Epidemiology suggests that diets high in fruit are beneficial to the health because they decrease the risk of cancer and cardiovascular disease. Interestingly, it is not understood why fruit is protective, although many theories have been put forward. Currently, it is thought that the beneficial effects stem from the high concentrations of antioxidant polyphenols which are present in plants. However, recent research has questioned the belief that the rise in total plasma antioxidant status following fruit ingestion is on account of the intake of polyphenols. However, the high fructose intake may cause a concomitant rapid increase in the levels of plasma uric acid, a known potent in vivo antioxidant. Around half of the antioxidant capacity of plasma is though to result from uric acid, and so it is unknown as to the extent that polyphenols contribute to the rise in antioxidant status following fruit ingestion.
To investigate the relative antioxidant effects of uric acid and polyphenols from fruit, researchers1 have fed 12 healthy subjects either 1L of clear apple juice or 1L of clear apple juice without polyphenols. The positive and negative control were provided by feeding either 1L of cloudy apple juice or 1L of water. Following ingestion, subjects had their plasma tested for total plasma polyphenolics, quercetin and uric acid. The ferritin-reducing ability of plasma (FRAP) and the 2,2-diphenyl-l-picrylhydrazyl (DPPH) radical scavenging activity of serum were also tested. All three juice drinks increased FRAP and DPPH scavenging activity, with peak values of antioxidant status appearing 1-hour post-consumption. This increase in antioxidant status was accompanied by a rise in plasma uric acid levels, but not total polyphenols or quercetin. There was no difference between the plasma antioxidant status of clear apple juice with or without polyphenols or the cloudy apple juice.
While apple juice therefore has the ability to increase the antioxidant status of plasma in humans, this action is not related to the presence of polyphenols. Neither total polyphenol nor quercetin plasma levels showed associations with plasma antioxidant status. Apple polyphenols have antioxidant status in vitro, and it can be demonstrated that this is due to the presence of polyphenols. However, these polyphenols are extensively conjugated in enterocytes prior to absorption and this modifies their antioxidant ability. The amount of fructose consumed by the subjects in this study was around 90g. Fructose is rapidly metabolised in hepatocytes by fructokinase where it enters glycolysis and produces large amounts of ATP and the release of purine bases and uric acid. However elevated plasma uric acid is a risk factor for cardiovascular disease because of its deleterious effects on the endothelial lining, and so the results of this study may need further examination.
RdB
