Two packs side by side, both with magnesium. One says 53 %, the other 107 %. Does that mean the second one is twice as good? Or twice as risky?
Neither. The percentage on the back says nothing about quality. It says how the amount contained relates to a fixed reference value. Once you have understood that, you compare food supplements in thirty seconds instead of thirty minutes.
What the reference intake is
The abbreviation NRV stands for nutrient reference value. It is a value that is the same across Europe for an average adult and serves exactly one purpose: comparability. It is not a recommendation for you personally and not an ideal figure.
| Nutrient | Reference intake per day |
|---|---|
| Vitamin C | 80 mg |
| Magnesium | 375 mg |
| Zinc | 10 mg |
| Iron | 14 mg |
| Folate | 200 µg |
| Biotin | 50 µg |
| Vitamin D | 5 µg |
| Vitamin K | 75 µg |
| Vitamin B12 | 2.5 µg |
The percentage is nothing other than the amount contained divided by this value. 1,000 mg of vitamin C come to 1,250 %, because 1,000 divided by 80 is twelve and a half.
Why some values look so high
Three reasons, and none of them is marketing.
The reference value is a lower limit. It describes what is needed for an adequate supply, not what is ideal. For some nutrients, expert recommendations sit clearly above it.
Uptake is limited. With vitamin B12, the regular transport route only lets a small amount through per single dose. To arrive at a meaningful amount via the second, passive route, the dose has to be high. 20,000 % sounds absurd and is simply the arithmetic consequence.
Some reference values are old. The value for vitamin D of 5 µg dates from a time before today's data. That is why D supplements look extremely highly dosed on paper.
Where high percentages really are a signal
With water-soluble vitamins such as C and the B vitamins, the body excretes what it does not use. With fat-soluble vitamins and with minerals it is different, there the body stores them. Iron is the clearest example: it is stored, not excreted. With nutrients like these, a high percentage is not a mark of quality, but a point at which it is worth taking a look at your own status.
So the rule of thumb is neither more is better nor less is better, but: it depends on the nutrient.
The three numbers that really count when you compare
Percentages alone are not enough, because they always refer to the daily intake amount, and that differs from product to product.
- Amount of the nutrient per daily intake amount, not per capsule or tablet. Some products are calculated with two or three units per day.
- How long it lasts in days, that is, the contents divided by the daily intake amount.
- Price divided by that number of days, which gives you the price per day. Only this figure is comparable between two products.
We have worked through the same calculation for powders using the example of collagen, where the serving size decides the price per day.
The most common mistake when reading labels
The big number on the front often refers to the compound, not to the nutrient. Magnesium citrate contains around 16 % magnesium, iron bisglycinate around one fifth iron. A figure such as 1,000 mg on the front therefore says little about how much magnesium is actually in there.
The value that counts is on the back in the nutrition table, in the line with the name of the nutrient. Everything before that is packaging.
Back to the two packs from the beginning
53 % and 107 % for magnesium only mean this: one daily intake amount delivers a good half of the reference intake, the other a little more than the full amount. Which one suits you depends on how much you already cover through your diet. And neither of these two numbers tells you which one is better value. Only the price per day does that.
Sources: reference intakes according to Regulation (EU) No 1169/2011 (FIC), Annex XIII.
Food supplements are not a substitute for a balanced, varied diet and a healthy lifestyle.
