While vitamin C is known to have a role in cancer prevention, researchers have found that vitamin D plays a key role in regulating cell proliferation, which also has a role in cancer prevention. A 40-year study in the United Kingdom in May 2018 found that vitamin D levels were associated with the incidence of cancer. The group with the lowest levels of vitamin D in the body had the highest rate of cancer, while the risk of cancer decreased relatively as vitamin D levels increased. Daily doses of vitamin D cut the probability of developing breast, colon and ovarian cancers in half. So vitamin D can prevent cancer, is the more you take the better? A study by British researchers RobertScragg,Kay-TeeKhaw and others found that too much vitamin D supplementation did not prevent cancer. The study’s insights were published in the July 2018 issue of the New England Journal. They recruited 5110 participants, aged 50 to 84 years, in New Zealand from 2011.The 5110 participants were randomized into two groups to receive either vitamin D3 (n=2558) or placebo (n=2552). Two of the participants dropped out for other reasons and 5108 participants (n===5108) were included in the analysis. The mean age of the 5108 subject population was 65.9 years, 58.1% were male, and the European ethnic population was 4253 or 83.3%. The rest were West Asian or South Asian populations. The initial dose in the oral vitamin D group was a total of 200,000 IU per month, followed by a total of 100,000 IU per month. 438 participants in the vitamin D group were randomly sampled during the observation period, and the mean follow-up concentration of vitamin D was consistently greater than 20 ng/mL. vitamin D body levels were higher than in the placebo group. The number of participants with a diagnosis of primary malignancy (excluding non-melanoma skin cancer) was counted at the end of the observation period until 2015. Cancer occurred in 165 (6.5%) of the 2558 participants in the vitamin D group and 163 (6.4%) of the 2550 participants in the placebo group, and the rate of cancer was higher in the vitamin D group than in the placebo group with a total oral dose of 100,000 IU per month. This shows that vitamin D supplementation can prevent the occurrence of tumors, but too much vitamin D supplementation cannot prevent cancer, the exact mechanism of which is not clear, as the Chinese saying goes, “Too much is not enough”. In this study, it is suggested that high dose of vitamin D supplementation is inappropriate, and the 2008 American Osteoporosis Prevention and Treatment Guidelines recommend 800-1000 IU vitamin D supplementation per day for people over 50 years of age.