Type 2 diabetes, which is blamed for over three million deaths each year, is on the increase and various food pundits and politicians say they can cure it if people would just ban trans fats or sodas or whatever they happen to be against this year.

And then there is genetics. There are genetic variants that have been associated with it but why wouldn't they have been eliminated by natural selection? Obviously if they had some other value but it has been shown that genetic regions associated with increased risk of type 2 diabetes were unlikely to have been beneficial to people at stages through human evolution.

To try to explain its persistence, biologists previously hypothesized that during times of 'feast or famine' throughout human evolution, people who had advantageous or 'thrifty' genes processed food more efficiently. But in the modern developed world with too much food, these same people would be more susceptible to type 2 diabetes.

Tests for positive selection in the genetic regions associated with type 2 diabetes. The red dotted line showing the significance threshold. DOI: 10.1016/j.ajhg.2013.12.010. Credit and link: Wellcome Trust Sanger Institute

"This thrifty gene theory is an attractive hypothesis to explain why natural selection hasn't protected us against these harmful variants," says Dr. Yali Xue, lead author of the study from the Wellcome Trust Sanger Institute. "But we find little or no evidence to corroborate this theory."

The team tested this hypothesis by examining 65 genetic regions that were known to increase type 2 diabetes risk, the most detailed study of its kind.

If these harmful variants were beneficial in the past, the team would expect to see a genetic imprint of this in the DNA around the affected regions. Despite major developments in tests for positive selection and a four-fold increase in the number of genetic variants associated with diabetes to work with, they found no such imprint.

"We found evidence for positive selection in only few of the 65 variants and selection favoured the protective and risk alleles for type 2 diabetes in similar proportions," notes Dr. Qasim Ayub, first author and also from The Wellcome Trust Sanger Institute, "This is no more than what we would expect to find for a random set of genomic variants."

This finding poses an intriguing and still unanswered question: if the harmful genetic variants that are associated with type 2 diabetes were not beneficial in the past, then why have they not been eliminated?

"Geneticists have been able to identify multiple genetic risk variants for many common diseases, such as type 2 diabetes," says Professor Mark McCarthy, co-author from the University of Oxford. "However, the big challenge, if we are to use this information to improve human health, lies in understanding how it is that these differences in DNA sequence lead to an altered risk of disease. Understanding how variants that appear, in the current environment, to have largely harmful consequences, have become so frequent will be an important step on this path towards better treatment and prevention"