Spinach (Spinacia oleracea) is a flowering plant in the family Amaranthaceae, native to central and southwestern Asia. It is an annual plant (rarely biennial), which grows to a height of up to 30 cm. Spinach may survive over winter in temperate regions. The leaves are alternate, simple, ovate to triangular-based, very variable in size from about 2-30 cm long and 1-15 cm broad, with larger leaves at the base of the plant and small leaves higher on the flowering stem. The flowers are inconspicuous, yellow-green, 3-4 mm diameter, maturing into a small hard dry lumpy fruit cluster 5-10 mm across containing several seeds.

In popular folklore, spinach is a rich source of iron. In reality, a 60 gram serving of boiled spinach contains around 1.9 mg of iron (slightly more when eaten raw). A good many green vegetables contain less than 1 mg of iron for an equivalent serving. Hence spinach does contain a relatively high level of iron for a vegetable, but its consumption does not have special health connotations as folklore might suggest.

Ultimately, the bioavailability of iron is dependent on its absorption. This is influenced by a number of factors. Iron enters the body in two forms: nonheme iron and heme iron. All of the iron in grains and vegetables, and about three fifths of the iron in animal food sources (meats), is nonheme iron. The much smaller remaining portion from meats is heme iron (Williams, 1993).

This larger portion of dietary iron (nonheme) is absorbed slowly in its many food sources, including spinach. This absorption may vary widely depending on the presence of binders such as fiber or enhancers, such as vitamin C. Therefore, the body's absorption of non-heme iron can be improved by consuming foods that are rich in vitamin C. However, spinach contains high levels of oxalate. Oxalates bind to iron to form ferrous oxalate and remove iron from the body. Therefore, a diet high in oxalate (or phosphate or phytate) leads to a decrease in iron absorption.

The myth about spinach and its high iron content may have first been propagated by Dr. E. von Wolf in 1870, because a misplaced decimal point in his publication led to an iron-content figure that was ten times too high. In 1937, German chemists reinvestigated this "miracle vegetable" and corrected the mistake. It was described by T.J. Hamblin in British Medical Journal, December 1981.

Spinach also has a high calcium content. However, the oxalate content in spinach binds with calcium decreasing its absorption. By way of comparison, the body can absorb about half of the calcium present in broccoli, yet only around 5% of the calcium in spinach. Oxalate is one of a number of factors that can contribute to gout and kidney stones. Equally or more notable factors contributing to calcium stones are: genetic tendency, high intake of animal protein, excess calcium intake, excess vitamin D, prolonged immobility, hyperparathyroidism, renal tubular acidosis, and excess dietary fiber (Williams, 1993).

Types of spinach

A distinction can be made between older varieties of spinach and more modern varieties. Older varieties tend to bolt too early in warm conditions. Newer varieties tend to grow more rapidly but have less of an inclination to run up to seed. The older varieties have narrower leaves and tend to have a stronger (although more bitter) taste. Most newer varieties have broader leaves and round seeds.

There are 3 basic types of Spinach:

• Savoy has dark green, crinkly and curly leaves. It is the type sold in fresh bunches in most supermarkets. One heirloom variety of savoy is Bloomsdale. Bloomsdale is also somewhat bolt resistant.

• Flat/smooth leaf spinach has broad smooth leaves that are easier to clean than savoy. This type is often grown for canned and frozen spinach, as well as soups, baby foods, and processed foods.

• Semi-savoy is a hybrid variety. It has slightly crinkled leaves. It has the same texture as savoy, but it is not as difficult to clean. It is grown for both fresh market and processing. Five Star is a widely grown variety and has good resistance to running up to seed.

Other species called spinach

The name spinach has been applied to a number of leaf vegetables, both related and unrelated to spinach:

Related species

• Chard (Beta vulgaris, Amaranthaceae), also known as spinach beet or perpetual spinach.
• Orache (Atriplex species, Amaranthaceae), also called "French spinach" or "mountain spinach".
• Good King Henry (Chenopodium bonus-henricus, Amaranthaceae) and other Chenopodium species, also called "Lincolnshire spinach".

In Indonesia, the word bayam is applied both to certain species of amaranth commonly eaten as a leafy vegetable, and to spinach, which is rarely seen, only in certain supermarkets but well known from Popeye cartoons.

Unrelated species

• New Zealand spinach (Tetragonia, Aizoaceae).
• Water spinach (Convolvulaceae).
• Malabar spinach (Basellaceae).
• The greens of various nightshade, legume and cucurbit species are also known as spinach, wild spinach, African spinach, "Thermadorian spinach" or morogo (in Southern Africa).