Eating three dairy servings a day has no impact on a child's body mass index (BMI), according to research by the University of Sydney.

Dairy is regarded as a primary source of calcium and the research shows it is linked to the lowering blood pressure.

"Our research shows eating the recommended amounts of dairy foods ... is not linked to weight gain and is associated with lower blood pressure in children aged eight and nine," said Associate Professor Tim Gill on Wednesday.

According to the Dairy Australia, three serves of dairy foods every day in any combination ensures most children will get the calcium they need.

A serve is described as one glass of milk (250ml), two slices of cheese (40g) or one small tub of yogurt (200g).

The research was conducted by the Boden Institute of Obesity, Nutrition, Exercise and Eating Disorders at the university. It aimed to examine the association between dairy consumption at 18 months of age and BMI and blood pressure at eight years of age.

In children who consumed 2.9 serves of dairy or more each day, there was an added benefit of lower blood pressure, which reduced the risk of heart disease in adulthood.

However, Dairy Australia says primary school children are not eating enough dairy to ensure recommended calcium intake.

This worsens as they get older and teenage girls are at particular risk, with 90 per cent not getting enough calcium.

The association says only a third of mothers ensure their children consume dairy every day and almost 40 per cent think dairy is essential only for children under the age of five.