Absorption of most vitamins and minerals takes place largely in the duodenum and jejunum. Dissolution of complex vitamin and mineral supplements can take up to 30 minutes in the presence of acid in the stomach. Fat soluble vitamins, such as beta carotene and vitamin E and D, require the presence of fat in the diet to enable adequate absorption. Therefore in order to maximise the absorption, vitamin and mineral supplements should be taken with food to enable full dissolution and to delay and slow down transit through the GI tract. This routine also helps to maximise the tolerance of supplements. Absorption of iron and other minerals from both foods and supplements can be restricted by the presence of phytates (wholegrain cereals and unleavened wholegrain breads) as well as by tannins (present in tea and coffee) so it is advisable to limit the consumption of these at the same time as consuming iron rich foods or supplements.

Calcium from cow's milk can also significantly affect the absorption of iron and so for maximum effectiveness iron tablets should not be taken with a drink of milk.