I think it comes down to what you mean when you say "man was meant" to have a particular diet. It seems evident from anthropology that man developed with an omnivore diet. He may not have developed as Homo sapiens sapiens with any other kind of diet. So here we are, as Homo sapiens sapiens, the thinking man, who can choose how he will live. We have all these choices, they can be confusing. But unless you're looking at it from a religious perspective (what God "meant" us to do, or be), using the word "meant" is inaccurate. Natural selection doesn't "mean" us to do anything, as far as the science can show us. Some behaviors such as omnivorousness enabled us to continue to live as a species through tough times (like the Ice Age). But we don't live like that now. We can, as thinking beings, consider how we want to live, even consider how our choices could be beneficial to us as a species. For instance, it's clear that much of how we eat meat now as a culture or group of societies isn't beneficial to the Earth's life systems. Overstocking grazing herds of animals or confinement factory raising of animals has a demonstrably damaging effect on ecosystems and even on a wider scale such as the global climate. So we can see how we live now, how we eat, and ask, is this beneficial in the short term and in the long term? It seems clear that how we live and eat now as a society is not beneficial to us in either the short term or the long term. If we could all live like hunter-gatherers we would certainly be doing less harm, even if we were eating meat, but the fact is we don't live that way nor can we live that way anymore. It's up to us to find a way of life that is beneficial where we find ourselves today.