The increase in human plasma antioxidant capacity after apple consumption is due to the metabolic effect of fructose on urate, not apple-derived antioxidant flavonoids

Abstract

Regular fruit consumption lowers the risk of cardiovascular diseases and certain cancers, which has been attributed in part to fruit-derived antioxidant flavonoids. However, flavonoids are poorly absorbed by humans, and the increase in plasma antioxidant capacity observed after consumption of flavonoid-rich foods often greatly exceeds the increase in plasma flavonoids. In the present study, six healthy subjects consumed five Red Delicious apples (1037 ± 38 g), plain bagels (263.1 ± 0.9 g) and water matching the carbohydrate content and mass of the apples, and fructose (63.9 ± 2.9 g) in water matching the fructose content and mass of the apples. The antioxidant capacity of plasma was measured before and up to 6 h after food consumption as ferric reducing antioxidant potential (FRAP), without or with ascorbate oxidase treatment (FRAP AO) to estimate the contribution of ascorbate. Baseline plasma FRAP and FRAP AO were 445 ± 35 and 363 ± 35 μM trolox equivalents, respectively. Apple consumption caused an acute, transient increase in both plasma FRAP and FRAP AO, with increases after 1 h of 54.6 ± 8.7 and 61.3 = 17.2 μM trolox equivalents, respectively. This increase in plasma antioxidant capacity was paralleled by a large increase in plasma urate, a metabolic antioxidant, from 271 ± 39 μM at baseline to 367 ± 43 μM after 1 h. In contrast, FRAP and FRAP AO time-dependently decreased after bagel consumption, together with urate. Consumption of fructose mimicked the effects of apples with respect to increased FRAP, FRAP AO, and urate, but not ascorbate. Taken together, our data show that the increase in plasma antioxidant capacity in humans after apple consumption is due mainly to the well-known metabolic effect of fructose on urate, not apple-derived antioxidant flavonoids.