In a recently published systematic review by Bjelakovic et al. on 78 randomized clinical trials on antioxidants supplementation including selenium, β-carotene, vitamin C, vitamin A, and vitamin E, not only have no favorable effects been observed, but additionally, mortality rates have risen (Bjelakovic and Gluud, 2007; Bjelakovic et al., 2012). Surprisingly, it has been shown that antioxidant supplementation may increase the risk of skin malignancy in women (Hercberg et al., 2007)... It has been shown that increased oxidative stress could enhance prevalence of malignancies by direct cellular damage, Seifirad et al. (2012); Lu et al. (2013); Seifirad and Masoudkabir (2013) however, as mentioned above oxidative stress when applied as immune system arms could protect organisms from invading pathogens and malignant cells (Weel et al., 1996)... It has been shown that high intake of tea or coffee that are rich in flavonoids in pregnant women might increase the risk of central nervous system tumors and childhood leukemia (Strick et al., 2000; Paolini et al., 2003; Plichart et al., 2008).
Antioxidants speed cancer in mice http://www.nature.com/news/antioxidants-speed-cancer-in-mice-1.14606
A study in mice has found that two commonly used antioxidants — vitamin E and a compound called N-acetylcysteine (NAC) — speed the growth of lung cancer rather than curb it... If anything, they thought that NAC might slow the tumours slightly, says Lindahl. Instead, the control tumours grew three times faster than expected... The team decided to dig deeper, and expanded its study to include another common antioxidant, vitamin E. The researchers fed either NAC or vitamin E to the mice, using doses of 5 or 50 times higher than the daily recommended amount for mice. Human dietary supplements often have 4 to 20 times the recommended daily intake of vitamin E for humans, says Lindahl. The results for the two antioxidants were similar: tumours grew about three times faster than those in animals that did not receive the treatment. Treated mice also died from their cancers about twice as quickly as untreated mice.
Antioxidant Induces DNA Damage, Cell Death and Mutagenicity in Human Lung and Skin Normal Cells https://www.nature.com/articles/srep03169
We show that epigallocatechin gallate (EGCG) as an exemplary antioxidant induced significant death and DNA damage in human lung and skin normal cells through a reductive mechanism... We found that EGCG was much more toxic against normal cells than H2O2... EGCG induced DNA double-strand breaks and apoptosis in normal cells and enhanced the mutation frequency... High intake of flavonoids (tea or coffee) during pregnancy is suspected to increase the risk of infant leukemia and childhood malignant central nervous system tumours. Moreover, researchers also observed that plant extracts such as phytoestrogens (Genistein, coumestrol, quercetin, zearalenone and resveratrol) induced genotoxicity and mutagenesis in mammalian cells, suggesting the possible involvement of mutagenicity in initiating phytoestrogen-induced carcinogenesis
Flavonoids modulate comet assay responses to food mutagens in human lymphocytes and sperm.
The flavonoids, silymarin, myricetin, quercitin, kaempferol, rutin and kaempferol-3-rutinoside have been examined in combination with the food mutagens... in the Comet assay... These compounds alone have been shown to produce positive responses in the Comet assay, as have the food mutagens.