A hunch. It seems to me that evolution would have made it relatively easy for species to adapt to different foods, as continued survival was so important. If it were more difficult, then one would have expected the Neanderthals to spend little time on the glaciers and go migrating to warmer climes.
Interesting hunch, and I've wondered about how much of a factor regional adaptations are too. Yet adapting to a new food is not the same thing as losing an existing adaptation to a food. From what I've read, once species are well adapted to a food, they don't tend to lose that adaptation for a long time to come, especially if there are microbes that help them utilize the food, and particularly when the food does not contain high levels of toxins, such as with fruit, which you yourself have pointed out multiple times. This would help explain how the Inuit were able to gorge on berries in the summer without a problem from the carbs. It also may help that mother's milk tends to be fairly carby (I have seen reports of roughly 40% carbs on avg, though it varies depending on the mother's diet and other factors), thus keeping a carby element in every diet, along with fresh liver and other sources.
BTW, I read that the most northerly Neanderthals and others didn't live on the glaciers. Rather, they lived near them, in the grass-lush areas fed by the mineral-rich waters of melting glaciers. It's a minor point, but I know that you value precision in your language, so I thought I'd try to help.
I do know about the Fuegians and also the Ice Man Wim Hof. Through them and my own personal experience I know that some cold adaptation is possible, even in the very short term. JeuneKog and Iguana were also correct that the Fuegians also used other warming strategies besides diet, like fire, and also squatting, huddling, and animal skins in the coldest weather, and also tended to stay near the coasts, benefiting from the warming effects of the ocean. It remains to be proven that the partial cold adaptation they developed made tropical fruits a harmful food for them (or any other population).
I know it's only an n=1, but I've actually found that since I learned of a way to improve my carb tolerance somewhat and incorporate more carby and prebiotic-rich foods into my diet, including some tropical fruits (and my past comments show I was skeptical of the degree of tropical-fruit love of the Instinctos and Wai dieters--I still think they may tend to overdo it and I don't think that tropical fruit are a necessity for reasonably good health, but I feel less strongly about it now and I understand better some of the logic behind their claims) that my cold tolerance has actually improved, rather than worsened, and my body temperatures throughout the day tend higher (granted there is the confounding factor of my cold-training, but it is much less intensive than what Wim Hof does). It wasn't something I would have expected in the past, because when I first greatly increased my fat intake and greatly lowered my carbs, I felt warmer (but that effect gradually wore off and I slowly became colder again). There are few people on this planet more Europid than me. If I can find a way to tolerate some tropical fruits, then I suspect that most people can, though this is also just a hunch. Who knows what fate will befall me down the road.
I do not claim that Europids, as a whole have a higher body-temperature than Negroids. But perhaps a few Europids have some Neanderthal characteristics which help them to withstand heat more? it is simple logic. I was pointing out that if it takes such a long, difficult effort to adapt to African or non-African foods or whatever, then other physical changes caused by adaptation to the different climates would have also occurred by then as well.
Pardon me, but now I'm confused. Do you think "it's relatively easy for species to adapt to different foods" or that "it takes such a long, difficult effort to adapt to African or non-African foods or whatever"?
One reason I'm a bit skeptical of your hunch is that I have a higher-than-avg level of known Neanderthal genes, and below-avg glucose tolerance, yet I'm nonetheless able to eat some tropical fruits and have been eating more of them in recent months, and my overall health markers were actually improved in my last lab test. Maybe this only applies to me, but then again, maybe not. When I see other Europids, such as Brady, Danny Roddy, Yuri, Yuli, Miles, Lowenherz, Stas86, Klowcarb, and others struggling in the longer term with VLC/ZC/keto and reporting improvements after moderating their diets, plus accumulating scientific research and my own experience, it adds up to be enough to admit that my hunch was wrong in my early comments of this forum that humans, or at least Europids like me, might be best described as facultative carnivores. I think you argued at the time that omnivore was a more accurate descriptor (perhaps in part because you were focused with arguing against William and other VLCers)? If so, I think you were right and I was wrong, assuming no big contradictory revelations in future scientific research.