Two products containing the same nutrient can differ in strength by a large multiple while sitting on the same shelf. The reason is that supplement doses are not derived from a single clinical standard.
Reference intakes are not treatment doses
Population reference values are set to meet the needs of nearly everybody in a healthy group, and they describe intake from the whole diet rather than from a pill.
Doses used to correct a diagnosed deficiency are often far larger and are prescribed for a defined period with follow-up testing.
A retail product may be built around either logic, or around neither, and the label rarely makes clear which it is following.
Upper limits are the real ceiling
Regulators publish tolerable upper intake levels representing the highest daily amount unlikely to cause harm in the general population.
For some nutrients that ceiling sits far above the reference intake, leaving a wide band in which products can legally be formulated.
For others the gap is narrow, and those nutrients are where high-dose products carry the most real risk, particularly the fat-soluble vitamins that accumulate in tissue.
Absorption falls as dose rises
Many nutrients are absorbed by transporters that saturate, so doubling the dose does not double what enters the bloodstream.
Beyond the saturation point the surplus passes through or is excreted, which is why very high doses of water-soluble vitamins produce expensive urine rather than proportional benefit.
Manufacturers can still print the larger figure on the label, since declarations describe content rather than what the body takes up.
Marketing rewards the larger number
A shopper comparing two products has little basis for choosing other than price and the figure on the front, and the larger figure reads as better value.
This produces a ratchet in which competing products escalate in strength without any clinical reason for the increase.
The pattern is most visible in categories where a nutrient has attracted attention, since demand there is driven by coverage rather than by measured need.
Stacking pushes totals past the ceiling
Multivitamins, fortified foods and single-nutrient products all contribute to the same daily total, and people frequently take several without adding them up.
Exceeding the upper limit is far easier through combination than through any single product, and some interactions matter, such as high zinc intake impairing copper absorption.
Anyone taking prescribed medication, pregnant, or considering high-dose products should have the combination reviewed by a pharmacist or doctor, since several supplements interact with common drugs.