Image copyright Thinkstock Image caption Abdominal obesity increased by nearly 50% between 1999 and 2012

US waistlines are continuing to expand, despite obesity appearing to be reaching a plateau, data suggests.

Research suggests the average adult waist size in the US increased by more than 1in (2.5cm) between 1999 and 2012.

But figures for obesity as defined by body mass index (BMI) had changed little over the same period, the US Centers for Disease Control and Prevention said.

Factors such as lack of sleep may be to blame for growing waistlines.

The researchers, led by Dr Earl Ford, analysed national data on nutrition and health in the US from 1999-2000 to 2011-12.

At a time when the prevalence of obesity may have reached a plateau, the waistlines of US adults continue to expand US researchers

The study included more than 32,000 men and women over the age of 20.

The researchers found:

Average waist circumference increased from 37.6in (95.5cm) to 38.8in (98.5cm)

The average waist circumference for men is now 40in (101cm)

The average waist circumference for women in now 38in (96cm)

The average increase in men was 0.8in (2cm); in women 1.5in (3.8cm)

Abdominal obesity (tummy fat) increased from 46.4% of the population in 1999-2000 to 54.2% in 2011-2012.

The team compared the findings with obesity data.

The researchers found obesity calculated from body mass index (BMI) did not change significantly between 2003 and 2012.

"At a time when the prevalence of obesity may have reached a plateau, the waistlines of US adults continue to expand," the researchers told the Journal of the American Medical Association (JAMA).

"The reasons for increases in waist circumference in excess of what would be expected from changes in BMI remain speculative, but several factors, including sleep deprivation, endocrine disruptors, and certain medications, have been proposed as potential explanations."

The research backs recommendations that routine measurement of waist circumference is a key step in controlling obesity.

According to NHS Choices, the risk of some health problems is affected by where body fat is stored, as well as weight.

Carrying too much fat around the middle (waist) can increase the risk of developing conditions such as heart disease, type 2 diabetes and cancer.

Amy Thompson, senior cardiac nurse at the British Heart Foundation, said more people were carrying extra weight around the middle, including in the UK.

"This increases your risk of heart disease and stroke, but losing weight and reducing your waist size is doable," she said.

"Try cutting down on the calories and getting more active, but don't try to lose weight too quickly.

"Slow and steady weight loss - around 1-2lb [1kg] each week - is healthier."