I don't think as a society we know nearly as much about nutrition as we like to think we do. For one thing, I don't believe that there is a one-size-fits-all diet for everyone, and we should really stop pushing that sort of agenda onto people. When you think about it, it's astounding that humans seem to have this need to be told what we should or shouldn't eat. As far as I know, we're the only animal on the planet that has this need.
For the better part of 30 years, I followed the conventional wisdom that one should limit their fat intake, and enjoy only lean meats, whole grains, and plenty of fruits and vegetables. I also exercised regularly. For many people this recommendation works great. It didn't for me, though.
I believed the conventional wisdom. But despite this, I eventually turned into an overweight, type 2 diabetic with GERD.
For my wife and I, we've found that eating pasta, sugar (even fruit), rice, and starchy foods, and yes, "healthy whole grains," we gain weight and feel awful. Other people don't. Good for them. What bugs me is when some "authority" comes along and says we absolutely need to eat those things in order to be healthy. I can tell you without any doubt, that I look and feel much, much better without those foods in my diet.
Fat is another thing. Again, we have so-called "experts" who say that saturated fat isn't good for you, and that we should limit red meat intake. I call BS on that. In the last year and a half, I have changed my diet to include plenty of saturated fat (as well as unsaturated and monounsaturated fat), and by eating these foods am easily able to maintain my weight and blood sugar. Despite the experts telling me these foods will kill me, my blood profile looks better than it ever has, or certainly better than it has in the last 15 years (I only have records dating back that far). While I'm at it, I'll let you in on another little secret: I now consume about 50% more calories than I ever did while I was eating "healthy." Since adopting lifestyle changes, my diabetes has completely regressed and 90 pounds has melted away (my wife has lost 45 pounds and gotten her cholesterol under control). So I'm a convert.
At my last checkup a few months ago, my own doctor said (paraphrasing), "I don't know exactly what you have been doing differently since last year, but keep it up. Everything looks great." He knows a little about my eating habits because I told him. But he doesn't know everything. When I was diagnosed with diabetes, I had to see a nutrition counselor. I followed all of her recommendations to the letter for over a month and it didn't help one bit. Only by stepping out of the realm of what I call "conventional nutritional wisdom," did my health begin to return.
Despite this, I wouldn't recommend my diet to everyone. I think there are so many other variables involved, including age and genetics. While I do think there is a group of us who would probably benefit, I don't want to be "that guy." However, if someone asks me what I've done to get my health back, I have no problem sharing with them.