How to calculate the fulfilment of RDI


I would like to start a discussion on the exact interpretation of RDI’s.

I have encountered two different ways to calculate the amount of a substance. I will illustrate this with an example: potassium.

Most people agree that the RDI of potassium is 3,5 gram.

  1. One way to get there is to add 3,5 gram of potassium salt (KaCl), and call it a day.
  2. The other way is to take the percentage of mass of potassium into account when calculating how much potassium you need: Potassium is 39 gram per mol, Chloride is 35,5, so the percentage of potassium in KaCl is 39/(39+35,5) = 52%. This would mean adding slightly less than 7 gram om potassium salt to soylent.

I have observed that some people around here use the second method. Some use the first, but it is less common.

In the case of sulphur, Rob said in one of his posts that after his sulphur deficiency he added 2 gram of MSM per day. This number has spread it’s way througout the community, until one day I saw someone point out that to get 2 gram of sulphur, you need to add 5,7 gram MSM, because MSM is only 32/91 = 35% sulphur. Something has gone clearly wrong here!

That’s not the worst thing: my multivit uses the first method exclusively. For the mineral Iodide, whose RDI is 0,15 mg, it contains 0,15 mg potassium iodide, which is only 127/(127+39) = 77% iodide.

I have searched for the actual way the RDI is determined/measured, and have come up empty handed. I have found sources for either method, and the wikipedia article on RDI heavily references a physical book I don’t have acces to.

Does anyone have any trustworthy source regarding all this?
Is my multivit falling short?


The second method is the correct one. To see why, consider alternatives to your arbitrary choice of adding potassium salt. Imagine instead you added 3.5g of bananas. Clearly that’s not right.

If you see someone doing the math the first way raise a red flag, they might not have any chemistry experience.


My apologies for not being clear enough: of course I think the second method is the right one.

My concerns are more about the way The Authorities (Institute of Medicine in the US, European Science Advisory Network for Health for Europe, Gezondheidsraad for the Netherlands) calculate the RDI. I strongly hope that they also use the second method.

But my multivit says on the box that it contains 0,15 mg of potassium iodide to fulfil the 0,15 mg of recommended iodide, and that this is 100% of the RDI, which is 0,15 mg in Europe*. Is the manufacturer of my multivit wrong? Has the Dutch FDA (Voedsel- en Warenauthoriteit) missed this? Should I write to the manufacturer and/or the Dutch FDA?

Had I been full time on soylent this could have had grevious health consequences. If this were America, I probably could sue them :stuck_out_tongue:

*) This sentence is selfcontradictory, because 0,15 mg of potassium iodide is only 0,12 mg of iodide. See my first post for how I came to this calculation.


It certainly reads like they have screwed up somewhere.
This is the reference I use (being a USAnian) to fact check.

Reading that, if you’re eating other foods, you’re almost certainly getting enough, and if you’re not taking in 1100 mcg then you are unlikely to be harmed.


Another possibility is that one of the other ingredients in the vitamins contains some iodine and something else, and that the KI is just there to bring the total to .15mg. It’s hard to say without seeing the ingredients list.

If you’re concerned it’s definitely worth sending a letter to ask how they do their calculations.


I cheated. I went to and built a nutritional profile there.


I use the second method but I’m getting most of my vitamins from spirutein and now ill have to check to see if they did. I kind of just assumed the bottles nutritional data was the bioavailable amount


I took a look at some multi-vitamins and multi-minerals at Vitamin Shoppe yesterday, and they all seem to do this same thing: Listing the amount of the compound as satisfying 100% of the RDI or RDA or AI value, when in fact the math disagrees.

I’m now wondering if there’s a flaw in the labeling law that allows them to get away with this, and being cheap, they do.