Fine tuned maybe, but with the goal of "stay alive just about long enough to be useful", we're pretty awfully constructed. For example, we need ultraviolet light to not get rickets but it can also give us cancer. Our fine-tuned digestive tract contains an organ which occasionally bursts. My wisdom teeth have decided that they'd rather grow forwards than up.
Evolution gets you to functional, it doesn't get you to "good".
To suggest that what we've been doing in the past is optimal seems ridiculous to me. I'm not saying I think this gruel is a good idea, but the argument "it's not natural" holds no weight.
No, but our bodies have been fine-tuned through evolution over the last million years, up until probably the last 20,000 or so. Your digestive tract is different from a cow, snake, or wolf for a reason.