It’s not difficult to provide a boat-load of physiological evidence that we’ve evolved over hundreds of thousands of years to be omnivorous. We don’t synthesize many of the micro-nutrients we need, so we have to obtain them in our diet. A balanced diet (i) involves eating no McSlop, processed foods, candies, ice-cream, etc. etc. etc. and (ii) getting a wide range of inputs. That’s why vegan diets are catastrophically bad over the long term for adults, and far worse for growing children. It’s also why any fad diet like an all-meat approach is also, over the longer term, not only silly but harmful.
That said, this is evolution in action. If people want to self harm by smoking, sawing off a limb, or adhering to a foolish diet, provided they don’t hurt anyone else and don’t expect the rest of us to pay higher premiums to cover the cost of their medical bills, they should be allowed to do whatever foolishness they want. Modern society has insulated us from the consequences of our follies (all those safety features in cars, all that modern medicine) that it’s about time we found ways to rebalance the scales. Because, let’s face it, planking and eating TidePods aren’t going to do the job on their own.