Here we report increases in plasma vitamin C and increases in total antioxidant capacity (as indicated by H2O2-induced DNA damage) as well as a trend towards decrease in oxidised pyrimidines and oxidised purines. Previously, we reported increases in mean plasma vitamin C concentrations of up to 26% after suplementation with 3 green kiwifruits per day for 3 weeks . There were no restrictions in either study as to when during the day the fruit should be eaten, and in practice the last fruit was consumed 10 h or more prior to blood sampling. Peak plasma vitamin C concentrations are reached a few hours after supplementation and then decline , and so the 14-16% increase in vitamin C seen here is all the more impressive. Measured a short time after kiwifruit consumption, the concentration would presumably have been much higher. This increase in plasma concentration of reduced and total ascorbic acid indicates that (golden) kiwifruit may be an effective booster of antioxidant status, especially since vitamin C activity is likely to be enhanced by acting in concert with other phytochemicals from fruit.
After both one and two fruits per day resistance towards H2O2-oxidation increased by ~30%. The reduced level of strandbreaks may in part be caused by the increased intake of vitamin C. At two fruits per day there was a borderline significant inverse correlation between plasma levels of reduced vitamin C and lymphocyte strandbreaks, but this association was not present at one fruit per day. Resistance towards H2O2-oxidation seemed to increase somewhat more following consumption of yellow kiwifuit than after consumption of green kiwifruit , even if a direct comparison cannot be performed between the two studies.
Supplementation with one kiwifruit per day for four weeks protected lymphocytes against DNA base oxidation, as indicated by a decrease of more than 20% in the number of FPG-sensitive sites measured with the comet assay. This effect was not seen after supplementation with two kiwifruits per day. The lack of a clear dose-response effect is not unprecedented; in our previous study with green kiwifruit , protective effects against DNA oxidation did not increase with higher daily doses of fruits. This may represent a saturation effect, if the active ingredient in one kiwifruit is sufficient to exert a maximal protective effect. We presently have no idea of the identity of the active ingredient.
Endonuclease III-sensitive sites were also reduced after supplementation by about one quarter. The reduction was significant after two kiwifruits per day but not after one. There was no significant correlation between endonuclease III-sensitive sites and H2O2-induced DNA damage or plasma vitamin C, indicating that the reduction in endonuclease III-sensitive sites may be caused by specific modulating compounds of the kiwifruits rather than by its antioxidant content per se.
The lack of any effect of kiwifruit supplementation on lymphocyte DNA repair - either BER or NER - is unexpected, since a stimulation of BER was the most pronounced effect in the previous kiwifruit study . Significant effects of green kiwifruit, and of a high fruit and vegetable diet, on both BER and NER, have been seen in another recent intervention trial , and so we can hypothesise that golden kiwifruit may lack specific ingredient(s) that can modulate DNA repair. The lack of significant effects of golden kiwifruit supplementation on the expression of key DNA repair genes is consistent with the lack of effect on repair enzyme activities. Lack of effect of antioxidant rich whole food on repair enzyme activities have been reported previously. Despite reduced DNA damage and increased resistance towards H2O2 induced strand breaks, Rizo et al. reported no effect of broccoli consumption on the expression of DNA repair enzymes .
There is increasing evidence that some natural components can reduce levels of specific CVD risk factors by modulating platelet function [11, 30]. In recent years, research groups including ours have reported inhibitory effects of different fruits on platelet aggregation . Potent anti-platelet-aggregation factor(s) are present in tomatoes and green kiwifruits [11, 24, 25]. Extracts of these fruits inhibit platelet aggregation in response to ADP, collagen, and thrombin both in vitro and in vivo [11, 21, 25].
In the present study, consuming 1 golden kiwifruit per day for 4 weeks reduced whole blood platelet aggregation. This effect disappeared during the washout period (data not shown). Consuming 2 golden kiwifruits generally did not inhibit whole blood platelet aggregation. (Only with 7.5 μM ADP was there a marginally significant decrease.) There is no obvious explanation for this lack of effect, but it corresponds to the lack of dose response in relation to markers of DNA oxidation, discussed above - though there is no reason to assume the operation of a common molecular mechanism. The lack of effects of 2 golden kiwifruits per day could be caused by unknown confounders.
Our in vitro platelet aggregation experiments suggest that green and golden kiwifruit extracts inhibit both ADP and collagen-induced whole blood platelet aggregation (to different degrees). The fruit extract may contain a wide variety of different types of compounds that have anti-platelet activity in vitro and that affect different mechanisms of activation and aggregation. We have earlier shown that anti-platelet effects of fruits including green kiwifruits and tomatoes are independent of their antioxidant activity [11, 16, 25].
Consumption of golden kiwifruit reduced plasma triglyceride levels without affecting cholesterol levels: original levels were restored after the washout period (data not shown). Lowering of plasma triglycerides by kiwifruit was observed despite the fact that the volunteers maintained their regular diet during the supplementation period. Such effects of fruits and vegetables have been reported before . Our data imply that both green and golden kiwifruits might have cardio-protective effects.
A possible limitation of the present study was the use of healthy subjects. The protective effect of antioxidant supplementation may be more pronounced in subjects exposed to increased levels of oxidative stress, such as e.g. endurance athletes, diseased or old people . Moderate sample size may be another limitation. Since this was a cross-over intervention any possible weak seasonal variation would cancel out in the final analysis, but the inclusion of a control group could have improved our knowledge of seasonal variations in the measured biomarkers.
To summarise, this intervention trial, with biomarkers relevant to both cancer and CVD indicates that one kiwifruit per day is as effective as two per day, when considering both effects on DNA oxidation, and platelet aggregation. The concentration of plasma triglycerides is decreased by golden kiwifruit, with no parallel effect on cholesterol.