Clinically the docs say B12 and iron deficits, along with some trace elements. Omega 3/6 balance issues also common. Potassium/iodine issues, thyroid issues, calcium metabolism, immune system oddities. That's secondhand: I'm not a clinician, and can only observe a general affect of poor health - easily tired, pallor, an appearance of frailty even when (as is common) not pudgy or notably skinny. It's all anecdotal, in my case. But the anecdotes reinforce each other. A good vegetarian diet takes some experience, some doing. And these are teenagers.