Infants with weights of less than 1500g require 4 mg/kg per day.[105][106] 1962 The whey:casein ratio was made similar to human milk because producers were aware that human milk contains a higher ratio of whey protein, and cow's milk contains a higher ratio of casein.[105] 1984 Taurine fortification was introduced because newborn infants lack the enzymes needed to convert and form taurine.[105][107] Late 1990 Nucleotide fortification was introduced into infant formula because nucleotides can act as growth factors and may enhance the infant immune system.[105] Early 2000 Polyunsaturated fatty acid fortification was introduced.