Additional Math Pages & Resources

Thursday, November 3, 2011

Comparison Pricing for International Units

Today's blog is concerned with comparison price shopping for vitamins. I have compared prices before, about two years ago when pricing a cup of tea, and last summer in showing how our Excel Math curriculum helps kids learn to compare item pricing. This post is a bit more complex.

I have been taking vitamin D (ordered by my regular doctor) because I can't spend too much time in the sun (according to my skin doctor). Most of us get vitamin D from food sources and from sunlight. A few people, in some sunny areas, get adequate amounts of vitamin D directly from the sun, via UV rays which trigger vitamin D synthesis in their skin. The great majority of us who lead indoor lives need more D than we get from sun exposure.

I have learned that buying vitamins is a bit like shopping for a mobile phone plan, or buying a new mattress. In other words, complicated!

Vitamins are "measured" in International Units. This measuring system has nothing to do with the International System of Units (what we Americans tend to call "metric"). Bear with me while I explain this:

What is an International Unit?
An IU is a unit of measure describing the quantity of a substance that achieves a certain amount of biological activity in the body. It is used with vitamins, hormones, medications, and other biologically-active materials. Different biological preparations may have the same effect in the body; the purpose of the IU is to be able to compare them. Any formulation with the same biological effect (regardless of weight or volume) contains the same number of IUs. 

How do we determine an IU and who does this?
The World Heath Organization has a special committee that prepares a reference compound (mixture) of a biological material and arbitrarily determines the number of IUs contained in it. They also specify the laboratory procedure used to compare other formulations with equal effect as the reference compound.

Thanks to this process, one IU of vitamin E is the same as any other IU of Vitamin E. However, there is no equivalence or direct comparison between IUs of Vitamin E with those of vitamin A or D, or insulin or penicillin.

In the case of Vitamin D, one IU is the biological equivalent of a very small amount (2.5 percent of a microgram, or 0.025 μg) of D3 cholecalciferol or D2 ergocalciferol. Our bodies need a lot more than 1 IU per day - more like hundreds of thousands of IUs.

How we decide the Recommended Daily Allowance (RDA)  and who does this?
Or to put another way, how many IUs of any substance you may need to consume each day? That is a question I cannot answer today. Do you mind if we tackle that tomorrow?

Now where were we? Oh yes, shopping.

In this case the common comparison of price per item is useless, because one pill can contain varying amounts of Vitamin D. And there are many different quantities in a bottle. You can purchase liquid, gel, chewable tablet and other forms.

I decided to create a comparison table to see how many IUs I can get for a dollar. Here's my table comparing four  brands and various quantities and strengths of dosage. The green is the best value for an IU and the red is the most expensive per IU. The red choice is 13 times more costly per IU than the green!

 Vitamin Brand/Type Int'l Units Quantity Cost $ Calculation IUs Per $1
 Nature's Bounty D3 10,000 60 19.99 (10000 x 60) ÷ 19.99 =  30,015
 Nature's Bounty D3 5000 300 15.55 (5000 x 300) ÷ 15.55 =  96,463
 Nature's Bounty D3 2000 300 14.99 (2000 x 200) ÷ 14.99 =  26,684
 Nature's Bounty D3 1000 480 16.50 (1000 x 480) ÷ 16.50 =  29,090
 Nature Made D3 5000 90 10.70 (5000 x 90) ÷ 10.70 =  42,056
 Nature Made D3 2000 220 8.90 (2000 x 220) ÷ 8.90 =  49,438
 Nature Made D3 1000 560 7.99 (1000 x 560) ÷ 7.99 =  70,087
 Nature Made D3 400 240 12.89 (400 x 240) ÷ 12.89 =  7,447
 Thorne D3 25,000 60 21.60 (25,000 x 60) ÷ 21.60 =  69,444
 Thorne D3 10,000 60 16.50 (10,000 x 60) ÷ 16.50 =  36,363
 Wellesse D3 Liquid 1000 48 6.29 (1000 x 48) ÷ 6.29 =  7,631

If you want more information, here's a place where all kinds of units are defined and compared.