Nutrition is confusing. It seems like every fad diet is supported by science and then refuted by the same science two weeks later. At the risk of posting yet another nutrition article I recommend this article from the Atlantic. It goes into the research behind nutrition science and produces a not too surprising conclusion.
If you don't have time to read it allow me to summarize, your mother's advice was correct, eat your veggies and lay off the junk food.
Traditional Chinese martial arts for mind/body development