 While some groups and individuals have called for more human testing of GM food, multiple obstacles complicate such studies.The General Accounting Office (in a review of FDA procedures requested by Congress) and a working group of the Food and Agriculture and World Health organizations both said that long-term human studies of the effect of GM food are not feasible.The reasons included lack of a plausible hypothesis to test, lack of knowledge about the potential long-term effects of conventional foods, variability in the ways humans react to foods and that epidemiological studies were unlikely to differentiate modified from conventional foods, which come with their own suite of unhealthy characteristics.