A vitamin that leans on the dairy aisle
Riboflavin, vitamin B2, powers the enzymes that turn food into energy. It rarely makes headlines because outright deficiency is uncommon in rich countries. The reason it matters for vegans is simply where it concentrates, per the NIH fact sheet, a public-domain US government document.
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Foods that are particularly rich in riboflavin include eggs, organ meats (kidneys and liver), lean meats, and milk
Milk in particular is one of the biggest contributors to riboflavin intake in the typical American diet. Remove dairy and eggs and you remove a large slice of most people's supply, which is why the fact sheet names vegans directly.
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Vegans and those who consume little milk in developed countries are also at risk of riboflavin inadequacy
The plant sources, and who should pay attention
Being on the watch list is not the same as being deficient, and the fix is ordinary food. Fortified breakfast cereals, nutritional yeast, almonds, soy products, mushrooms, spinach and other leafy greens all carry riboflavin, and fortified plant milks put it back into the glass. The group the fact sheet singles out for extra care is vegan athletes, whose training raises the need.
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vegetarian athletes are at risk of riboflavin deficiency because of their increased need for this nutrient and because some vegetarians exclude all animal products
The practical read: a vegan eating fortified cereals or nutritional yeast, almonds, soy and greens is well covered, and a vegan training hard should make sure at least one solid riboflavin source shows up daily.