A landmark study has found there may be more to the western world's obesity epidemic than first thought.

The study, published in Proceedings of the Royal Society B, found that weight gain may be linked to a number of issues beyond diet and lifestyle; including the possibility that weight gain is related to chemicals in the human body, or of infectious-origin.

The scientists, from nine research centres around the globe, observed the weight gain trend in feral and domestic animals that were living in close proximity to humans.

The study of more than 20,000 animals found that their body weight was increasing uniformly, despite their different environments, raising the possibility of "several as-of-yet unidentified and/or poorly understood factors" in weight gain.

These factors may include a virus or genetic factors beyond that of changes in the underlying DNA sequence.

The findings appear to contradict the common belief that the obesity epidemic is caused almost entirely by our changing diet and lifestyle factors.

"This finding may eventually enhance the discovery ...of other factors that have contributed to the recent rise in obesity rates", the study claims.