One of the most important supplements for a breast cancer patient is the hormone melatonin. Melatonin inhibits human breast cancer cell growth (Cos et al. 2000) and reduces tumor spread and invasiveness in vitro (Cos et al.1998). Indeed, it has been suggested that melatonin acts as a naturally occurring anti-estrogen on tumor cells, as it down-regulates hormones responsible for the growth of hormone-dependent mammary tumors (Torres-Farfan 2003).
A high percentage of women with estrogen-receptor-positive breast cancer have low plasma melatonin levels (Brzezinski et al. 1997). There have been some studies demonstrating changes in melatonin levels in breast cancer patients; specifically, women with breast cancer were found to have lower melatonin levels than women without breast cancer (Oosthuizen et al. 1989). Normally, women undergo a seasonal variation in the production of certain hormones, such as melatonin. However, it was found that women with breast cancer did not have a seasonal variation in melatonin levels, as did the healthy women (Holdaway et al. 1997).
Low levels of melatonin have been associated with breast cancer occurrence and development. Women who work predominantly at night and are exposed to light, which inhibits melatonin production and alters the circadian rhythm, have an increased risk of breast cancer development (Schernhammer et al. 2003). In contrast, higher melatonin levels have been found in blind and visually impaired people, along with correspondingly lower incidences of cancer compared to those with normal vision, thus suggesting a role for melatonin in the reduction of cancer incidence (Feychting et al. 1998).
In tumorigenesis studies, melatonin reduced the incidence and growth rate of breast tumors and slowed breast cancer development (Subramanian et al. 1991). Furthermore, prolonged oral melatonin administration significantly reduced the development of existing mammary tumors in animals (Rao et al. 2000).