Here’s a big list of studies about vitamin D and winter sickness rates:

Distribution of Month of Birth of Individuals with Autism Spectrum Disorder Differs from the General Population in the Netherlands (2017)

Conclusion: This study of 3,478 autistic children in the Netherlands found that children conceived between autumn and winter have a higher risk for developing autism, with conception in October producing the highest risk. The researchers hypothesize that this may be due to lack of sun exposure during the first trimester of pregnancy leading to vitamin D-deficiency. This study also acknowledges that autism rates have been increasing in the Netherlands over the years and this rise has coincided with the medical establishment advising people to avoid sun exposure.


Month of Conception and Risk of Autism (2011)

Conclusion: This study of 6,604,975 children born from 1990 to 2002 in California found that those conceived between December and March had an increased risk of developing autism. These also happen to be the months when population vitamin D levels are the lowest, due to decreased cutaneous vitamin D synthesis during winter when the sunlight is weakest. Could vitamin D-deficiency during the first trimester of pregnancy be the culprit behind autism?


Association of Autistic Spectrum Disorder with Season of Birth and Conception in a UK Cohort (2010)

Conclusion: “Seasons of conception and birth were compared in children with and without ASD” in the UK. It was found that children with autism are disproportionately born in the Spring, when most people’s vitamin D levels are the lowest. The results also suggested that children born in the spring have roughly double the risk of being autistic compared to children born in the summer.


Global Breast Cancer Seasonality (2010)

Conclusion: 2,921,714 breast cancer cases from 64 different global regions were analyzed. There is a consistent, world-wide increase in breast cancer diagnoses in the spring and fall. This increase is even stronger in regions farther away from the equator. Fall is when most people’s vitamin D levels begin dropping as the solar radiation starts to get weaker going into winter. Spring is when most people’s vitamin D levels are at their lowest, as they are coming out of winter and this is when it’s most difficult, or completely impossible, for many people to produce their own vitamin D from sun exposure.


Variation in Season of Birth in Singleton and Multiple Births Concordant for Autism Spectrum Disorders (2008)

Conclusion: This U.S. study of 1,068 autism cases found that children conceived during autumn and winter are at a higher risk of developing autism, adding support to the hypothesis that maternal vitamin D levels during the first trimester of pregnancy may be a critical factor in the development of autism.


Epidemic Influenza and Vitamin D (2006)

Note: Extremely comprehensive overview of the evidence linking vitamin D deficiency, sun exposure deficiency, and seasonal changes in solar radiation on influenza rates.


Epidemiology of Community-Acquired Pneumonia in Edmonton, Alberta: An Emergency Department-Based Study (2005)

Conclusion: In Edmonton, Alberta, winter brings a significant increase in the rate of community-acquired pneumonia cases. Winter is when people’s vitamin D levels tend to start dropping.


Timing of Birth and Risk of Multiple Sclerosis: Population Based Study (2005)

Conclusion: People with multiple sclerosis are more commonly born after winter, when population vitamin D levels are typically at their lowest.


Influenza and Pneumonia Hospitalizations in Ontario: A Time-Series Analysis (2004)

Conclusion: In Ontario, the influenza and pneumonia case rates significantly increase during winter and significantly decrease during summer. These rates correlate with the seasonality of people’s vitamin D levels.


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.


Epidemiology and Seasonality of Respiratory Tract Virus Infections in the Tropics (2003)

Conclusion: For tropical regions (in Asia, Africa, and South America), there has been an observed increase in respiratory viral infections during the rainy seasons.


Effect of Sunlight and Season on Serotonin Turnover in the Brain (2002)

Conclusion: A study of 101 healthy men found that their serotonin levels were lowest during winter, when there’s less sunlight, but serotonin production in the brain “rose rapidly” as the amount of bright sunlight increased.


Seasonal Fluctuations in Hospitalisation for Pneumonia in Finland (2001)

Conclusion: Finland experiences an increase in pneumonia cases during winter time, when people’s vitamin D levels tend to start dropping.


Seasonal Variation in Hospital Admissions for Community-Acquired Pneumonia: A 5-Year Study (1999)

Conclusion: Community-acquired pneumonia rates increase in the winter and spring, when most people’s vitamin D levels decrease.


Seasonality of Hip Fracture and Haemorrhagic Diseases of the Newborn (1993)

Conclusion: Hip fracture rates for newborns reach their peak January-February and occur least July-August. These rates are in correlation with seasonal vitamin D level changes. Adequate vitamin D is essential for bone health.


Relation of Fractional 47Ca Retention to Season and Rates of Bone Loss in Healthy Postmenopausal Women (1991)

Conclusion: A study of 58 postmenopausal women over a two-year period found that their calcium retention was higher August-October. This is a time of year when people’s vitamin D levels tend to be highest. The study also found that their calcium retention was lower March-May, after coming out of winter when people’s vitamin D levels tend to be lower. Seasonal changes in calcium retention and bone loss are strongly correlated with seasonal changes in population vitamin D levels.


Do Children with Autism Have March Birthdays? (1990)

Conclusion: This study found that autistic children are disproportionately born in March, when most people’s vitamin D levels are lowest following the winter months when it’s either more difficult or completely impossible to make vitamin D from sun exposure.


Seasonal Variation of Lumbar Spine Bone Mineral Content in Normal Women (1983)

Conclusion: Women’s bones tend to be stronger at the end of summer compared to the end of winter. This correlates with the well-documented seasonal changes in population vitamin D levels.


The Role of Season in the Epidemiology of Influenza (1981)

Conclusion: Influenza tends to occur more in the winter time.


Epidemiology of the Hong Kong-68 Variant of Influenza A2 in Britain (1971)

Conclusion: Britain experiences increased influenza rates during winter time.


Seasonal and Annual Changes in the Calcium Metabolism of Man (1942)

Conclusion: It was observed and documented as early as 1942 that people’s calcium absorption rates reached their lowest level right after winter’s end and reached their highest level at the end of summer. It was speculated back then that this had something to do with seasonal fluctuations in the amount and strength of available sunlight and vitamin D, and, indeed, does correlate with more recent research into seasonal changes of vitamin D levels.


Also see studies on 

Latitude, Vitamin D Levels, and Disease Rates

Seasonal Changes in Vitamin D Levels


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