Natural food sources of vitamin B12 are limited to animal foods; hence vegetarians and vegans are at increased risk of vitamin B12 deficiency. However, the consumption of fortified foods (e.g. nutritional yeast), as well as taking vitamin B12 supplements can substantially reduce the risk of deficiency.
Exclusively breastfed infants of vegan women are also at increased risk of vitamin B12 deficiency, sometimes very early in life. This can occur even if the mother's own deficiency is only mild. Untreated vitamin B12 deficiency in infants can lead to neurological damage, failure to thrive, developmental delays, and anemia.