It is like the myth that you should drink 8 glasses of water a day. In this case the advice (from the WHO I think) was people needed 2 litres of water a day, which can be sourced from your normal food and beverage intake. This was altered to drinking 8 glasses of water a day and promoted by the bottled water industry. I get into discussions with people about it over and over but because it is all pervading I rarely make headway.
Another one is wheat grass . . though I find a more receptive audience for that one usually, except from the committed hippy types.