Doctors have known that low levels of vitamin D are linked to certain kinds of cancers as well as to diabetes and asthma, but new research also shows that the vitamin can kill human cancer cells. The results fall short of an immediate cancer cure, but they are encouraging, medical professionals say. JoEllen Welsh, a researcher with the State University of New York at Albany, has studied the effects of vitamin D for 25 years. Part of her research involves taking human breast cancer cells and treating them with a potent form of vitamin D. Within a few days, half the cancer cells shriveled up and died. Welsh said the vitamin has the same effect as a drug used for breast cancer treatment. Vitamin D enters the cells and triggers the cell death process," she [said]. "It's similar to what we see when we treat cells with Tamoxifen," a drug used to treat breast cancer. The vitamin's effects were even more dramatic on breast cancer cells injected into mice. After several weeks of treatment, the cancer tumors in the mice shrank by an average of more than 50 percent. Some tumors disappeared. Similar results have been achieved on colon and prostate cancer tumors in mice.