Well, since we're out to be objective here, let's talk about water.
Here is one of MANY references to the "8 glasses of water a day" myth:
This myth probably started when the Food and Nutrition Board of the National Research Council published a finding that recommended approximately 64 to 80 ozs. of water intake per average person. However, the next sentence of the report also stated that most of that comes from the food and drink we consume regularly.
Look, I'm not against water. I drink a lot of it myself. But the way people on the internet talk about it -- it's like water is some sort of magic potion that will guarantee weight loss. Not only that, but people also state things that categorically are untrue, such as the idea that other liquids don't count in your water intake (watermelon is 85% water, after all!), or that beverages like coffee are such extreme diuretics that they don't count either.
The whole thing does little more than remove people's focus from the real thing that affects their weight -- their energy balance. The same nonsense went around regarding the thermogenic effect of certain foods. Yes, eating a lot of celery won't hurt you, and neither will drinking 8 glasses of water a day, but come on -- this is NOT what will cause you to lose weight!
Or take this idea that we're all walking around under-hydrated. Well, if we're dehydrated, what about our ancestors 100 years ago. Talk about dehydrated! They didn't have anywhere near the access to clean water that we do today. And if our ancestors from a century ago were dehydrated, then our ancestors from 10,000 years ago must have been positively dry. And what about original mankind that evolved on the savannahs of Africa. If you believe we're all walking around dehydrated today, then it stands to reason that the entire human race for the sum total of its existence has been dehydrated. And if you believe that, then "dehydration" is in fact the normal human condition, and our genes have evolved to take care of that little problem.