Five Units, And Why Two Labels Never Compare Cleanly

The two easy ones

A milligram is a thousandth of a gram. A microgram is a thousandth of a milligram, so a thousandth of a thousandth. That is the whole of it, and 1 mg equals 1000 mcg.

The confusion begins because manufacturers sometimes write micrograms as µg, sometimes as mcg, and very occasionally — alarmingly — abbreviate to mg by mistake. Anything quoted in the thousands is almost certainly micrograms; anything quoted as a small decimal is almost certainly milligrams. A supplement carrying 5000 mg of anything would be a brick.

IU: the unit that means something different for every nutrient

International units do not measure mass at all. They measure biological activity, and the conversion is specific to each substance, which makes IU the single most misleading unit on a supplement shelf. Some worked examples:

Look at the two vitamin E rows. The same 400 IU on two bottles is 268 mg on one and 180 mg on the other, purely because one used the natural form and one the synthetic. This is not an obscure edge case; it is why the United States moved supplement labelling away from IU and toward mass units, and why older stock still on shelves is genuinely hard to compare against newer stock.

Vitamin D appears on the Gelagen panel as 10 mcg. In an older notation that identical amount is 400 IU, and a shopper comparing the two would reasonably conclude one bottle had forty times more.

DFE: the unit that adjusts for absorption

Dietary folate equivalents exist because synthetic folic acid absorbs considerably better than the folate naturally present in food. The conversion is:

• 1 mcg of food folate = 1 mcg DFE
• 1 mcg of folic acid taken with food = 1.7 mcg DFE
• 1 mcg of folic acid taken on an empty stomach = 2 mcg DFE

Which means Folate listed at 442 mcg DFE contains rather less than 442 micrograms of actual material. The DFE figure is the honest one, because it describes what your body receives rather than what went into the mixer. A label quoting plain micrograms of folic acid is quoting a number that flatters itself.

RAE: the same idea for vitamin A

Retinol activity equivalents solve an identical problem. Preformed vitamin A from animal sources is used more or less directly; beta-carotene from plants has to be converted, and inefficiently. So:

• 1 mcg retinol = 1 mcg RAE
• 12 mcg of beta-carotene from food = 1 mcg RAE
• 24 mcg of alpha-carotene from food = 1 mcg RAE

That twelve-to-one ratio is the reason nobody has ever developed vitamin A toxicity from carrots. It also explains why Vitamin A carries an upper limit expressed in preformed vitamin A specifically: the plant version is self-limiting and the animal version is not.

And the percentage beside all of them

The percent daily value compares one serving against a single reference figure chosen for a general adult population. It is not adjusted for your age, sex, weight, diet or pregnancy status. It is a shared yardstick, and that is the entire extent of what it claims to be.

Two consequences worth carrying. A very large percentage often reflects a very small denominator rather than a generous dose. And the number the percentage never shows you is the tolerable upper limit, which is the figure that actually constrains what you should take. Panels print the first and essentially never mention the second.

How to use all this in a shop

That takes about four minutes and it is the single most useful skill in this entire category. The panel pages here do the conversion work in advance for one product, which is a start.