Definition
A unit that expresses the amount of a substance by how strongly it acts in a living body, not by its weight. Each substance gets its own standard reference sample, and amounts are measured against it. That is why the actual weight of 1 IU is different for each substance.
What matters is the effect in your body, not the weight
Take 100 grams (g) of a mild green pepper and 100 grams of a fiery hot chili. The weight is the same, but the heat is worlds apart. Equal weight does not mean equal spiciness. That is why spiciness is sometimes given on its own scale that measures how hot something is, not how heavy.
Vitamins and hormones had a similar problem. When these substances were first discovered, it was hard to extract them in pure form or to work out their molecular structure. Even if you weighed a sample, you could not tell how much of the substance you actually wanted was inside.
So scientists changed their approach. They picked one officially recognized reference sample, then agreed on a rule: if a substance does this much of what the reference does in animals or cells, it counts as this many units. An amount defined by the strength of its effect is the International Unit (IU). Today, international bodies such as the World Health Organization (WHO) look after these reference samples.
Why does 1 IU weigh a different amount for each substance?
One meter (m) is the same length whether you measure a wooden stick or an aluminum pipe. But IU has no fixed weight that works for every substance. Put 1 IU of vitamin A and 1 IU of vitamin D on a scale, and their actual masses are very different.
For example, about 0.025 micrograms (µg) of vitamin D make 1 IU, while about 0.3 micrograms of vitamin A (retinol) make 1 IU. Same 1 IU, yet the weights differ by more than ten times.
The reason is that IU is a yardstick set separately for each substance. The IU for vitamin A is defined by the vitamin A reference sample, and the IU for vitamin D by the vitamin D reference sample. So 1 IU of one does not mean the same size of effect as 1 IU of the other, and you cannot compare IU numbers across different substances.
Even within one substance, a different form can change the IU conversion. For vitamin E, the natural form and the synthetic form have different weights per 1 IU.
To be more precise: how IU differs from the SI
Having "International" in the name does not mean IU belongs to the International System of Units (SI) used throughout physics. SI units such as the meter (m), kilogram (kg), and second (s) are base scales designed so that the same yardstick applies to anything.
IU, by contrast, was defined from the start by comparing the effect a substance has inside a living body against a reference sample. Rather than measuring one objective physical quantity, it is a practical way of counting how much a substance works, according to that substance's own standard.
As it became possible to extract substances in pure form and identify them, labels for some ingredients have been moving away from IU toward weights such as micrograms (µg). That is why, for the same vitamin, you can see tables written in IU and tables written in µg side by side, depending on the country and the era.
Common misconceptions
1 IU can be converted directly into the same weight for every nutrient.
The weight of 1 IU differs from substance to substance. 1 IU of vitamin D is about 0.025 µg, and 1 IU of vitamin A is about 0.3 µg. That is because each substance has its own reference sample.
The International Unit (IU) is an official unit in the International System of Units (SI), like the meter or the kilogram.
The names sound alike, so they are easy to mix up, but they are different things. The SI is the standard system for measuring physical quantities. IU is a separate unit that rates the strength of an effect for each substance against its own reference sample.
Where you meet it
A nutrition label that lists vitamin D in IU instead of weight. The same product may list it in µg on a label in another country.
Substances like insulin and heparin, which come from living sources and were hard to isolate in pure form. Their amounts were recorded for a long time in units such as IU.
The Scoville scale for chili heat, which measures not weight but how much you must dilute a chili before the heat disappears. It shares IU's idea: counting an amount by the strength of its effect.
In one sentence
The International Unit (IU) rates the strength of an effect, not weight, against a reference sample set for each substance. That is why 1 IU weighs a different amount for each substance, and why it is not the same thing as the International System of Units (SI).