Apple phytochemicals

Apples contain a large concentration of flavonoids, as well as a variety of other phytochemicals, and the concentration of these phytochemicals may depend on many factors, such as cultivar of the apple, harvest and storage of the apples, and processing of the apples. The concentration of phytochemicals also varies greatly between the apple peels and the apple flesh.Some of the most well studied antioxidant compounds in apples include quercetin-3-galactoside, quercetin-3-glucoside, quercetin-3-rhamnoside, catechin, epicatechin, procyanidin, cyanidin-3-galactoside, coumaric acid, chlorogenic acid, gallic acid, and phloridzin (Figure 2). Recently researchers have examined the average concentrations of the major phenolic compounds in six cultivars of apples. They found that the average phenolic concentrations among the six cultivars were: quercetin glycosides, 13.2 mg/100 g fruit; vitamin C, 12.8 mg/100 g fruit; procyanidin B, 9.35 mg/100 g fruit; chlorogenic acid, 9.02 mg/100 g fruit; epicatechin, 8.65 mg/100 g fruit; and phloretin glycosides, 5.59 mg/100 g fruit [46].The compounds most commonly found in apple peels consist of the procyanidins, catechin, epicatechin, chlorogenic acid, phloridzin, and the quercetin conjugates. In the apple flesh, there is some catechin, procyanidin, epicatechin, and phloridzin, but these compounds are found in much lower concentrations than in the peels. Quercetin conjugates are found exclusively in the peel of the apples. Chlorogenic acid tends to be higher in the flesh than in the peel [47].Because the apple peels contain more antioxidant compounds, especially quercetin, apple peels may have higher antioxidant activity and higher bioactivity than the apple flesh. Research showed that apples without the peels had less antioxidant activity than apples with the peels. Apples with the peels were also better able to inhibit cancer cell proliferation when compared to apples without the peels [32]. More recent work has shown that apple peels contain anywhere from two to six times (depending on the variety) more phenolic compounds than in the flesh, and two to three times more flavonoids in the peels when compared to the flesh. The antioxidant activity of these peels was also much greater, ranging from two to six times greater in the peels when compared to the flesh, depending on the variety of the apple [31]. This work is supported by Leontowicz et al [48] who found that rats consuming apple peels showed greater inhibition of lipid peroxidation and greater plasma antioxidant capacity when compared to rats fed apple flesh.Many of these phytochemicals from apples have been widely studied, and many potential health benefits have been attributed to these specific phytochemicals. The procyanidins, epicatechin and catechin, have strong antioxidant activity and have been found to inhibit low density lipoprotein (LDL) oxidation in vitro [49]. In mice, catechin inhibits intestinal tumor formation and delays tumors onset [50,51]. Sawa et al. (1999) found that chlorogenic acid has very high alkyl peroxyl radical (ROO•) scavenging activity. Compared to about 18 other antioxidant compounds (including quercetin, gallic acid, α-tocopherol), chlorogenic was second only to rutin [52]. Since ROO• may enhance tumor promotion and carcinogenesis, chlorogenic acid may add to the protective effect of apples against cancer. Chlorogenic acid has been found to inhibit 8-dehydroxy-deoxyguanosine formation in cellular DNA in a rat model following treatment with 4-nitroquinoline-1-oxide [53].Quercetin is also a strong antioxidant, and is thought to have potential protective effects against both cancer and heart disease. Briefly, quercetin has been found to down regulate expression of mutant p53 in breast cancer cells, arrest human leukemic T-cells in G1, inhibit tyrosine kinase, and inhibit heat shock proteins [54]. Quercetin has protected Caco-2 cells from lipid peroxidation induced by hydrogen peroxide and Fe2+ [55]. In mice liver treated with ethanol, quercetin decreased lipid oxidation and increased glutathione, protecting the liver from oxidative damage [56]. Recently, it has been found that high doses of quercetin inhibit cell proliferation in colon carcinoma cell lines and in mammary adenocarcinoma cell lines, but at low doses quercetin increased cell proliferation (20% in colon cancer cells and 100% in breast cancer cells) [57]. However, low doses of quercetin (10 uM) inhibited cell proliferation in Mol-4 Human Leukemia cells and also induced apoptosis [58]. Quercetin inhibited intestinal tumor growth in mice, but not in rats. Low levels of quercetin inhibited platelet aggregation, calcium mobilization, and tyrosine protein phosphorylation in platelets [59]. Modulation of platelet activity may help prevent cardiovascular disease.

Both animal and cell culture studies show that there is an association between the polyphenolic compounds found within apples and a wide variety of effects that may help prevent chronic disease. This supports the hypothesis that it is the phytochemicals found in fruits, especially apples, that impart healthy benefits. More research is still needed to clarify the effects of these compounds in vivo. In order to examine the effects of these compounds in vivo, it is necessary to understand the bioavailability of the specific compounds, and the bioavailability of these compounds within the fruit matrix.