A large number of factors have been cited in the recent past to explain why, since the late 1970's, children have been getting heavier: a) older mothers; b) air-conditioning, c) medications, d) less sleep, and e) environmental contaminants (e.g., endocrine-disruptors). Now, add to the list adenovirus-36.
Recent studies of Korean children and American and Italian adults have shown that obese people are more likely to have antibodies against adenovirus-36 than normal-weight individuals. A new study of children 8 to 18 (admittedly a rather small sample of 67 obese and 57 normal weight children) found 19 with antibodies to andenovirus-36; 15 were obese and 4 were not. The weight differential was very large.
Support for the thesis is found in animal studies. Chickens, mice, rats, and monkeys infected with the virus all get fat even though the animals do not eat more or exercise less than before they were infected. Also, laboratory tests have shown that adult stem cells infected with the virus produce more fat cells, and those fat cells store more fat than normal.
Still, epidemiology is not proof of causation. The laboratory and animal studies provide support for the thesis, but differences in physiology and biochemistry limit their persuasive power. Further, the lack of exercise and higher fat diet certainly are contributing factors even if there is a causal or semi-casual relationship. For example, a study of military personnel did not find a relationship, which could be explained by an incentive that such personnel have to maintain their physical condition.
The study can be found at http://pediatrics.aappublications.org/cgi/content/abstract/peds.2009-3362.