Why do babies need vitamin D drops?
Because vitamin D is the one nutrient humans are not designed to get from food, and infants in the UK cannot make enough of it from sunlight for much of the year.
Why it is unusual. Most vitamins come from diet. Vitamin D is primarily made in the skin, when UVB radiation converts a cholesterol derivative into a precursor that the liver and kidneys then activate. Food sources are limited — oily fish, egg yolk, fortified products — and it is difficult to obtain sufficient amounts from diet alone.
Why UK infants are at particular risk:
Latitude. From roughly October to March, sunlight in the UK contains too little UVB at the required wavelength for skin synthesis to occur at all, regardless of how bright the day is. This is a fact of the sun's angle, not of cloud cover.
Babies should be kept out of direct sunlight anyway, particularly under six months, which removes the main source.
Breast milk contains relatively little vitamin D, and its level depends on the mother's own status — which is frequently low for the same reasons.
Rapid skeletal growth means demand is high precisely when supply is lowest.
Darker skin produces vitamin D more slowly for a given exposure, and covering for cultural or religious reasons reduces it further, so risk is higher in some groups.
What the deficiency causes. Vitamin D is required for calcium absorption. Without it, bone mineralises poorly — rickets in children, causing soft, deformed bones, delayed growth and sometimes seizures from low calcium. Rickets was largely eliminated and has re-emerged in the UK, which is why the guidance exists.
The current UK advice, in outline: a daily supplement for breastfed babies from birth; formula-fed babies generally do not need one while taking a sufficient quantity of formula, because formula is fortified; and a supplement for children aged one to four, with adults advised to consider one in autumn and winter.
Healthy Start vitamins are available free to eligible families. Check current NHS guidance and dosage, and speak to your health visitor or GP about your own child.