Re: How is RDA calculated and what' the interval? E.g. Potassium??

Aloha, Mikael,

     Let me start by providing some of the information that I give to my 
introductory nutritions students, here at the University of Hawai`i at 
Manoa, where I have been for 15 years, on the Human Nutrition faculty.  
Also, I will be speaking from the US perspective, since this is what I 
know best.  Other countries (e.g., Sweden) have similar systems, but each 
is a bit different.

     The RDA (Recommended Dietary Allowance) is set for each nutrient, 
separately...and can differ by age group, gender, and (for women) 
reproductive status.  Actually, the RDA is part of a broader set of 
indices, now called the Dietary Reference Intakes, which includes AI 
(Adequate Intake) and EAR (Estimated Average Recommendations).  The AI is 
used when there isn't enough good scientific information to set an actual 
RDA OR when there is a lot of disagreement amongst experts as to what is a 
correct level (as was the case recently with the RDAs for calcium and 
phosphorus) 

     The RDA for any nutrient is set high...high enough to cover about 98% 
of the normal, healthy population (based on gender, age, etc.).  If you 
know any statistics, it is set at the mean intake of the population + 2 
standard deviations above the mean.  Thus, for almost all people, it is 
more than is necessary to maintain health...has a safety factor built in.  
In the US, we don't have RDA ranges...just single numbers...so, I'm not 
sure what the lower number might mean (in Sweden?) in other countries' 
recommendations. Maybe minimums?

    In addition, for each nutrient, the DRI Committee from the Food and 
Nutrition Board (part of the US National Academy of Sciences)uses some set 
of physiologic measures to set the RDA.  For example, one of the 
indicators for setting the RDA for vitamin D was the serum level of one 
form of the vitamin.

     Regarding your comment about potassium:  The US recommendation for 
potassium is not an RDA, but rather still a minimum daily requirement 
(sort of the least that is safe).  It is set for 1600-2000 mg/day for 
maintaining adequate potassium stores and fluid values.  A diet high in 
fruits and vegetables should provide adequate potassium.  The old Daily 
Value (still used on our food labels) is 3500 mg/day for an adult, which 
sounds like the value you mention.  If you eat enough fruits and 
vegetables, you should easily get this amount.  If you eat fewer fruits 
and vegetables, then you may get only about 2000 mg.  If you eat huge! 
amounts of fruits and vegetables, you might get as much as 8000-11000 
mg/day!! So, I guess the message is "eat your fruits and vegetables."