Table 2.

Estimated efficiency of conversion of β-carotene to vitamin A in animals, defined as μg of β-carotene required to form 1 RE (1 μg retinol). The n value given is the total number of animals in the study.

Species	Conversion ratio	Study design	References
Herbivorous mammals
Cow	8.33	Recommendation extrapolated from study in lambs [122].	[123, 212]
Sheep	5.56–8.33	Recommendation based on review of available studies.	[218]
Sheep	7.36	Depletion/repletion study of BC from corn silage in lambs (n = 56); hepatic VA used to indicate VA status.	[123]
Horse	6–10	Recommendation based on review of available studies.	[215]
Horse	>10	Depletion/repletion study with BC supplement in water-soluble beadlet form; serum retinol and relative dose response test used to indicate VA status (n = 45). Supplementation with 215,00 μg BC/d did not restore VA status, while 21,500 RE as retinyl palmitate did (though not completely).	[151]
Omnivorous mammals
Rat	2	Recommendation based on review of available studies.	[156]
	6	Depletion/repletion study using dry gelatin beadlet form of BC as supplement; hepatic VA used to indicate VA status.	[157]
Mongolian gerbil	6–13	Depletion/repletion study using water-soluble beadlet form of BC as supplement (n = 80); hepatic and renal VA used to indicate VA status.	[158]
Human	2 in oil 12 in food	Recommendation based on review of available studies.	[166]
Human	2.4	Stable isotope study administering physiological doses of 13C-BC in oil to children for ≤10 wk (n = 35); concurrent administration of 13C-retinyl acetate allowed estimation of BC bioefficacy from serum response.	[75]
Human	9.1	Stable isotope study administering single oral doses of 2H-BC to adults (n = 22); concurrent administration of 2H-retinyl acetate allowed estimation of BC bioefficacy from serum response.	[78]
Human	14.8 in carrots; 20.9 in spinach	Subjects consumed single meal of intrinsically labeled 2H-spinach and 2H-carrots (n = 14); concurrent reference dose of 13C-retinyl acetate allowed estimation of BC bioefficacy from serum response.	[77]
Pig	6.7	Recommendation based on review of available studies.	[216]
Pig	13–27	Depletion/repletion study of BC from corn (n = 171); hepatic VA used to indicate status.	[124]
Pig	40	Gilts were fed low VA diet for only 4 weeks, and there was no indication that they were truly depleted (n = 36); supplemented with 100 mg BC in water-soluble beadlets; hepatic VA used to indicate status.	[169]
Carnivorous mammals
Ferret	>15	Depletion/repletion study with BC supplement from water-soluble beadlets (n = 145); hepatic VA used to indicate status.	[183]
Birds
Chicken	2	Recommendation based on review of available studies.	[195]
Chicken	6	Depletion/repletion study using dry gelatin beadlet form of BC as supplement; hepatic VA used to indicate VA status.	[157]
Cockatiel	>2	Depletion/repletion study in which chicks born from breeding pairs fed VA-devoid diet consumed VA or BC (synthetic) supplemented diets for 38 d (n = 27); hepatic VA used to indicate status.	[198]