A quarter of the young Kiwi women who took part in a nutritional survey are lacking in a vitamin essential for brain health.
Of the respondents in the Adult Nutrition Survey, 23% of females aged 19 to 30 did not get enough vitamin B12, essential for healthy blood and brain development.
The survey, funded by the Ministry of Health, was conducted by Otago University and looked into the eating habits of 4721 Kiwis.
Animal products like fish, meat, milk and eggs were the usual sources of B12, so vegans had a particularly raw deal, nutritional experts said. Although red meat was the best source, B12 was also added to products like Marmite and energy drinks.
"It is important for blood functions, so it matters most if you're a young woman and pregnant," survey director and lecturer Dr Winsome Parnell said.
"If it's not corrected in young women and they get pregnant, there's an increased risk of congenital birth defects."