Researchers say a baby's DNA can be affected by what their mother eats before pregnancy and one gene is especially sensitive to these changes.

A mother's diet around the time of conception could permanently change the function of a gene that influences immunity and cancer risk in her child, scientists have discovered.

Previous studies by the same researchers showed that a child's DNA can be affected by what their mother eats before pregnancy, but they have now hit upon a gene called VTRNA2-1 as being particularly sensitive to these changes.

VTRNA2-1 is a tumour suppressor gene which also affects how the body responds to viral infections.

The research, which was carried out in West Africa but involved a team from the London School of Hygiene & Tropical Medicine, compared 120 pregnant women who conceived at the peak of either the rainy or dry season in rural Gambia and therefore, ate very different diets.

While a child's genes are inherited directly from their parents, how these genes are expressed is controlled through "epigenetic" modifications to the DNA.

Scientists explained that the most commonly studied epigenetic modifications are chemical marks placed on the DNA of genes known as methylation, that can prevent the message from being read and which can be influenced by an individual's environment.

Because methylation requires a defined set of nutrients, what a mother eats before and during pregnancy can affect the setting of these tags, with potentially permanent consequences for her child's gene function.

Lead author, bioinformatician Matt Silver of the Medical Research Council International Nutrition Group at the London School of Hygiene & Tropical Medicine, said: "By studying babies conceived to mothers eating very different diets in the dry and rainy seasons in rural Gambia we could exploit a natural experiment.

"Our results show that the methylation marks that regulate how VTRNA2-1 is expressed are influenced by the season in which babies are conceived."

His colleague, Andrew Prentice, said: "We think this is the first concrete evidence that a mother's diet before pregnancy can affect the disease risk of her child by rewriting a tiny portion of its epigenome.

"Because this gene plays a key role in controlling response to viral infections and offering protection against certain cancers, the potential implications are enormous."