Here are some studies about vitamin D, latitude, and disease rates:

25-Hydroxyvitamin D in African-Origin Populations At Varying Latitudes Challenges the Construct of a Physiologic Norm (2014)

Conclusion: The vitamin D levels of 2500 black adults from Chicago, Jamaica, Ghana, South Africa, and Seychelles (500 subjects from each territory) were measured. Those living closer to the equator had higher vitamin D levels compared to those farther away from the equator. Chicago subjects, farthest away from the equator, had the highest rate of deficiency.


Association Between Latitude and Breast Cancer Incidence in Mainland Australian Women (2014)

Conclusion: This study of breast cancer cases by latitude in Australia found that the south half of the country (farther from the equator) has more than double the rate of breast cancer cases compared to the north half of the country.


Sun Exposure, Vitamin D Receptor Genetic Variants, and Risk of Breast Cancer in the Agricultural Health Study (2014)

Conclusion: Sun exposure habits and breast cancer rates were evaluated in 31,021 ladies for a decade. Out of this data set, researchers compared 293 breast cancer cases with 586 matched controls and found that women who get 1+ hours of sun exposure daily have a significantly lower risk of breast cancer compared to women who get less sun exposure. “Our results suggest that sun exposure may be associated with reduced risk of breast cancer.”


Autism Prevalence in the United States with Respect to Solar UV-B Doses: An Ecological Study (2013)

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.


The Epidemiology of Multiple Sclerosis in Scotland: Inferences from Hospital Admissions (2011)

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 Prevalence of Headache May Be Related With the Latitude: A Possible Role of Vitamin D Insufficiency? (2010)

Conclusion: Study found that populations closer to the equator (where sun exposure, bright light exposure, and population vitamin D levels are the highest) have a noticeably lower prevalence of tension and migraine headaches compared to populations farther away from the equator.


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.


Relationship Between Low Ultraviolet B Irradiance and Higher Breast Cancer Risk in 107 Countries (2008)

Conclusion: This study looked at breast cancer rates in 107 different countries and found that countries closer to the equator, getting more UVB sunlight, enjoy significantly lower incidence rates of breast cancer compared to countries father away from the equator.


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).


Circannual Vitamin D Serum Levels and Disease Activity in Rheumatoid Arthritis: Northern Versus Southern Europe (2006)

Conclusion: The vitamin D blood levels of 118 European (Italian and Estonian) rheumatoid arthritis patients were measured during both winter and summer. The patients living in the more southern latitude Italy had significantly higher vitamin D levels at both times than those living in the more northern latitude Estonia.


Serum 25-hydroxyvitamin D concentrations in girls aged 4–8 y living in the southeastern United States (2006)

Conclusion: This study of 168 young girls living in the southeastern U.S. found that those with lower vitamin D levels had a lower bone mineral content, white girls have higher vitamin D levels than black girls (due to the increased melanin preventing vitamin D synthesis), their vitamin D levels dropped in the winter, and their levels increased in the summer. These girls also were found to have higher vitamin D levels compared to girls living at higher latitudes. Despite the black girls having lower pre-vitamin D levels, they had a higher bone density than the white girls. This adds support to findings of people of darker-skinned races possibly have differing vitamin D endocrine systems.


Do Sunlight and Vitamin D Reduce the Likelihood of Colon Cancer? (2005)

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.


Nutritional Rickets Among Children in the United States: Review of Cases Reported Between 1986 and 2003 (2004)

Conclusion: This meta-analysis of 22 studies on the prevalence of rickets in American children found that 83% of kids with rickets were black and over 90% were breastfed without any supplementary vitamin D source. People with darker skin types, especially those residing in more northern latitudes, are almost universally are found to have the lowest vitamin D levels and make up a disproportionately higher amount of vitamin D-deficiency diseases and symptoms due to their high melanin content acting as a natural sunscreen blocking the UVB necessary for vitamin D production.This analysis also found that most of the rickets cases were diagnosed in the winter and spring, when population vitamin D levels are the lowest. Five of the studies analyzed that reported cases of white children developing rickets were all from extreme northern states Washington, Minnesota, New York, Connecticut, and New Hampshire, where it is physically impossible for anyone to synthesize vitamin D from sunlight for roughly half of the entire year from autumn to spring.


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.


Vitamin D Deficiency and Bone Health in Healthy Adults in Finland: Could This Be a Concern in Other Parts of Europe? (2001)

Conclusion: The vitamin D levels of 328 middle-aged southern Finnish people were analyzed (202 women, 128 men) following the winter of 1998. Overall 86% of women and 56% of men were found to have insufficient levels, with 28% of women and 26% of men being severely deficient. Researchers concluded that this is primarily the result of Finland’s high 60 degree Northern latitude which provides inadequate sunlight for maintaining adequate vitamin D levels year-round or at all. The researchers also concluded that this is also a problem for all parts of Europe except southern Europe.


Geographic Variation of MS Incidence in Two Prospective Studies of US Women (1999)

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.


Prevalence of Vitamin D Insufficiency in an Adult Normal Population (1997)

Conclusion: This study analyzed the vitamin D levels of 1,569 adults in 20 different French cities. The northern populations had a much higher rate of vitamin D-deficiency than the southern populations.


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).


Sunlight and Breast Cancer Incidence in the USSR (1990)

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.


Also see studies on 

Seasonal Vitamin D Level Changes

Winter Sickness Rates


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