I recently suggested to a friend that yoghurt might be a healthier food than milk. After looking at nutrition labels in the supermarket, my friend called me to say he was concerned that yoghurt had a fairly high sugar content. I then looked at the labels on several brands of yoghurt/yogurt, including whole-milk, low-fat, and non-fat plain yoghurts, and found that almost all of them had at least 15 g of sugar per cup (the exception had 10 g of sugar, but listed modified food starch as an ingredient, and had a total carbohydrate content of 17 g per cup).
As I suspected, the main explanation is that store-bought yoghurt is only partly fermented; most of the lactose (milk sugar) in the milk remains in the yoghurt. An Austalian dairy-industry-sponsored health website confirms this, and adds that the lactose will decrease with time while yoghurt is stored in a refrigerator. An alternaive medicine website recommends making yoghurt at home and fermenting it for 24 hours to get rid of all the lactose.
The possible health benefits of consuming yoghurt instead of milk may stem from the presence of live bacteria, the reduction or elimination of lactose, the reduction of animal proteins, and the presence of bacterial proteins and peptides. Unfortunately, the yoghurt in the supermarket seems to be only partly yoghurt, and the rest milk thickened or congealed with additives, such as pectin and gelatin.