Breast milk would be perfect for babies if we lived like our distant ancestors in central Africa where the sun shines brightly almost everyday and we wore only light clothing or perhaps nothing at all. As it is breast milk of women in Britain and most of Europe contains very little vitamin D and not enough to keep babies in optimum health.
The sun is not strong enough in northern Europe to make any vitamin D for half the year and our cloudy climate means that even in summer we often get little vitamin D from one week to the next. So some 80% of the population have sub-optimal vitamin D levels of the sunshine vitamin in winter and even in summer some 50% are sub-optimal. Read more Vitamin D deficiency in babies in UK