Placebo effect doesn’t necessarily mean a biological change hasn’t occurred. This isn’t the first instance of an assumed correlation between vitamin B deficiencies and depression, also noted it’s been found that Vitamin B12 can act as a modulator for the gut microbial ecology, B7 (biotin), B9 (folic acid) and B12 (cobalamin) being the most pronounced of all produced in a healthy gut, which is also believed to contribute significantly to our neural chemistry. In essence a higher presence of vitamins B leads to a healthier gut, less inflammation and a healthier brain (goodbye depression) of course other factors contribute to depression as well.