Complex sentence: Another set of concerns is that some functional beverages contain ingredients that have not been sufficiently studied for health benefits, safety, and dosage or have higher levels of a certain ingredient, like caffeine, a large amount of which is associated with heart disease and cancer.
Split into simpler sentences: Another set of concerns is that some functional beverages contain ingredients that have not been sufficiently studied for health benefits, safety, and dosage. At the same time, many functional beverages have higher levels of a certain ingredient, like caffeine-which, when consumed in large amounts, is associated with heart disease and cancer.