These substances have little nutritive value. They may be considered as food adjuncts, rather than foods proper.

Of fungi, mushrooms are the most commonly eaten. They are prized for their delicate flavor. Chemical analysis shows a high percentage of nitrogen, but although reported as protein, it is largely in an indigestible form.

Algae and lichens are much used as food in some parts of the world and high claims are sometimes made for their nutritive value, but digestion experiments show that, although they have a similar carbohydrate content to other succulent vegetable foods, these carbohydrates are not attacked by the ordinary digestive enzymes of the alimentary tract.

The most important alga, from the dietetic standpoint, is Irish moss. It is commonly used in making jellies or soothing beverages for invalids, but it has no nutritive value whatsoever.

The lichen most used as food is Iceland moss. It has frequently been recommended as a food for diebetics, but it is practically indigestible.