Long-term effects on retina of rhesus monkeys fed taurine-free human infant formula.

Abstract

Research in the last 20 years has clearly established that taurine is essential for the normal development and maintenance of photoreceptor cell structure and function. This has been demonstrated best in cats as first described by Hayes et al. 5, 6 These carnivores are dependent on a dietary supply of this amino acid, because of insufficient biosynthetic capacity due to the low activity of the enzyme, cysteine sulfinic acid decarboxylase. Cats become taurine-depleted when deprived of dietary taurine and develop central retinal degeneration. In rhesus monkeys and humans, dependency on dietary taurine appears to be age-dependent. In human infants, formulas low in taurine prompt a postnatal drop in plasma taurine concentration, unlike taurine-supplemented formulas or human milk, which is rich in taurine2, 9. Human infants, children and adults receiving taurine-free parenteral nutrition for a prolonged time showed significant reductions of taurine in plasma, urine and blood cells3, 14, 21, 22, 24, in some cases accompanied by ophthalmoscopic and electroretinographic abnormalities which could be reversed by taurine supplementation1, 3, 4, 20. In rhesus monkeys, we have demonstrated that taurine-free human infant formula feeding from birth led to 50–60% reductions in plasma taurine concentration after 3 and 6 months in comparison to the same formula supplemented with taurine, and that these differences were no longer present at 12 months. Light and electron microscopic examinations revealed that taurine-de-prived monkeys at 3 months of age show morphological changes in the retina and visual cortex compared with monkeys fed the taurine-supplemented formula8, 12. The retinal changes regressed with increasing age, but were still present in most taurine-deprived monkeys at 12 months, while the densities of rod and cone pigments, measured by fundus reflectometry, were significantly reduced in the taurine-deprived monkeys at 6 months but not at 12 months7. The present study was undertaken to ascertain whether or not the morphological changes in the retina persists for a substantial period beyond the time at which tissue taurine concentrations appeared to approach normal levels in the taurine-deprived monkeys.