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 . Where there is muck there really is brass with scientists claiming that millions can be made by extracting gold from human waste . In fact as well as gold , faeces contains platinum , silver and other metals along with rare elements such as palladium and vanadium . Researchers now hope to 'mine ' sewage reducing the need to mine raw metals from the ground , while reducing pollution by recycling human waste . Previous research estimated the waste from one million Americans could contain as much as $ 12 million ( £8.7 million ) worth of metals . In 1999 the UK produced over 1,130,000 tonnes of sludge dry solids . If Britain 's waste was mined it could be worth $ 762 million ( £510 million ) a year . Dr Kathleen Smith of the US Geological Survey said : 'If you can get rid of some of the nuisance metals that currently limit how much of these biosolids we can use on fields and forests , and at the same time recover valuable metals and other elements , that 's a win-win . ' 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 . '
Researchers collected sewage samples from 71 cities across the US . They analysed ' microbiome ' of each city - a key indicator of health . Heaviest city was St Joseph , however , with an obesity rate of 37.4 % . the say website provides information without compromising privacy .