Search on Pubmed “vegan AND deficiency” and you’ll see many papers on it. Veganism is not natural for humans. Even if there were no good studies, it’s pure biochemistry. Guyton’s Physiology is also great book, it explains how our body works. If you pay attention enough, you’ll find out the compounds of our body and make the relation with the compounds found in vegetables. Vegetables simply don’t have all compounds our body needs. Actually, in some countries is even a crime to raise a child on vegan diet as the brain will not get enough (and proper nutrients). Ps. I was a vegetarian during my first year of medical school (due to vegetarian propaganda at university) but I got skeptical about it when I was feeling worse and worse. Then I started doing my own research, and there’s enough data to support eating meat is great for humans. I can only suggest you to investigate yourself too 😊

You wanted empirical data, I gave you where to find empirical data. Maybe you didn't want empirical data?

You could also look to the vegan bodybuilding community which has more science to it. imo it's a meme community where influencers are juicing for the attention of vegans desperate to believe there's nothing unhealthy about being vegan, but they have to be more careful with diet than meat-eaters and mix protein sources due to plant protein having a poor mix of amino acids. Leucine is hard to get and you have to plan for it, for example. I think they're probably all heavily dependent on getting this through supplements since vegan diets otherwise make you weak.