If you read the marketing claims for probiotics and supplements, and an alarming number of papers that have made spurious claims to feed the fad, you might think gut bacteria were the magic bullet for a lot of diseases.

A new paper says they even determine whether or not your jeans fit this week. Pizza and exercise are hereby absolved. Instead, the  types of microbes that grow in our body, influenced by our genetic makeup, influences whether we are fat or thin, according to a paper in Cell.

The authors make their epidemiological correlation by studying twins and the Christensenellaceae bacterial family, which is more common in lean individuals and they determine is 'heritable'. They say that a member of this class of bacteria, Christensenellaceae minuta, protected against weight gain when transplanted into mice.

This will get mainstream media attention because the authors hint that it offers the possibility of personalized probiotic therapies optimized to reduce the risk of obesity-related diseases based on an individual's genetic makeup - sure, there are trillions of gut bacteria so most likely blindly firing yogurt into the stomach does nothing, but beliefs are also why organic food will be a $100 billion industry in 2015. Probiotics has a lot of upside; despite having no tangible benefit yet, it is already $28 billion just in the US.

Genetic data was 171 identical and 245 fraternal twins, whose genomes have been sequenced at King's College in London and are stored in its "TwinsUK" registry. This gave them a way to rank which microbes in the human gut are heritable.

With twins raised in the same households, "you can assume that environmental influences are going to be very similar to one another," said Cornell graduate Julia Goodrich, the paper's first author. When Goodrich analyzed the microbe populations in the twins' fecal samples, she found that identical twins, who have the same genetic makeup, had more similar gut microbiotas to each other than did fraternal twins, who share half the same genes.