Vitamin C is used as a dietary supplement because of its antioxidant activity, although a high dose (500 mg) may act as a pro-oxidant in the body1,2. Here we show that 100 g of fresh apples has an antioxidant activity equivalent to 1,500 mg of vitamin C, and that whole-apple extracts inhibit the growth of colon- and liver- cancer cells in vitro in a dose-dependent manner. Our results indicate that natural antioxidants from fresh fruit could be more effective than a dietary supplement.
Apples of the Red Delicious variety were extracted using 80% acetone and their content of phenolics and flavonoids determined3,4: the extracts contained 290.2±4.2 and 219.8±1.8 mg phenolics, and 142.7±3.7 and 97.6±3.9 mg flavonoids per 100 g apples with and without skin, respectively. There are known to be more phenolics in the skins of apples than in the flesh, and quercetin glycosides are found only in the skins5.
We measured the total antioxidant activity of apples by using the total oxyradical-scavenging capacity (TOSC) assay6 and found that apples with skin had a higher TOSC value than apples without skin ( Fig. 1a). The total antioxidant activity of 1 g apples with skin was 83.3±8.9 TOSC (μmol vitamin C equivalents) — that is, the antioxidant value of 100 g apples is equivalent to 1,500 mg of vitamin C. Given that the average vitamin C content in fresh apples with skin is 5.7 mg per 100 g (ref. 7) and that the total antioxidant activity of 0.057 mg vitamin C (in 1 g of whole apples) is only 0.32 TOSC (Fig. 1a), then almost all of the antioxidant activity in apples must be due to phytochemicals.
We treated a colon-cancer cell line, Caco-2, with extracts equivalent to 0, 5, 10, 20, 30, 40 and 50 mg ml−1 apples for 96 hours (the treatment time for maximal response). Cell proliferation was inhibited in a dose-dependent manner after exposure to apple-extract concentrations above 20 mg ml−1 (Fig. 1b): at 50 mg ml−1, inhibition was 43±1% and 29±4.1% for apples with skin and for apples without skin, respectively.
We also tested the effect of apple extracts on the proliferation of another cancer-cell line, HepG2 human liver-tumour cells. We found that apple extracts at 50 mg ml−1 inhibited the proliferation of these cells as well, by 57±0.21% and 40±0.64% for apples with and without skin, respectively (Fig. 1c). The extracts of apple with skin could thus significantly (t-test, P=0.031) reduce tumour-cell proliferation compared with extracts of apples without skin. No cytotoxicity of the apple extracts was seen at any of the concentrations tested (data not shown).
We suggest that this strong inhibition of tumour-cell proliferation in vitro could be due to apples' combination of phytochemicals (phenolic acids and flavonoids), as these are natural antioxidants. It has been proposed that the consumption of whole fruits may provide the antioxidant balance needed to quench reactive oxygen species8 which have been implicated in tumorigenesis9. Phytochemicals in apples other than ascorbic acid seem significantly to enhance their antioxidant properties and their capacity to inhibit the proliferation of tumour cells in vitro.
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Total Phenolics, Tannins and Antioxidant Activity in Twenty Different Apple Cultivars Growing in West Himalaya, India
Proceedings of the National Academy of Sciences, India Section B: Biological Sciences (2019)