Here are a couple of studies on smoking, a couple on B vitamins, a couple on vitamin D, and an intriguing study on iron.  The smoking papers underline the varieties of damage this habit produces.  So the Pasco et al study shows that, for women, being a smoker is associated with double the risk of developing subsequent major depression.  The Strandberg research challenges any notion of "Eat, drink and be merry for tomorrow we die".  This study of 1658 men reports that "During the 26-year follow-up of this socioeconomically homogeneous male cohort, HRQoL (quality of life) deteriorated with an increase in daily cigarettes smoked in a dose-dependent manner.