I know from personal experience that half that dietary advice is bullshit. When I was first diagnosed with hypertension in 2001, they told me to cut my salt intake to less than 2000mg per day. My wife and I went overboard, and not only did we eliminate added salt and cut all the packaged foods that are high in sodium, we switched everything we could to food that were naturally low in sodium. So I went from putting salt on just about everything to having almost no salt at all in my diet. After a year it had made absolutely no change in my blood pressure, and eventually we have stopped worrying about salt. Same thing with fat - tested with borderline high cholesterol, and doctors and nutritionists told us "get rid of butter." Years later I see lots of studies saying margarine is worse than butter after all. Over time I've gone back and forth with low-fat diets and my normal fat intake and never see a swing in the cholesterol levels... because some people are genetically pre-disposed to produce high amounts of cholesterol in their liver (like me), and what they eat doesn't really affect their cholesterol levels as long as you don't choke down a tub of lard and a few sticks of butter every day.
And don't get me started on gluten-free crap. I knew when that whole thing started it was some kind of fad. Now the doctor who published the original study that started the gluten-free craze has updated his findings, and has reversed his position by saying that gluten-free diets have no special benefits for people who don't suffer from celiac disease. Which just confirms the suspicion I had all along that someone diagnosed with celiac disease got some advice to cut out wheat and wheat products, which means that they stopped eating bread and a lot of other highly processed foods. They probably lost a bunch of weight, and got a lot healthier as a result, and all of their friends would ask, "what did you do?" To which, that person would reply, "I'm on this amazing gluten-free diet!" And then all these people who didn't have celiac disease started demanding gluten-free everything. They may have actually felt better, but, again, it was because they had stopped eating crap and paid more attention to their diet, not because gluten-free is some magical cure-all.
I strongly believe that the effects of certain foods and substances within foods, and even medications to an extent, vary drastically from person to person. Moderation in all things is the key.
