Grapefruit, Seville oranges, bergamot, and possibly other citrus also contain large amounts of naringin. It can take up to 72 hours before the effects of the naringin on the CYP3A4 enzyme are seen. This is problematic as a portion of grapefruit contains enough naringin to inhibit the metabolism of substrates of CYP3A4. Naringin is a flavonoid which contributes to the bitter flavour of grapefruit. Citrus varieties that don't usually contain much bitter naringin in flesh tend to have higher concentrations in rind, which is sometimes also consumed, apparently the compound also serves as insecticidal.