We're definitely considering adding native support for tracking additional vitamins (such as Vitamins D and K), as well as supplements, or medications. In the meantime, many users use the Notes area to record these additional items.
As an alternative, you can create a new Meal category called "Vitamins and Supplements". You can then create a custom food entry in the food database to represent your vitamin or supplement, and log this item to your diary when you consume it.
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