A breakthrough involving New Zealand scientists points to new ways of handling the developed world's obesity crisis.
People who became obese were thought to be victims of bad genes which caused them to pack on the kilos.
But the scientists' discovery suggests destiny may be reversed through good nutrition in early childhood.
The scientists from Auckland University's Liggins Institute and their colleagues in Britain are examining how a mother's diet in pregnancy can determine if a baby will become obese in adulthood and suffer heart disease and diabetes.
The Government is deeply concerned about New Zealand's rapidly rising rate of obesity and the predicted increase in diabetes that this will fuel.
More than 20 per cent of adults are obese, double the rate 30 years ago, and a further 35 per cent are overweight.
The scientists have shown that if a mother is undernourished, her children's bodies are set up to cope with a life of scarcity.
But the energy-dense "hamburger and milkshake" diet of modern Western society is likely to result in children who are likely to become fat - unlike those from mothers who eat a balanced diet during pregnancy.
In a study by the Liggins Institute, Southampton University and AgResearch to be published this week in a United States journal, the researchers describe molecular changes that can occur after dietary intervention in early childhood.
Liggins director Professor Peter Gluckman, one of the researchers, said it also showed that genetic switches set in the womb could be reversed by nutritional changes in early childhood.
"It changes the way we should think about tackling the obesity epidemic," he said from Britain last night.
"It's probably the most important intellectual breakthrough we've made in understanding development."
In the experiments, the newborn offspring of well-fed and undernourished female rats were dosed with leptin, a hormone that signals to the body when it has eaten enough. When the young rats became adults, the long-term effects were measured by checking genes that regulate metabolism in the liver.
Rats from well-fed mothers reacted to leptin in the opposite way to those from undernourished mothers.
Professor Gluckman likened the process to female honey bees developing as either queens or workers, depending on whether they were fed royal jelly as larvae.
"This is the first suggestion that this fundamental biological process operates in mammals, and has major implications for addressing issues such as obesity," hesaid.
"Not everyone is the same - gene switches have been moved in early development to make some more or less sensitive to fat in the diet."
Co-researcher Dr Alan Beedle said the study, to be published in the Proceedings of the National Academy of Sciences, dispelled the common idea that a person's life course was set by their genetic make-up at birth.
"It's really development, and modifiable factors during development, that can change how we grow and what diseases you are susceptible to as an adult," he said.
The study's message for developed nations was that mothers should eat a balanced diet during pregnancy, with the right amounts of protein and vitamins.
And if a fetus was under-nourished in the womb, it might be possible to detect that by a blood test at birth and correct the condition with good nutrition.