I just finished my initial 6 BCG treatments, which took about 9 weeks. My treatments fell over a period which included Thanksgiving, my 50th birthday, Christmas and New Year, so there was a lot of food -- not necessarily the healthiest kind -- readily available.
FWIW, what worked best for me was to follow the Michael Pollan adage "Eat food, not too much, mostly plants," which is how I eat most of the time, but I really cleaned up my act during BCG. I found that if I ate a big, heavy meal, then I woke up feeling really sluggish the next day. I also cut out all alcohol, because even just having a small glass of wine made me feel miserable the next morning.
As far as supplements, I take a multivitamin every day just as a backup, even though my diet is generally full of nutrient-rich foods. Since starting BCG, I started taking it before bedtime. Dr. Lamm recommends a multivitamin called "Oncovite," and that is supposed to be taken at bedtime. I know that water soluble vitamins pass through your body, so I guess that the idea is your vitamin-rich urine sits in your bladder as you sleep where it can provide some benefit. Maybe it's a bunch of hooey, but since I take a multivitamin anyway, it's just as easy to take it at bedtime.
I did not give up coffee, however. I do have my limits!