Funnily enough, same goes for buying anything else. Take cars. Why do BMW and Mercedes advertise endlessly, for example, while some marques like Mazda hardly ever do? The answer is that Mazdas are so reliable, they don’t need to advertise, personal recommendation sells them, while BMW and Mercedes have to sell on image.
As soon as a product is advertised, stop buying it, is a very good guide. If something was that good, it wouldn’t need advertising. With food and health, the message is totally true. Eat butter, not margarine. Eat natural fruit juice not sugar-loaded carton juice. And so on, and so on.