If there's one thing you should always remember about science, it's that fact and truth are established after multiple studies converge on an answer. Even after that, further research might turn over what you thought was true of the other studies, because you were looking at it through too narrow of a lens. Single papers offer evidence, but rarely do they offer firm truths.
This reality is compounded by another factor in certain fields, like nutrition and medical science. There are business interests trying to sell you a product, and proof that a product is healthy is obviously a great selling point. On the other side, if you're an activist, proof that a product is bad can be just as useful. All of this can add bias.
What the Health, a new documentary on Netflix produced by Joaquin Phoenix, takes advantage of these realities. The documentary cherry-picks studies to "prove" that any animal-based product will cause cancer or diabetes.
Of course, we don't know for sure whether giving up animal products is actually good for your health in the long run. Some studies show that vegetarians and vegans live longer, but others don't, as Melody Ding outlines in The Conversation. Those that do suffer from a general health consciousness bias. Those folks that avoid meat and animal products could simply be living longer because they care more about their lifestyles in lots of different ways. They might also be younger and wealthier.
What does that mean for you? Well, avoid meat if animal welfare or environmentalism is important to you. Otherwise, if you're really interested in living longer (which is what everyone ultimately wants, right?) cross your fingers and hope your parents live for a long time. We use this quote a lot, but S. Jay Olshansky once told Gizmodo, "In the world of ageing sciences, if you want to live a long life, choose long-lived parents."
After that, don't smoke, and focus on a diet that minimizes processed foods and includes more vegetables. Maybe eat less, get out more, and focus on a real healthy lifestyle change like biking to work every day or giving up all desserts. Past that, you might have to rely on research or expertise backed by less or less reliable data that will point you in the direction the expert thinks best reflects the existing evidence... and maybe their ethics.