For centuries, female fleshiness has been prized in Sudan, with women encouraged to be plump. But many Sudanese women now aspire to being slim - and gyms are taking off in a big way.

Inside a large sweaty hall, about 20 women are taking an aerobics class. US dance-pop music blares from the speakers.

You could be in just about any city in the world. Only there is not a man in sight.

Sudan is a conservative country and just a few years ago, this kind of scene would have been virtually unthinkable.

"Many more people want to exercise," says fitness trainer Amal Ahmed.

"They all want to be slim… They want to slim their tummy," she adds.

Sama Style health club is in an upmarket area of Khartoum. It's one of about 30 gyms that have sprung up in the capital in the past few years - most of them aimed exclusively at women.

The gym also has a swimming pool, treadmills, bikes and other exercise machines - but nothing beats aerobics for losing weight, says Ahmed.

Demand is high, and classes are full.

"Married women, young ladies, all ages come," she says. "They love it."

The traditional preference for women to have thick legs remains

Obesity is a big - and growing - problem in Sudan, and health concerns are one motivation for women to work out.

"They don't want to be fat so they don't get diseases. They are afraid of arthritis and back pain," says Ahmed.

But there has been a shift culturally too. "In Sudan, fat has become not wanted. Sudanese want to be like the people outside," she says.

Weight loss never used to be a priority here. As little as two generations ago, the custom was to fatten up Sudanese girls before their weddings.

People living at the time of the ancient Kush civilization, which ended in AD350, favoured full bodies, says folklore historian Sadia Elsalahi - especially thick hips and thighs.

Until the 1930s, Sudanese parents would marry off their daughters as young as 11 or 12 years old, says Elsalahi, when their bodies hadn't fully developed.

Find out more The World is a co-production of BBC World Service, Public Radio International and WGBH

Airs weekdays on more than 300 radio stations across the US and Canada Listen to The World Download a podcast

"To make a girl seem older, they made her bigger and fatter."

When a girl got engaged, her family would cut a large hole in the centre of a bed. The girl would sit in the hole for a whole year being fed fatty foods and drinks.

When she grew big enough to fill the hole, she would be considered ready for the wedding.

There was an economic incentive to fattening up too - a bigger bride would mean a larger payment of gold for her family.

Then for the smoke bath... After the fattening up, a bride-to-be would sit in a smoke bath of burning perfumed acacia wood, twice a day for 40 days in the lead up to her wedding. During that period, she wouldn't wash. Her body would be basted with aromatic oils until a thick layer formed on her skin.. On day 40, the thick sooty layer would be peeled off, revealing glowing skin underneath. That layer - the "banooteya" - would be folded up and sent to the groom's family, who would weigh it. Whatever the weight was, the groom would match in gold for the bride. So the larger the surface area of the woman's body, the more gold the family would get.

That was then.

Now the men are the only ones who will be left who will be fat! Amal Ahmed , Fitness trainer

"The idea of being fat is our fathers' and grandfathers'" says Nusaiba Abdelaziz, a student studying at Afhad University for Women in Khartoum.

Abdelaziz and her friends - all management students - are sitting under a tree during a break from class.

"I'm one of those girls that want to get skinny," says Hanaa Mubarak. But she says her mother keeps sabotaging her plans with calorie-rich, full-fat Sudanese cooking.

"My mum says, 'Hanaa, eat more of this, drink a lot of that, you need to get fatter.' But my skirt is tight. I want to feel lighter like the rest of the girls."

"I like my body, but if I can get skinnier I will," adds her friend Marwa Salahadeen. "I want to be really skinny."

Image caption Nusaiba, Hanaa, Marwa and friends outside university

Most men, they agree, now want to be with a woman who is slim.

"They see superstars on TV like Rihanna and Beyonce, and so we want to be like that," says Tibyan Yaseen.

In recent years, a new term "style" has entered the vocabulary in Sudan. The word is used - in English - to refer to a woman who is slim and pretty.

And these young women all want to be "style".

There has been a significant shift in the way that young women feel about their bodies, says, Nafisa Bedri, a professor at Afhad University for Women who has researched obesity and body image in Sudan.

Her studies show that many have a skewed view of themselves - describing themselves as overweight, when they are underweight, and vice-versa.

When you turn on the TV in Sudan, you're bombarded with adverts for weight-loss products, with images of svelte Lebanese and Egyptian women flashing across the screen, and this undoubtedly has an effect, says Bedri.

"The media has created this image among young women that they have to be very thin - they want to be skinny like the models they see in magazines and on satellite television."

Just as in the West, it's now very common for women to lose weight - rather than gain it - in time for their wedding day, she says.

While the idea of what constitutes the ideal body shape is changing in Sudan, the traditional preference for women to have thick legs remains.

Image caption A painting by Abdulaziz Farah of a 1950s Sudanese bride

Some women go to great lengths to try to achieve this, says Bedri - even injecting their legs with insulin or cortisone bought on the black market.

At the gym, the women taking the aerobics class finish and dry off. Some head to the equipment room for some muscle toning.

Others veer towards the pool for a quick swim before they leave.

Trainer Amal Ahmed says if demand keeps going up as it has, they will start more classes, for younger girls.

"Now the men are the only ones who will be left who will be fat!"

Hana Baba was reporting for The World - a co-production of the BBC World Service, PRI and WGBH

You can follow the Magazine on Twitter and on Facebook