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The U.S. Department of Agriculture considers the peanut to be a legume, which is to say a “pea.” Most diet, nutrition, and recipe books call it a nut. So which is it? Here’s the best short answer I can give: It’s a pea that behaves like a nut.
The peanut originated in South America, but once Europeans were introduced to it, they quickly carried it to other warm areas of the globe, including parts of Africa. From Africa, it was brought to what is now the southeastern United States, where the Kongo word nguba became “goober.”
Botanically, the peanut (Arachis hypogaea) is a member of the legume family, which also includes many other food crops such as peas, beans, licorice, carob, and alfalfa (as in sprouts). The peanut plant looks very much like any pea or bean plant – a sprawling or bushy little herbaceous annual (i.e., non-woody plant with a natural life span of no more than a year). It does have one unique characteristic, however: When an individual flower is done blooming, its stalk bends down and the tip burrows into the soil. The peanut pod forms underground. This is the origin of another of the peanut’s common names, the groundnut.
I assume that the main reason why most people classify peanuts with nuts rather than with its fellow legumes is that it is generally eaten more like a nut than like a bean. Consider serving size, for example. Dr. D’Adamo defines a serving of legumes as a cupful (about 240 ml). Depending on the variety, a cupful of cooked beans typically provides 10 to 16 grams of protein (a nice amount for the main protein source in one meal) and from 1 to 3 grams of fat.
Peanuts are a much more concentrated source of nutrients: It would take only about 1/2 cup (120 ml) of dry-roasted peanuts to supply a comparable 14 grams of protein – but that amount would also supply 28 grams of fat! This concentration and the high fat content explain why a serving of peanuts is generally considered to be a very nutlike 1/4 cup.
I had always assumed that the peanut’s amino-acid profile, on the other hand, was typical for a legume, and was planning to say so here until I double-checked Frances Moore Lappe’s classic book Diet for a Small Planet (1971). It turns out that while most other legumes are a good source of Isoleucine and an excellent source of Lysine (which makes them a good complement to most grains) the peanut is a relatively poor source of both. Once again, this makes it a somewhat better fit in the nut category.
Notice that I do not say “the nut family.” Unlike legumes, nuts do not constitute a botanical family; they’re more of a culinary category. Almonds, for example, are actually in the rose family, along with peaches and apricots. Cashews and pistachios are from the same family as mangos and poison ivy. Hazelnuts and filberts are from the birch family. Pine nuts and pignolias really do come from pine trees. About the closest thing there is to a “nut family” is the walnut family, which includes all types of walnuts as well as pecans and hickory nuts.
So if the almond is a nut in the rose family, the filbert is a nut in the birch family, and the pignolia is a nut in the pine family, then why can’t we just say that the peanut is a nut in the legume family?