Grapefruit (Citrus paradisi) is a sub-tropical tree of the family Rutaceae and its citrus fruit.

The evergreen tree is usually found at around 5-6 m tall, although it can reach 13-15 m. The leaves are dark green, long (up to 15 cm) and thin. It produces 5 cm white four-petalled flowers. The fruit is yellow-skinned, largely oblate and ranges in diameter from 10-15 cm and has an acidic yellow segmented pulp. It is also known as the pomelo or the toronja. Varieties include the white grapfruit and the red, of which the 1929 US Ruby Red (of the Redblush variety) has a patent. The fruit has only become popular from the late 19th century, before that it was only grown as an ornamental plant. The US quickly became a major producer of the fruit, with plantations in Florida and Texas.

The fruit was discovered in the 1750s probably in Barbados. It had developed as a natural cross of the pummelo (citrus grandis) with the sweet orange (citrus sinensis), it is rather closer to the first than the second. Further crosses have produced the tangelo (1905), the minneola (1931) and the sweetie (1984). It was not called the grapefruit until the 1800s, previously it was known as the shaddock, its current name is in reference to the clusters of the fruit on the tree. It was not distinguished from the pummelo botanically until the 1830s when it was given the name Citrus paradisi and its true origins were not determined until the 1950s. This has led to the official name being altered to Citrus X paradisi.

Grapefruit can have a number of interactions with drugs, often increasing the effective potency of compounds such as cyclosporine and tacrolimus. Grapefruit seed extract is a strong antimicrobial with proven activity against bacteria and fungi.

Grapefruit formed a core part of the so-called grapefruit diet, under the pretext of being able to increase the metabolism and burn fat.

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