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The number of pizza take outs compared with gym memberships may give you some idea of a city's obesity rates.

But scientists say to really understand an area's weight problems you need to go underground into the grimy depths of the sewage network.

Here, they claim, raw waste can be analysed to reveal the urban 'microbiome', offering glimpses into disease trends on a wider scale.

Humans harbour tremendous amounts of bacteria in their gastrointestinal tract and gut bacteria. Lifestyle differences can alter the human gut microbiome, and microbial community composition of an individual with obesity. Now scientists say this community can be analysed from a community's sewage system

Studies of the human microbiome, the collection of trillions of microbes living in and on the human body, have gained traction during the last decade.

Scientists are hoping to identify a 'healthy microbiome' by finding one or more bacterial community types that may be linked with healthy individuals.

In a recent study by the University of Wisconsin-Milwaukee, sewage samples were collected from 71 cities across the US.

According to the researchers, a microbial study of that sewage helped them predict each city's obesity rates with 81 to 89 per cent accuracy.

The skinniest city sampled was Steamboat Springs, Colorado, with an obesity rate of 13.5 per cent, while the heaviest was St. Joseph, Missouri, with a rate of 37.4 per cent.

While scientists have known this for some time, this is the first study to reveal the same holds true when these microbiomes are mixed together in the sewage network. Using computer power, the team were able to to rapidly sequence microbial populations and look for patterns in the results

This is because humans harbour tremendous amounts of bacteria in their gastrointestinal tract and gut bacteria.

Lifestyle differences can alter the human gut microbiome, and microbial community composition of an individual with obesity.

While scientists have known this for some time, this is the first study to reveal the same holds true when these microbiomes are mixed together in the sewage network.

The study used a technique known as oligotyping which exploits subtle variations gene data.

Using computer power, the team were able to to rapidly sequence microbial populations and look for patterns in the results.

Overall, scientists compared the gut bacterial community profiles of 137 healthy adults provided by The Human Microbiome Project to the bacterial community profiles of more than 200 sewage samples.

'This method is similar to trying to create a map of a geographical region,' explains Murat Eren, an Assistant Research Scientist at the MBL, and one of the authors of the study.

'The way we have been working with microbiomes of individuals has been similar to driving around and mapping the streets and structures of a city in a detailed manner.

'This approach takes our efforts to a much larger scale.

'In this sense it is similar to taking one big aerial picture of a city, trading off intricate details of a small number of well-described streets for broader insights and larger patterns.'

The researchers say the use of oligotyping, which provides greater sensitivity, allowed them to better explain the distribution of very closely related bacterial organisms to compare microbiomes among 71 human populations.

'The sewage samples of 71 cities do not tell us anything specific about 'individuals' who live in those cities' added Eren.

'However, only using sewage samples, we were able to differentiate these cities based on their estimated level of obesity.

'This approach can be beneficial to answer various public health questions while not compromising the privacy of individuals.'