Vitamin A and Carotenoid Nutrition of Cockatiels
By Elizabeth A. Koutsos, Ph.D.
Mazuri Exotic Animal Nutrition/PMI Nutrition Intl, LLC
Vitamin A is essential for normal vision, cellular differentiation, reproduction, and immune function. The nutrient requirements of birds for vitamin A are often extrapolated from the known requirements of commercial poultry species that are designed to maximize growth or egg production. Another important factor when considering the requirements of commercial poultry is that these species of birds are precocial and hatch with feathers, open eyes, and are ready to find food and water. In contrast, cockatiels and many other companion and exotic avian species are altricial and hatch without feathers, with eyes closed, and rely on parents for food, water and shelter. This means that the chicks are hatching at a very different developmental stage, which may have implications for the nutrient levels that they require at hatch. However, requirements for commercial poultry are often the only available resource to begin to establish nutrient recommendations for companion and exotic birds.
The use of carotenoids as a source of vitamin A in a bird diet may be ideal for several reasons. First, if a bird eats plant-based diets in the wild, they may be better equipped to handle carotenoids in their diet as opposed to pre-formed vitamin A which comes from animal sources. Second, since the body regulates the conversion of carotenoids to vitamin A, the potential for toxicity is generally lower than with pre-formed vitamin A. However, it is important to keep in mind that some animals do not have the metabolic capacity to convert carotenoids to vitamin A. Therefore, research needs to be conducted to determine if an animal can convert carotenoids to vitamin A before making dietary changes. Important factors to consider are the animals’ wild-type feeding strategy, the type of carotenoid being used, and the animals’ ability to convert that carotenoid into vitamin A.
An examination of these factors in cockatiels reveals that these birds are able to utilize carotenoids as a source of vitamin A. The cockatiel is granivorous in the wild, primarily feeding on seeds, although consumption of insects and leaves has also been reported. Domesticated seeds are generally very low in vitamin A, and so supplementation of a seed-based diet is critical (for a variety of other nutrients in addition to vitamin A). In the wild, therefore, it is likely that cockatiels obtain vitamin A from other sources, including insects (retinol or carotenoid sources) or plant parts like leaves and fruits (carotenoid sources). Given their wild type diet, it is likely that 1) cockatiels have not evolved to metabolize large quantities of dietary retinol, and 2) cockatiels have the metabolic capacity to use carotenoids as a source of vitamin A.
The first hypothesis, that cockatiels have not evolved to metabolize large quantities of vitamin A, was examined in adult cockatiels (non-reproducing adult females, 2-3 years of age) that were fed a range of dietary vitamin A levels (0, 2,000, 10,000 or 100,000 IU of vitamin A/kg diet) for 9-23 months. Vitamin A was supplied in the form of retinol (retinyl palmitate), and the diet did not contain any other sources of vitamin A or carotenoids. This research resulted in several valuable findings. First, cockatiels suffer from overt vitamin A toxicity when dietary levels reach 100,000 IU vitamin A/kg diet. In fact, birds fed 100,000 IU vitamin A/kg diet had to be removed from the trial after about 9 months of feeding, due to poor feather quality, significantly elevated liver retinol levels, alterations in vocalization behavior, and some liver pathology. The alterations in vocalizations were very dramatic; birds fed 100,000 IU vitamin A/kg diet had higher frequency of vocalizations and more frequent (5 times more frequent) vocalizations compared to birds fed lower levels of vitamin A. Second, after ~ 23 months of feeding, birds fed 0 IU vitamin A/kg diet were still not showing clinical signs of vitamin A deficiency, although their immune system showed signs of vitamin A deficiency with reduced production of antibodies, and liver retinol levels were declining. Third, although birds fed 10,000 IU vitamin A/kg diet did not have clinically significant signs of vitamin A toxicity, their liver retinol levels continued to rise, and they did have some subtle behavioral changes. Taken together, this research demonstrates that cockatiels are well-adapted to storing vitamin A in their livers, such that they are able to survive on vitamin A deficient diets for a very long period of time, assuming their previous vitamin A status was good. Vitamin A toxicity was reached more quickly, and even birds fed 10,000 IU vitamin A/kg diet seemed to be approaching signs of vitamin A toxicity after 23 months of feeding. The optimal level of retinol in the diet for adult, non-reproducing female cockatiels was ~ 2800 IU/kg.
In summary, the wild-type diet of cockatiels would suggest that these animals have not evolved with a diet containing large amounts of retinol. Research supports that these birds are very efficient at storing vitamin A in their livers for later use. This results in susceptibility to vitamin A toxicity if dietary levels are too high. At the same time, vitamin A deficiency is also possible, and immune dysfunction occurs before other clinical signs are noticeable. Therefore, it is critical that your bird receive adequate levels of vitamin A, and that diets and supplements are stored properly in order to maintain dietary vitamin A levels over time. One source of vitamin A that can reduce the threat of vitamin A toxicity is β-carotene, which was shown to be an effective source of vitamin A in the cockatiel. Further research with other companion bird species is needed to determine their ability to utilize β-carotene as a vitamin A source.
Klasing, K.C. 1998. Comparative Avian Nutrition. CAB International. New York, NY.
Koutsos, E.A., Tell, L.A., Woods, L.W. and K.C. Klasing. 2003. Adult cockatiels (Nymphicus hollandicus) at maintenance are more sensitive to diets containing excess vitamin A than to vitamin A-deficient diets. J. Nutr. 133:1898-1902.
Koutsos, E.A. and Klasing, K.C. 2005. Vitamin A nutrition of growing cockatiel chicks (Nymphicus hollandicus). J. Animal Physiology Animal Nutr. 89:379-387.
Vitamin A, like vitamins D, E and K, is fat-soluble, which means that it is stored in fat deposits and in the liver of an animal. Fat-soluble vitamins cannot be excreted in urine, and so are difficult to excrete from the body. As a result, toxicity of fat-soluble vitamins is of much greater concern than for water soluble vitamins. Vitamin A tends to be of greatest concern, because the level of appropriate intake (i.e., the level that prevents deficiency but doesn’t cause toxicity), is very narrow. Also, vitamin A can be very instable to heat, light and improper storage conditions. This may result in high levels of dietary fortification, in order to avoid degradation of vitamin A to a level that may cause deficiency. Dietary fortification is necessary for complete diets, as well as seed based diets. Seeds, in general, are deficient or devoid of vitamin A and if not supplemented, may result in vitamin A deficiency. Therefore, supplements for seed-based diets contain vitamin A, and often at levels that are quite high (25,000 to 100,000 Dietary sources of vitamin A include the active form, retinol, which can be provided in purified retinol supplements (such as retinyl palmitate), or as a component of animal-based products in the diet. Precursors to Vitamin A, carotenoids, may also be provided in the diet from plant-based sources. For example, one molecule of β-carotene may be converted to two molecules of retinol (Figure 1). About 50 of the 600 known carotenoids can be converted to vitamin A. Carotenoids have other functions, including pigmentation of skin and feathers (generally red, yellow and orange colors), and immune functions.
To address the second hypothesis, that cockatiels have the metabolic capacity to use carotenoids as a source of vitamin A, growing cockatiel chicks (who will have the highest vitamin A requirements of any life stage) were hatched from eggs laid by breeding pairs fed a vitamin A deficient diet. These breeding pairs were fed just long enough (90 days) to create vitamin A deficiencies in the chicks, but still have normal fertility and hatchability. Once chicks hatched, parents were fed diets containing either 0 IU vitamin A/kg diet, 4000 IU/kg diet of vitamin A from retinol, or 2.4 mg β-carotene/kg diet. These diets were fed (by the parents) to the chicks until fledging. This trial showed that chicks fed 0 IU vitamin A had signs of vitamin A deficiency by 5 weeks post-hatch, including body weight loss and poor feathering, and their immune tissues were very poorly developed. In contrast, chicks fed either 4000 IU retinol/kg diet or 2.4 mg β-carotene/kg diet had similar body weights and normal feathering patterns (Figure 2). Finally, liver vitamin A levels were much higher for chicks fed 4000 IU vitamin A/kg diet, suggesting that the β-carotene diet would have reduced risks of vitamin A toxicity for long-term feeding. This research demonstrated that cockatiels can utilize β-carotene to meet vitamin A requirements based upon their successful growth and feathering, but also because the enzyme needed for conversion of β-carotene to vitamin A was expressed in the cockatiel intestine.