A study of infant bones from a deserted medieval village has given backing to the ancient nursing nostrum that "breast is best".

Evidence from Wharram Percy in the Yorkshire Wolds, abandoned when almost everyone was killed by the Black Death, shows that unweaned children were as healthy as their modern counterparts.

Malnutrition, disease and other curses of peasant life in the 10th to 14th centuries set in when children left the breast - which appears to have happened later than is usually the case today.

Results from nitrogen isotopes in the bones show children were still taking breast milk at 18 months, although by then their diet included food and water, much of which was sub-standard or contaminated.

"Stunted growth really started after this point," said Simon Mays, a human skeletal biologist with English Heritage, who has carried out the study with archaeologists from Bradford and Oxford universities.

"Conditions thereafter were so poor that adults in Wharram Percy continuing to grow until their late 20s, in order to make up for the slow start, as opposed to the modern figure of about 18 years old."

The study, the latest contribution to a huge archive of discoveries from Wharram Percy, which has been studied since Victorian times, backs up theories that the medieval countryside was even more unhealthy than squalid, crowded towns.

Dr Mays said: "Growth rates of infants at Wharram Percy suggest conditions even worse than those of slum dwelling Victorian workhouse children."

Breastfeeding was a rare but inevitably short-lived natural defence against the general bleakness of life in the village, where only a church and dozens of grassy hummocks remain.

Dr Mays said: "It promoted infant health because milk contains important natural ingredients that strengthen the immune system. But in medieval times it also enabled children to avoid contaminated food and water. This was a major source of disease in villages like Wharram Percy."

The analysis used new techniques in analysing the isotopes, including a mass spectrometer which weighed individual atoms. From the mass of scientific data, a picture of "a terrible struggle for existence" emerged.

Dr Mays said: "While being breastfed, these infants grew as well as modern babies, but when it stopped, the environment made its baleful impact. Extended breastfeeding shielded children from the very high level of infant mortality we might otherwise expect to see."

The findings also suggest that medical advice, which in medieval times included belief in the Roman doctor Soranus's recommendation of extended breast-feeding, was being followed at village level. Evidence confirms that feeding and the use of wet-nurses was the norm.

Belinda Phipps, chief executive of the National Childbirth Trust, said:"Women in the medieval period had the advantage of living in a culture that was particularly supportive of breastfeeding and where experienced breastfeeders could offer help to the new mother. Sadly, that's very different from now."