Here are some studies about vitamin D, latitude, and disease rates:
Conclusion: Autism rates are lower in the southern United States, which get more vitamin D-producing UVB solar radiation, compared to the northern United States, which get less UVB.
Gestational Exposure to Urban Air Pollution Related to a Decrease in Cord Blood Vitamin D Levels (2012)
Conclusion: A study of 375 mother-newborn pairs in Poitiers, France and Nancy, France found that pregnant women living in Nancy, with it’s higher air pollution and higher latitude, are more likely to be vitamin D-deficient and give birth to vitamin D-deficient newborns – due to the air pollution blocking the UVB light necessary for cutaneous vitamin D synthesis, the higher latitude of Nancy providing less UVB, and higher air pollution making people more reluctant to go outside. 86% of all newborns in the study had insufficient vitamin D levels and 28% were severely deficient. Infants born in summer had the highest vitamin D levels, followed by infants born in autumn and spring, with infants born during winter having the lowest vitamin D levels.
Conclusion: There is an increased rate of multiple sclerosis cases in Northern Scotland compared to Southern Scotland.
Exposure to Ultraviolet Radiation: Association with Susceptibility and Age at Presentation with Prostate Cancer (2011)
Conclusion: “A positive association between latitude and prostate cancer mortality has been interpreted to indicate that ultraviolet radiation (UVR) protects against development of this cancer.”
The Association Between Ultraviolet B Irradiance, Vitamin D Status and Incidence Rates of Type 1 Diabetes in 51 regions Worldwide (2008)
Conclusion: Incidence rates of Type-1 diabetes are generally higher at latitudes farther away from the equator where there is less UVB radiation necessary for vitamin D production in humans. Type-1 diabetes incidences decrease in regions closer to the equator.
Could Ultraviolet B Irradiance and Vitamin D be Associated With Lower Incidence Rates of Lung Cancer? (2008)
Conclusion: Lung cancer incidence rates were investigated in 111 different countries. Lung cancer rates were found to be higher in latitudes farther from the equator and in areas with higher cloud cover. These would also be areas of the world that receive less solar UVB radiation, which is necessary for vitamin D production.
High Prevalence of Vitamin D Insufficiency in Black and White Pregnant Women Residing in the Northern United States and Their Neonates (2007)
Conclusion: A study of pregnant women in Pittsburgh found that roughly 50% of them, and later their babies, were vitamin D insufficient.
Solar ultraviolet-B exposure and cancer incidence and mortality in the United States, 1993-2002 (2006)
Conclusion: For the period of 1993 to 2002, satellite-measured solar UV-B levels, 3+ million cancer incidences, and 3 million cancer deaths in America were analyzed. There was a correlation between lower UVB levels and an increased rate of “bladder, colon, Hodgkin lymphoma, myeloma, prostate, rectum, stomach, uterus, and vulva” cancer, and to a lesser extent for “breast, kidney, leukemia, non-Hodgkin lymphoma, pancreas, and small intestine” cancer.
The Association of Solar Ultraviolet B (UVB) with Reducing Risk of Cancer: Multifactorial Ecologic Analysis of Geographic Variation in Age-Adjusted Cancer Mortality Rates (2006)
Conclusion: A study of American cancer mortality rates from 1950-1969 and 1970-1994 found that cancer mortality rates have been higher in areas of American with weaker UVB solar radiation (i.e. northern latitudes).
Conclusion: This epidemiological study found that between 1959 and 1961, the American states that received the most average daily solar radiation (southern latitude states) had the lowest mortality rates for colon cancer and the northern latitude states had significantly higher colon cancer mortality rates.
Incidence of Juvenile-Onset Crohn’s Disease in Scotland: Association with Northern Latitude and Affluence (2004)
Conclusion: Children in Northern Scotland have been found to have higher incidences of Crohn’s disease compared to children in Southern Scotland.
A Systematic Review and Meta-Analysis of Northern Hemisphere Season of Birth Studies in Schizophrenia (2003)
Conclusion: Schizophrenics tend to be born more in winter and spring compared to other seasons, when vitamin D-creating UVB radiation is less available. They also tend to be born more in latitudes farther away from the equator, which get less UVB radiation.
Ecologic Analysis of Some Immune-Related Disorders, Including Type 1 Diabetes, In Australia: Latitude, Regional Ultraviolet Radiation, and Disease Prevalence (2003)
Conclusion: A positive correlation has been found between type 1 diabetes rates and latitude in Australia.
Conclusion: There are more incidences of multiple sclerosis in the northern regions of the United States that the southern regions, which are closer to the equator and enjoy more vitamin D-creating UVB solar radiation.
Geographic Patterns of Prostate Cancer Mortality. Evidence for a Protective Effect of Ultraviolet Radiation (1992)
Conclusion: There are lower prostate cancer mortality rates in the southern regions of America compared to the northern regions due to the south receiving stronger sunlight with more vitamin D-producing UVB radiation.
Geographic Variation in Breast Cancer Mortality in the United States: A Hypothesis Involving Exposure to Solar Radiation (1990)
Conclusion: The U.S. breast cancer mortality rate is lower in the South and Southwest regions of America (which are closer to the equator and get more UVB) than it is in the Northeast (farther away from the equator, less UVB).
Conclusion: “ The pattern of increased breast cancer incidence in regions of low solar radiation in the USSR is consistent with the geographical pattern seen for breast cancer mortality in the US and worldwide.”
Influence of Season and Latitude on the Cutaneous Synthesis of Vitamin D3: Exposure to Winter Sunlight in Boston and Edmonton Will Not Promote Vitamin D3 Synthesis in Human Skin (1988)
Conclusion: Due to the unavailability of UVB solar radiation in the high northern latitudes during winter, it is impossible for humans to synthesize their own vitamin D between November and February in Boston and between October and March in Edmonton, Alberta.
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