Is ductal carcinoma in situ really breast cancer? can lifestyle, nutrition or supplements affect its course or occurrence?

DCIS: We dont consider dcis as an invasive breast cancer- rather we consider this as a precancer condition./ a non-invasive breast cancer- localized only on the duct or stage t0. You are right -there are a lot of factors that perhaps can contribute to development of cancer- like lifestyle/habits ( smoking/alcohol, lack of exercise) , nutrition/diet ( certain food/amino acids that is carcinogenic) etc.

YES: Ductal carcinoma in situ means that the cancer is there in the duct but it has not invaded into the surrounding tissues of the breast. This means it was found early. Can living a healthy lifestyle help? Yes it can but it cannot fix it. Get this removed before it becomes a more progressive invasive cancer.

Yes and No: Ductal carcinoma in situ is basically a cancer which has not yet become invasive. The only way to treat this is to get rid of it before it turns into an invasive cancer. Because it is detected early, surgery provides a high rate of cure.

Precancerous: Carcinoma in-situ is just one step short of cancer. There is no scientific evidence for or against life style changes altering the course of in-situ lesions. You may engage in life style changes that promote a healthy life style but should not omit conventional treatment. Steve jobs low grade cancer became fatal while he tried to treat it by alternative methods. See your doctor.

Another point: In addition to all of the other answers, there are some new ideas about this problem. The danger with this disease is that you are more likely to get invasive cancers. There is a recently developed test that can give a much better idea of who will get invasive cancer, and who benefits from radiation.