et al., 1998). Earlier studies reviewed in Update 1998, Update 1996, and VAO— and cited in Appendix B —show an inconsistent but weakly positive association between exposure measures and Type 2 diabetes mortality.
Positive associations are reported in most of the morbidity studies identified by the committee. Several studies that used Type 2 diabetes morbidity as an outcome measure have been published since the last Veterans and Agent Orange review: studies of male and female Vietnam veterans from Australia; a NIOSH study of U.S. chemical workers; the Air Force Health Study (Ranch Hand study); and a separate examination of the Ranch Hand comparison group. One of these studies did not show a positive association: the survey of female veterans from Australia indicated 5 self-reported cases of diabetes where 10 were expected (Commonwealth Department of Veterans' Affairs, 1998b). However, the survey of male Australian veterans of Vietnam (Commonwealth Department of Veterans' Affairs, 1998a) did find a statistically significant excess of self-reported diabetes—2,391 cases were reported when 1,780 were expected. The NIOSH (Calvert et al., 1999) and Ranch Hand comparison group (Longnecker and Michalek, 2000) studies both reported an elevated incidence of diabetes in individuals who had high levels of serum dioxin relative to others examined in that study. The primary analysis in the Air Force Health Study (AFHS, 2000) showed nearly identical diabetes incidence in Ranch Hand veterans and the matched comparison group. Despite this negative finding, the study is considered suggestive because dose–response relationships between dioxin levels and diabetes incidence were observed in several other analyses that controlled for confounding variables. In presently unpublished material provided by AFHS researchers, additional analyses were carried out that support the 2000 report findings (Michalek, 2000b). The committee encourages the researchers to seek publication of these results in a peer-reviewed journal so that they can be fully evaluated.
Although some of the risk estimates in the studies examined by the committee are not statistically significant and, individually, studies can be faulted for various methodological reasons, the accumulation of positive evidence is suggestive. The committee does not believe that publication bias plays a crucial role in this tendency in the data.
Presently available data allow for the possibility of an increased risk of Type 2 diabetes in Vietnam veterans. It must be noted, however, that these studies indicate that the increased risk, if any, from herbicide or dioxin exposure appears to be small. The known predictors of diabetes risk—family history, physical inactivity, and obesity —continue to greatly outweigh any suggested increased risk from wartime exposure to herbicides.