Johhny says he doesn't listen to nutritionists because they change their minds on a daily basis, as well as their advice. He says that everyone's reaction to food is different anyway, so he knows what suits him best.
Yesterday, eggs were bad for you. Today, they are healthy.
In the morning, carbohydrates were the basis of a healthy diet. In the afternoon, you should run like hell from them.
It's easy to start believing nutrition to be an unreliable science changing its postulates on a daily basis. That it's broken and that Johnny is right.
Except that postulates don't change. However, something does.
What people believe in
What changes is the laymen and/or quasi-professional interpretation of scientific data. This interpretation is generally based on lack of knowledge, on gullibility, fashion, clickbait, and pretty much everything other than the real state of the matter.
Knowledge is expanded and built upon itself. We almost never acquire scientific data that significantly changes the existing consensus. We have never found any data that made us understand that everything we've ever known about a healthy diet is wrong. They say sensationalism sells books...
Nutrition as a science
It's not fair to glorify nutrition science either, at least not in its current state.
Scientific method is the best known way to pursue the truth. Imitating the event. Isolating external factors. Observing. Interpreting. Replicating. Science, the way it is being done, does have quite a room for improvement. From reducing the motivation of scientists for producing significant results at any cost, to enhancing the motivation of those included in peer review process.
Truth be told, nutrition is a complex branch of science (diet shouldn't be complex). Effects of diet are way more subtle than the laypeople think and the scientists and nutritionist want. It's difficult to notice and quantify the effect which can be noticed only after a period of 20 years, and which is obscured by many known and unknown external variables.
One also needs to admit not knowing. We know enough to give a quality dietary advice, but we don't know everything. And that's ok. One shouldn't invent answers but rather continue looking for them with patience.
Is nutrition science is broken?
I work in science. I like science. I believe in the quality of knowledge produced by good science. Nutrition science isn't broken, but we must be aware of its flaws, and not take every result for granted.
How to gain more quality knowledge? More science. More quality science. But science, not guesswork.