Yes, scientific research is hard because bodies are complex, environmental impacts are significant, and no methodology no matter how rigourous can account for the compounded determinants of results.



Much of the discussion on diet neglects the important things, like calories, macronutrient (protein, fat and carbs) distribution, micronutrient profile, meal timing and how to distribute the nutrients across those meals and snacks; and finally, supplementation.



To me personally, people can determine for themselves if they include animal products or exclude them. We will likely find that any real food diet that is well balanced in all respects will enhance the health and longevity of human beings. There isn't any scientific evidence that vegans have better, longer lives due to that diet. Deb's example of the Inuit neglects the social determinants, the processed food, addictions, cultural decimation, mental health and so on. We have similar concerns with Australian Aboriginals, who were the epitome of health prior to colonisation, and we generally consider colonisation to be the leading cause of poor health outcomes.


To cut a long story short, if we really want to address the horrendous harms stemming from obesity and declining fitness in younger and younger people over time, we have to promote a overall cultural transformation so the 'way of life', normal day to day living, manifests trends toward decreasing obesity and increasing fitness levels.