It seems that Vitamin C and folic acid suffer the most from boiling rather than steaming. Boiling veg might lose you a quarter of vitamin C and about a third of folic acid, though steaming does lose some of each too, but not as much. Folic acid is one of those that it can be tricky to get enough of if you don't eat your greens, but I wouldn't be worrying too much about Vitamin C. Thing is, whether you steam or boil, you're still eating veg, which can't be bad.