From Wikipedia: “Spinach (Spinacia oleracea) is an edible flowering plant in the family Amaranthaceae native to central and western Asia.
It is an annual plant (rarely biennial), which grows up to 30 cm tall. Spinach may survive over winter in temperate regions. The leaves are alternate, simple, ovate to triangular, and 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 in diameter, maturing into a small, hard, dry, lumpy fruit cluster 5–10 mm across containing several seeds.”
Raw spinach, oxalic acid, iron, and calcium:
“Raw spinach contains oxalic acid, but the amount is dependent on various factors such as the age of the leaves. Younger leaves, for example, contain less oxalate. Oxalic acid can be harmful to your health; however, it breaks down into water, carbon monoxide, and carbon dioxide at temperatures over 150°C. This is because certain minerals such as calcium form large complexes that are not readily soluble, and this makes it difficult for them to be absorbed into the bloodstream via the intestine. In addition, oxalic acid damages tooth enamel and in larger quantities can also be one cause of kidney stones. However, eating foods that contain oxalic acid does not pose a risk to your health, as long as you eat them in moderation. As a comparison, the lethal dose of oxalate consumed orally is 600 mg per kg of body weight, which is equal to about 4.5 kg of raw spinach for someone who weighs 60 kg (132 lbs).*”
“Spinach, along with other green, leafy vegetables, contains an appreciable amount of iron, attaining 21% of the Daily Value in a 100-g amount of raw spinach. For example, the United States Department of Agriculture states that a 100-g serving of cooked spinach contains 3.57 mg of iron, whereas a 100-g ground hamburger patty contains 2.49 mg. However, spinach contains iron absorption-inhibiting substances, including high levels of oxalate, which can bind to the iron to form ferrous oxalate and render much of the iron in spinach unusable by the body. In addition to preventing absorption and use, high levels of oxalates remove iron from the body.
Spinach also has a moderate calcium content which can be affected by oxalates, decreasing its absorption. The calcium in spinach is among the least bioavailable of food calcium sources. By way of comparison, the human body can absorb about half of the calcium present in broccoli, yet only around 5% of the calcium in spinach.”
Types of spinach:
“The three basic types of spinach are:
“In a 100-g serving providing only 23 calories, spinach has a high nutritional value, especially when fresh, frozen, steamed, or quickly boiled. It is a rich source (20% or more of the Daily Value, DV) of vitamin A, vitamin C, vitamin K, magnesium, manganese, iron and folate. Spinach is a good source (10-19% of DV) of the B vitamins riboflavin and vitamin B6, vitamin E, calcium, potassium, and dietary fiber.”
“Spinach is often used to counteract bloating ..., and the seeds work as a laxative. ... Experiments have shown that spinach has hypoglycaemic properties. For more information, see Frozen chopped spinach.*”
Note (italics): * = Translation from a German Wikipedia entry