Choline, which helps transfer signals between nerve cells, is highest in dairy foods and meat.
Nutritionist Emma Derbyshire told a BMJ journal that people not eating those foods may not get sufficient choline.
But the British Dietetic Association said, with planning, it was possible to obtain enough from a vegan diet.
Choline is also linked to liver function.
Dr Derbyshire, an independent nutrition and biomedical science consultant, wrote in BMJ Nutrition, Prevention and Health that the UK was falling behind other countries by not recommending or monitoring dietary levels of the nutrient.
The US does have “adequate intake” levels of 425mg per day for women and 550 mg per day for men.
She suggested there had been research suggesting pregnant and breastfeeding women in particular had to ensure they had enough choline in their diets, because of suggested benefits for foetal brain development.
Dr Derbyshire said: “I’m looking in the first instance to raise awareness. But I also think if people are eating a plant-based diet, particularly if they are women of childbearing age, they should look at supplements.”