I do not dispute that fiber has health promoting benefits. I do feel that it’s a bit of a straw man from the original point, though. The discussion seemed to be that the less processed food, in the blue zones, was responsible for increases in longevity, independent from CI. From that perspective, I believe the animal studies, where diet composition is the same (degree of processing isn’t different) and CI is the IV, are useful. I believe they provide evidence that energy intake affects longevity independent processed food consumption.

Diet composition compared to energy intake has also been investigated in animals. Read the introduction to the linked study and the authors discuss previous findings demonstrating CR working independently from diet composition.

“We recently completed a longitudinal study in male C57BL/6J mice (B6) initiated at 4 months of age in which the effects of the two diet compositions under ad libitum (AL) and CR conditions were measured (Mitchell et al., 2019). Independent of diet, male B6 mice on 30% CR exhibited significant improvements in health, survival, and a delayed onset of cancer with respect to AL-fed mice.”

https://www.sciencedirect.com/science/article/pii/S1550413120302357

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As the authors noted, this can be seen in multiple species. In animal trials, caloric intake is often achieved by manipulating quantity of food, not type. This could suggest that energy would independently impact longevity, and that it's not simply a matter of eliminating highly processed foods. While animal studies are not the highest degree of evidence, they are often useful for determining mechanisms of action when confounding variables like this can be accounted for. This is not to say that diet quality does not also have an independent or synergistic effect on longevity, but it would point toward energy intake being a piece of the puzzle, at least.

Thank you for the in-depth response