These people aren't doctors and don't even play them on TV, but they still authoritatively dispense advice about how our bodies were made to handle all-meat diets or all-grain diets and outline information they think you should or should not do.
What happens is they read a blog or watched Dr. Oz or a P90X commercial, and are now regurgitating their confused recollection of it as if it were fact. They don't remember half the details, they just remember how excited they were to find out something boring they previously took for granted (eat less and exercise more and you lose weight) was really untrue, and something unexpected and exciting was true in its place!
Strangely enough, it seems to afflict moms more than any other segment of the population, probably because women are more interested in health news, and moms are more interested than any other population group in giving advice and meddling in people's lives (their kids' lives, specifically).
If you can't tune these people out, it can really take the wind out of your sails when you're starting out on a simple diet and exercise regimen and don't see much progress at first (which is normal), and start to doubt if you're on the right track. With people spouting specialized theories at you about how weight loss "really" works, and why what you're doing is never going to work, it's pretty easy to just stop going to the gym the first day you feel tired.