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.
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.
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.
