"Believing that your hypothesis must be correct before all the evidence is gathered encourages you to interpret the evidence selectively. This is human nature. It is also precisely what the scientific method tries to avoid. It does so by requiring that scientists not just test their hypotheses, but try to prove them false."
- Gary Taubes
Every day we can find new articles about the benefits of certain foods in preventing or treating chronic diseases. For instance, I've heard many times that coconut is good for proper thyroid function, or strawberries are good for the eyes, or papaya leaf extract prevents cancer, etc.
Then the question to ask is: What about the healthy populations without access to these foods? Why they do not develop chronic disease? Until a few decades ago, in Europe nobody had access to coconuts.
After years of research, I concluded that the answer to the question: What constitutes a healthy diet? is quite simple. We are not sick because we do not consume enough coconut or strawberries, or vitamins or fiber. We develop chronic diseases once sugar and refined carbohydrates become part of our daily diet.
"[...] 'Diseases of civilization' were rare to nonexistent among isolated populations that lived traditional lifestyles and ate traditional diets, [...] these diseases appeared in these populations only after they were exposed to Western foods - in particular, sugar, flour, white rice and maybe beer."
- Gary Taubes
" Maybe if these carbohydrates were added to any diet, no matter how replete with the essential protein, vitamins, minerals, and fatty acids, it would lead to chronic diseases of civilization. This would explain why the same diseases appeared after Westernization in cultures that lived exclusively on animal products - the Inuit, the Masai and Samburu nomads, Australian Aborigines, or Native Americans of the Great Plains - as well as in primary agrarian cultures like the Hunza in the Himalayas or the Kikuyu in Kenya."
- Gary Taubes
So no matter what the original diet was - based on plants or animal food - diseases of civilization were relatively absent from these populations until they adopted the Western diet.
Peter Cleave (author of The Saccharine Disease) was the first to suggest that these Western diseases (heart disease, obesity, diabetes, peptic ulcers, and appendicitis) are caused by a single, primary disorder, which he called "refined-carbohydrate disease".
This theory makes sense from an evolutionary and biochemical point of view as well.
About the potential dangers of sugar I highly recommend this article: Big Sugar's Sweet Little Lies by Gary Taubes and Kristin Kearns Couzens.