Relation of schizophrenia prevalence to latitude, climate, fish consumption, infant mortality, and skin color: a role for prenatal vitamin d deficiency and infections?

Abstract

Previous surveys found a large (>10-fold) variation in schizophrenia prevalence at different geographic sites and a tendency for prevalence to increase with latitude. We conducted meta-analyses of prevalence studies to investigate whether these findings pointed to underlying etiologic factors in schizophrenia or were the result of methodological artifacts or the confounding of sites' latitude with level of healthcare at those sites. We found that these patterns were still present after controlling for an index of healthcare--infant mortality--and focusing on 49 studies that used similar diagnostic and ascertainment methods. The tendencies for schizophrenia prevalence to increase with both latitude and colder climate were still large and significant and present on several continents. The increase in prevalence with latitude was greater for groups with low fish consumption, darker skin, and higher infant mortality--consistent with a role of prenatal vitamin D deficiency in schizophrenia. Previous research indicates that poor prenatal healthcare and nutrition increase risk for schizophrenia within the same region. These adverse conditions are more prevalent in developing countries concentrated near the equator, but schizophrenia prevalence is lowest at sites near the equator. This suggests that schizophrenia-producing environmental factors associated with higher latitude may be so powerful they overwhelm protective effects of better healthcare in industrialized countries. The observed patterns of correlations of risk factors with prevalence are consistent with an etiologic role for prenatal vitamin D deficiency and exposure to certain infectious diseases. Research to elucidate environmental factors that underlie variations in schizophrenia prevalence deserves high priority.

Fig. 1.

Schizophrenia Prevalence and Latitude by Continent and Infant Mortality. Note: South Asia sites were from India and Indonesia; those sites had higher infant mortality rates than all but one of the East Asian sites. For the regression lines in North America, those with infant mortality rates above 30 per 1000 were grouped as having a higher rate, those below 30 as having a lower rate. The slopes of linear regression lines were 0.15 for the East Asian sites, 0.22 for the African sites, 0.086 for the South Asian sites, 0.14 for the North American sites with lower infant mortality, 0.51 for the North American sites with higher infant mortality, and 0.48 for the European sites.

Fig. 2.

Schizophrenia Prevalence and Latitude by Fish Intake. Note: Error bars represent 95% confidence limits. The slopes of linear regression lines were 0.03 for sites with high fish intake, 0.21 for sites with moderate fish intake, and 0.17 for sites with low fish intake. For the subset of Scandinavian sites, the slope of linear regression for was 0.03 for sites with high fish intake and 1.57 for sites with moderate fish intake.

Fig. 3.

Schizophrenia Prevalence and Latitude by Skin Color. Note: Error bars represent 95% confidence limits. The slopes of the linear regression lines were 0.24 for those with lightest skin, 0.43 for those with intermediate skin color, and 0.31 for those with darkest skin.