Armchair athletics may not be an Olympic sport but it's the most popular activity in China this month, fuelling concerns about rising obesity rates.

Chinese media has even given its army of TV Olympic spectators a name -- otaku -- a Japanese word that means "venerable house" and usually refers to someone nerdy who is totally devoted to a hobby to the point of not leaving home.

Figures have shown about 840 million of China's 1.3 billion population tuned in to watch the August 8 opening ceremony of the Beijing Games and interest is expected to stay high to August 24.

Increased TV viewing, less physical jobs and a shift away from a traditional Chinese diet rich in vegetables and carbohydrates with little animal-sourced food to a more Western diet heavier in meat, eggs and dairy has piled on the pounds.

"Lots of mothers don't know what to feed their children anymore," said Zhao Hua, who was having lunch with her 6-year-old son Tanning at a massive McDonald's in the Olympics site.

"In the past it was good to be a bit fat because it was a sign of strength but now children are getting too fat."

Figures show about a quarter of Chinese adults are obese or overweight, which is lower than many other countries but has jumped from 13 percent in 1991 with forecasts it could double by 2028.

By comparison World Health Organization figures show 65 percent of adult Americans in 2005 were overweight or obese.

A University of North Carolina study, published in the July/August issue of the journal Health Affairs, showed of all developing countries, only Mexico's rate of obesity was growing faster than that of China.

The World Food Program says a 6-year-old boy in China is now 13 pounds (6 kgs) heavier and almost two-and-a-half inches (6.4 cms) taller than a 6-year-old was 30 years ago.

"We need to find the right investments and regulations to encourage people to adopt a healthy lifestyle, or we risk facing higher rates of death, disease, and disability and the related costs," warned researcher Barry Popkin.

The Chinese diet has changed, with hundreds of McDonald's and KFC outlets in China, but experts also blame a drop off in physical activity, with more cars and less bikes on the roads.

Chinese newspaper the People's Daily said China has fewer than seven training fields for every 10,000 Chinese, compared with 200 sports fields for every 10,000 people in United States, and Japan.

"Now all the teenagers just like to play video games and watch television and our children like McDonald's. It is not healthy," said Yu Yanbing who was tucking into some fries at McDonald's with his 3-year-old son Zixi.

(Editing by Alex Richardson)