Eating too much fat is bad for bone health - especially for those born small or too early, or whose mothers had poor nutrition during pregnancy.
That's according to just-published insights from researchers at the Liggins Institute and the University of Auckland, who analysed the bone structure of rats whose mothers were either fed normally or undernourished during pregnancy.
While it's known a high-fat diet raises risk for heart disease and metabolic diseases such as type 2 diabetes in adulthood, the new study, published in the journal Scientific Reports, suggests a further risk for bone health.
In the study, half of the rat offspring from each group of mothers were later fed a regular diet from weaning, the other half were fed a high-fat diet.
Compared to the other groups, the bones of the rats in the undernourished-mother-plus-high-fat-diet group had the lowest bone mineral content and fewer, more widely spaced bone plates - characteristics of reduced bone strength.
Past animal studies had shown that poor nutrition in pregnant mothers "programmes" the metabolism of their children in a way that raises their risk of later developing weight gain and obesity.
This study was the first to uncover a compounding effect of poor maternal nutrition and a high-fat diet in offspring on later bone health.
"This may have implications for thousands of children around the world, whose mothers have poor nutrition due to forces such as poverty and the obesogenic environment, and then who are exposed to high-fat Westernised diets in childhood," said study lead Professor Elwyn Firth, of the Liggins Institute and university's Department of Exercise Sciences.
"Other research shows kids with obesity are prone to fractures, and that osteoporosis in older people has its origins in young [fetal or postnatal] life."
The research team also investigated what changes in the early environment did to later bone health.
Earlier research by Liggins scientists has shown that treating offspring with the obesity-related hormone leptin, a hormone that regulates appetite and energy balance, could reverse metabolic programming, so that offspring grow up with normal body weight, fat mass and leptin levels.
They found that if new-born rats born to undernourished mothers and then fed a high-fat diet were treated with leptin for 10 days, the programming effects on bone structure were permanently reduced or even reversed.
But the same leptin treatment given to new-born rats on a high-fat diet whose mothers had eaten a regular diet had a negative effect on bone health - it reduced bone length.
This meant leptin reversed the effect of the mother's malnutrition, but not the effect of the offspring's own poor diet.
"At the moment, leptin is not a viable treatment option for babies, because we do not yet know what the safe levels are, and evidence like this suggests it could be detrimental for babies who are not low in leptin," Firth said.
Study co-author Professor Mark Vickers said leptin was not a treatment option for most adults with obesity, as they were less sensitive than normal to this hormone.
"But our study opens up other possible therapeutic avenues for children and once again highlights the importance of early life for optimising health later in life."
It was already known from previous research that exercise early in life could improve bone health and strength into adulthood, he said.
"It's also known that maintaining a certain leptin level in early life is essential for normal regulation of metabolism, and that food and exercise affect leptin levels.
"Perhaps in the future it will be possible to change leptin levels without an injection, by using nutrition and exercise, which may counteract the programming effects."
The team's next step will be to test different doses of leptin in rats, and investigate how exercise in early life affects leptin levels and adult bone health.
"Our study points to a deepening understanding of the skeleton as not only a structural organ, but as part of the hormonal system, which in turn powerfully influences metabolism," Firth said.
"And metabolism - what you do with energy from diet - is the crux of why some children and adults become overweight or obese."