Colorectal Cancer Patients on Chemotherapy Lack Enough Vitamin D:

Patients receiving chemotherapy for colorectal cancer are likely to have severe vitamin D deficiency and may need high-dose vitamin D supplements to bring their levels up to normal, according to a study published online October 2 in the International Journal of Colorectal Disease. (read the whole article)

by the way, I love this quote from the above study!
Dr. Fakih believes chemotherapy patients should have their vitamin D levels regularly tested—a practice that is not common in the United States. “The reason we test them is that there really is not a whole lot to lose here,” he says. “Replacing vitamin D in patients with vitamin D deficiency may also have positive effects on bone health, as well as on the cardiovascular and immune systems.”