RDA

In the UK, RDA in this context stands for Recommended Daily Allowance. See Diet and Fitness Today and The Medical Dictionary at the Free Dictionary. In the USA, it stands for Recommended Dietary Allowance; this is the term used by the US National Institutes of Health, and referred to by Wikipedia.

Either way, the A stands for Allowance and not for Amount. (And the R stands for Recommended and not Reference.)

I could not find one website that included the phrase "reference daily amount".