> humans have been eating for literally tens of thousands of years is causing depression and widespread health concerns

They literally haven't eaten meat, dairy and eggs 3-5x per day. This is a new phenomenon in the last half-century. And to go back even further, our biochemistry is millions of years old, and is frugivorous in nature. We have no physical adaptations to eating meat. We only started doing it recently as a means of survival when humans left their tropical niche.

> a controlled, healthy, low-calorie diet

Plant-based foods are naturally low in fat, which is calorie dense, so such a diet is in fact ideal for weight loss.