Calcium, iron, zinc, et al. To avoid deficiencies on a plant-based diet, you have to carefully manage your diet around them.
Anecdotally, my sister in law has been vegetarian for over a decade and just recently been diagnosed with anemia.
Anecdotally, my sister in law has been vegetarian for over a decade and just recently been diagnosed with anemia.
Also your sister in law hasn't been well informed - it's a known fact that vegetarian diets have less iron in them, and women especially should take an iron supplement because of menstruation combined with the diet. But it's just one pill.