Cofactor
A nutrient the body needs in order for another nutrient or enzyme to do its job.
A cofactor is a helper. Many vitamins and minerals act as cofactors, meaning an enzyme or another nutrient cannot function properly without them.
This is why some supplements are taken together: one is required for the other to work. Magnesium, for example, is a cofactor for hundreds of enzymatic reactions, including those that activate vitamin D.