Honestly the links between diet and health are always thin at best, but I don't know of any that strongly link a well-balanced vegan diet with reduced health. Usually, the opposite.
All the evidence seems to indicate that the less meat and dairy one eats, the healthier they are. I also used to believe vegans were unhealty until I did some research. As a personal anecdote, I've been vegan for about 4 years now and my health is better than ever.
As a personal anecdote, vegetarian didn't work for me! Persistent anemia, hair loss, fatigue; adding back some meat just fixed it.
Various diets are worth trying -- you never know until you try, and you learn a lot by trying. So for readers out there, try being vegan for a month: you'll learn a lot about what you eat and how to cook and you can observe how you feel.
Also worth noting: many traditional cultures embedded food restrictions into calendars and/or religious rules. Being vegan for a while before Easter is more or less a thing in some Christian groups; Ramadan involves fasting as a practice. These cultural practices, combined with the seasons in nature, make me wonder if we were built to be cyclically vegan/omnivore/berry-eater. This may be more sustainable for many.
Some evidence seems to suggest that it could negatively affect brain health. (See deficiencies in B12, Iron and Zinc.) I've even read anecdotes of people that have gotten diabetes going all wholefoods vegetarian.
My belief is that our biology is complex and specific to each individual to the point where we can't all share the same diet and expect the same effects on our bodies.
> I've even read anecdotes of people that have gotten diabetes going all wholefoods vegetarian.
There can be a problem with reducing fat and protein significantly while using carbs for the majority of caloric intake. This can lead to diabetes. But you can do this on an omnivore diet as well.
https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emis...
It's not the largest contributor, for sure. But it's sizable.
https://www.nature.com/articles/s41598-017-06466-8
Reducing meat intake _does_ sizably reduce overall emissions.
Honestly the links between diet and health are always thin at best, but I don't know of any that strongly link a well-balanced vegan diet with reduced health. Usually, the opposite.
https://academic.oup.com/ajcn/article/89/5/1627S/4596952
> "In general, vegetarians typically enjoy a lower risk of cardiovascular disease (CVD), obesity, type 2 diabetes, and some cancers [...]"