RE: RDA Sugar?
It's a GDA not an RDA.
Recommended Daily Amounts (or Reference Nutrient Intakes) refer to the minimum amount required for the body to function. So, the actual RDA for salt is about 3-4g (I think!) but we eat far more, so the Guideline Daily Amount (i.e. maximum limit) is 6g.
GDAs have been developed by Industry rather than the Government and are based upon scientific rationale.
As with any recommendations, they apply to the population as a whole and cannot be applied to any one individual with accuracy.