A re-evaluation of the minimum vitamin A requirement of growing-finishing lambs

The minimum vitamin A requirement of growing-finishing lambs was determined by depleting 24 Rambouillet lambs (15.0 +/- .59 kg) of their vitamin A stores and then feeding them for 16 wk one of six fixed intakes of vitamin A (2, 4, 8, 16, 32 or 64 micrograms retinol.kg live wt-1.d-1). Plasma retinol concentrations at the end of the depletion phase averaged 6.0 +/- .6 micrograms/dl. The relationship between vitamin A intake (micrograms retinol.kg live wt-1.d-1, X) and plasma retinol concentration (micrograms/dl, Y) at the end of 16 wk of supplementation was Y = 1.40 + 2.08X-.015X2 (r = .98). The relationship between vitamin A intake (micrograms retinol.kg live wt-1.d-1, X) and liver retinol concentration (log10 micrograms/100 g wet wt, Y) was Y = 1.34 + .058X-.00044X2 (r = .90). Cerebrospinal fluid (CSF) pressures were elevated in lambs fed the 2-, 4- and 8-micrograms levels of vitamin A intake. A broken-line regression model was used to describe the relationship between vitamin A intakes and CSF pressures, and to define minimum vitamin A requirements. The equation obtained was Y = 2.206-.292 (X-1.146) theta, r = .95, where X = vitamin A intake (log10 micrograms retinol.kg live wt-1.d-1) and Y = CSF pressure (log10 mm saline). In this model theta = 1 when X less than or equal to 1.146 and theta = 0 when X greater than 1.146.(ABSTRACT TRUNCATED AT 250 WORDS)