A recent study has tried to identify the key suspect, or suspects, which may be fuelling the obesity epidemic.
University of Alabama at Birmingham obesity researcher David B. Allison suggests that the root cause of obesity may be much more complicated than the conventional wisdom - too much food availability, too little opportunity to exercise.
Needing raw data, he tracked down previous studies of mammals, living with or around humans, which had lasted at least a decade. He found information, called data sets, on 12 groups of animals. Divided into male and female populations, he ended up with 24 data sets, containing information on more than 20,000 animals.
The data sets were varied but there was one constant - All 24 sets had seen overall weight gain in the population over time. Twenty-three of the 24 had seen an increase in the percentage of obese individuals in the group.
"And yet there was no single thread running through all 24 data sets that would explain a gain in weight. The animals in some of the data sets might have had access to richer food, but that was not the case in all data sets. Some of the animals might have become less active, but others would have remained at normal activity levels," said Allison.
"Yet, they all showed overall weight gain. The consistency of these findings among animals living in different environments, including some where diet is highly controlled and has been constant for decades, suggests the intriguing possibility that increasing body weight may involve some unidentified or poorly understood factors," he added.
Allison and Yann Klimentidis, a post-doctoral trainee in the School of Public Health, are beginning to look at alternative reasons for obesity beyond the usual suspects of increases in food intake:
Light. Studies have shown that subtle changes in the amount of time spent in light or dark environments changes eating habits.
Viruses. Infection with adenovirus-36 is associated with obesity, and the presence of antibodies to AD36 correlates to obesity in humans.
Epigenetics. Genetic modifications brought about by any number of environmental cues such as stress, resource availability, release from predation or climate change.
"When looking for ways to combat obesity in humans, we need to be more aware of all the possible alternative causes of obesity. If we can find causes for the weight gain seen in our animal subjects, we may be better able to apply that to coping with obesity in humans," said Klimentidis.
The study is published in the journal Proceedings of the Royal Society B.