Vitamin A is a group of unsaturated nutritional organic compounds that was discovered in 1913. Vitamin A is essential for normal growth and development in animals. Vitamin A stabilises the cell membrane, stimulates the synthesis of certain important proteins by acting on transcription and possesses an electron transfer capacity. These important cellular activities translate into growth differentiation and the proliferation of a wide range of epithelial tissues, bone, reproductive and embryonic development. Throughout the entire animal life cycle, an adequate physiological concentration of vitamin A in the body will play an instrumental role in the growth and differentiation processes at the cellular level. It is responsible for the formation, protection, and regeneration of mucous membranes, as well as skeletal development. It is also essential for improving resistance to infectious diseases and plays a major role in animal fertility. In addition, vitamin A circulating in the blood is taken up by the cells in the retina and is used as a part of the pigment for good vision, which is paramount to general animal well-being. Unfortunately, vitamin A cannot be synthesized by animals and must be obtained from the diet as preformed vitamin A and its esters. Vitamin A is found only in feeds of animal origin, for example liver, fish oil and fish meal. While it can be derived from dietary carotenoids, mainly beta-carotene in feed ingredients such as green plant material, supplementation of the vitamin A in the diet is essential to maintain optimal animal growth performance.

One of the major forms of vitamin A is retinol, a highly conjugated unsaturated hydrocarbon molecule with a terminal allylic alcohol (Figure 1). Such a structure is predisposed to undergo rapid oxidation and degradation. Hence, commercially available vitamin A products are typically retinyl esters of acetate, propionate or palmitate. The ester functionality is electron withdrawing, meaning that it will disperse away the electron density in the conjugated double bonds, rendering the molecule less sensitive towards oxidation. The ester group also serves as a "chemical cap" to block off any possible reactions at the reactive allylic alcohol moiety. Retinyl esters undergo hydrolysis by pancreatic lipase and an enzyme in the intestinal brush border. The intracellular retinol is re-esterified, packaged into chylomicrons and released into general circulation via the lymphatic system. As vitamin A comprises of a collection of organic molecules, there is no specific measure of vitamin A content in a matrix. The universally recognized quantification unit of vitamin A content is International Units (IU) in g/kg of material. One IU of Vitamin A is equivalent to 0.344 µg retinyl acetate.

Figure 1. Structures of retinol and retinyl acetate.

An effective vitamin A feed additive must satisfy two key criteria. First, it must maintain its stability as a feed additive, during storage as well as in feed and premixes. BASF Lutavit^® A 1000 NXT incorporates butyl hydroxytoluene (BHT) as an effective antioxidant that helps to retard radical initiated oxidation process. With increasing safety concerns over ethoxyquin (EQ), BHT is a safe and cost-effective alternative to EQ without compromising antioxidant efficacy to protect the vitamin A in a matrix. This is particularly important in feed and vitamin-mineral premixes, where the combination of multiple ingredients and harsh processing conditions could potentially catalyse the degradation of vitamin A. In one of our recent studies, we compared the stability of different vitamin A feed additives blended in stress premixes comprising of wheat middlings, choline chloride and trace minerals, then stored in open containers for 4 weeks at 40 °C and 70 % relative humidity (Figure 2). We found that BASF Lutavit^® A 1000 NXT has the highest recovery compared to other vitamin A products in the market containing BHT, tocopherol or EQ as antioxidants. This is attributed to the unique formulation of Lutavit^® A 1000 NXT and the stringent manufacturing process control at BASF to produce a high-quality product.

Figure 2. Stability of different vitamin A products in stress premix composition stored in open packages at 40 °C / 70 % relative humidity stored over 4 weeks.

The second requirement for a good vitamin A feed supplement is bioavailability. Retinyl acetate is a lipophilic molecule with a molecular weight of 328.5 g/mol. From a pharmacological perspective, transmembrane trafficking of retinyl acetate will have a low energy barrier. The limiting factor is therefore how homogeneously the active ingredient is dispersed throughout the feed mixture and readily available in the physiological condition for uptake. The technology behind Lutavit^® A 1000 NXT enables high flowability, low dustiness and uniform dispersion of the product particles in feeds and premixes. A recent vitamin A bioavailability trial was set up to compare Lutavit^® A 1000 NXT (at 3 inclusion rates 0, 20 and 40 mg/kg feed) to other products in the market (Figure 3). Birds were fed for 16 days, a vitamin A-free diet (depletion phase), followed by a 24- hours vitamin A supplementation phase (repletion period) and then a 48-hours vitamin A-free post feeding period. The birds were euthanised, and the liver samples were prepared within 6 hours for analysis. Lutavit^® 1000 NXT showed highest relative bioavailability compared to the other two products in the market.

Figure 3. Relative bioavailability of Lutavit^® A 1000 NXT compared to other vitamin A products in the market.

Conclusion

Supplementation of Vitamin A in diets is essential to ensure optimal growth development and good health of animals. Vitamin A has been predominantly used as in poultry, swine and ruminant diets. The inclusion rate of vitamin A in feed is dependent on factors such as type of basal diet, species, growth stage and feed storage and/or processing conditions. The key requirements for an effective vitamin A additive are good chemical stability and high bioavailability. Lutavit^® A 1000 NXT by BASF is a uniquely formulated vitamin A feed additive that demonstrates good flowability, excellent stability and high compatibility with other ingredients in feed and premixes, and most importantly, superior bioavailability.

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Article made possible through the contribution of BASF Animal Nutrition

Going back to basics with vitamin A in animal nutrition