Researchers collected sewage samples from 71 cities across the US . The researchers found that in some cities, such as New York and Los Angeles, there was a significant difference between how much of the water they used to flush toilets versus what came out through their taps. Heaviest city was St Joseph , Missouri , with an obesity rate of 37.4 % . Scientists say method provides data without compromising privacy .
Researchers collected sewage samples from 71 cities across the US . They analysed 'microbiome ' of each sample - a key indicator of health . Heaviest city was St Joseph , Missouri , with an obesity rate of 37.4 % . Scientists say method provides data without compromising privacy .