I don't understand the argument against fructose. It seems to mainly "the liver has to work to digest it", but isn't that true of a lot of other nutrients as well? I mean, that's the main function of the liver, so the fact that fructose engages the liver should not by default imply that it's bad.
We are indeed well adapted to consuming fruit. Even though modern fruit is bred to be much more sugary than its natural predecessors (ever tried eating a wild apple from a tree on a hike? Insanely sour), there does not seem to be any kind of health problem associated with consumption of raw fruit. We have a lot of metabolically sick people around the world - over a billion probably - but few of them, if any, have gotten to that point by eating too much fruit.
Fructose is not fruit, though. Even very sweet fruit like mango has only 16 per cent of fructose per weight. Fructose itself is 100 per cent, it is refined, concentrated stuff, from which all fiber has been removed.
Fiber that naturally occurs in fruit and veg slows down processing of fruit and veg in the digestive system - it takes fairly long (hours) to extract all the fructose from an eaten piece of fruit. As a result, the liver isn't overworked - it receives manageable quantity of fructose over a long period.
Now compare a situation when you drink a litre of sweetened beverage. That is a lot of fructose - you just drank an equivalent of seven? eight? apples in half an hour or so. And all that sugar comes in liquid form, so it enters your blood very fast, producing a huge spike. The liver is overwhelmed. It can cope somehow, but do this ten thousand times and some negative outcomes are bound to follow.
There is a scientist (Robert H. Lustig, professor of endocrinology at UCSF) who, more than a decade ago, published a long video called "Sugar: The Bitter Truth". It is stuffed with various biochemical observations and longish, but I found the information useful.
When the messaging is centered around "carbs are bad" or "fructose is bad" it will lead people to believe that fruits are bad for you. I've seen several health channels on youtube recommend not eating more than the equivalent of one apple per day in terms of fruits.
While people can and do consume excessive amounts of fat/oil, we are somewhat protected by the fact that fat has a very strong satiating effect, which makes it a little harder to completely pig out. Fructose has no satiating effect at all.
Generally, yes, anything in excess has dire consequences, but in some things those consequences manifest sooner than in others. Sweetened beverages as consumed today are very much a definition of excess: big packaging, a lot of sugar within, and their mass consumption is societally normalized.
That processing hinges on a healthy insulin response to keep blood sugar levels in check. Which for a large part of the population is probably not the case.
Again with a suitable amount of fibres to go with that glucose intake, the load can be levelled somewhat though.
The key is fiber. When you eat fruit, you consume fructose with fiber. Fiber hinders absorption of fructose. Many processed foods have a lot of fructose and no fiber, because fiber doesn't help the shelf life.
EDIT: here's a more expanded explanation from the same endocrinologist in another video https://www.youtube.com/watch?v=-s5szfPYKY4&t=4924s
Robert Lustig explains the issue here: https://robertlustig.com/fructose2/ and this [ https://journals.physiology.org/doi/pdf/10.1152/physrev.0001... ] provides a readable account of the problem if fructose consumption rises above a certain level.
Except it affects insulin levels. Rather than the brain.
You could maybe make an argument that the fruits we consume now are a result of hundreds of years of selective breeding and contain much more of it than indigenous plants.
This vast majority of not-fructose plays a great role in the overall digestion process.
Fructose is refined product. These tribespeople do not eat refined fructose, they eat fruit which is much more than just fructose.