Here’s a big list of studies about the documented seasonal changes in population vitamin D levels all around the world:

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.


Adolescent Girls in Maine Are At Risk for Vitamin D Insufficiency (2005)

Conclusion: 23 young Maine girls were part of a six-month vitamin D study. The mean vitamin D level dropped 28% during winter.


Vitamin D Deficiency and Seasonal Variation in an Adult South Florida Population (2005)

Conclusion: Even in South Florida vitamin D deficiency is a problem. 40% of 99 tested residents were deficient.


Prevalence of vitamin D deficiency among healthy adolescents (2004)

Conclusion: The vitamin D levels of 307 Boston teenagers were found to be 24% lower in the winter compared to summer.


Vitamin D Insufficiency Among Free-Living Healthy Young Adults (2002)

Conclusion: A study of 307 young adults in Boston found that incidences of vitamin D deficiency rose significantly in the winter.


Hypovitaminosis D in Healthy Schoolchildren (2001)

Conclusion: Vitamin D levels for pre-teen and teenage students at three different private schools in Lebanon were checked at the end of summer and at the end of winter. 65% were deficient at the end of winter, 40% were deficient at the end of summer.


Seasonal Changes in Plasma 25-Hydroxyvitamin D Concentrations of Young American Black and White Women. (1998)

Conclusion: This study checked the vitamin D levels of 51 black and 39 white Boston women (ages 20-40 years) four times over the course of one year at each season. At all time points, the black women had lower vitamin D levels. All women had the lowest levels at the end of winter and the highest levels during summer. Although both white and black women experienced rising vitamin D levels from spring to summer, the rise was smaller for black women.


Seasonal Prevalence of Vitamin D Deficiency in Institutionalized Older Adults (1997)

Conclusion: 155 elderly residents at three different Toronto nursing homes were found to have much lower vitamin D levels at the end of winter compared to the end of summer. Blood samples from March revealed that 60% of the patients were deficient.


Seasonal Variation in Vitamin D (1994)

Conclusion: People’s vitamin D levels tend to peak around the end of summer and significantly decrease throughout winter.


Effect of Vitamin D Intake on Seasonal Variations in Parathyroid Hormone Secretion in Postmenopausal Women (1989)

Conclusion: The parathyroid hormone levels of postmenopausal women peak following the end of winter and hit their lowest levels following the conclusion of summer. This strongly correlates with documented seasonal changes in population vitamin D levels.


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.


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.


Seasonal Changes in Serum 25-Hydroxyvitamin D Concentrations Among Asian Immigrants (1982)

Conclusion: Asians immigrants in the UK suffer lower vitamin D levels in winter.


Diet, Sunlight, and 25-Hydroxyvitamin D in Healthy Children and Adults (1979)

Conclusion: Vitamin D levels were higher after summer and lower after winter.


Seasonal Variation in Serum 25-Hydroxycholecalciferol in Healthy People (1978)

Conclusion: Vitamin D levels were checked in 232 South African elderly femoral neck fracture patients. The blood tests performed closest to the hospital admissions found that the majority of them were deficient. Further study and follow up found that their vitamin D levels were highest in the summer and autumn months.


Also see studies on

Winter Sickness Rates

Latitude, Vitamin D Levels, and Disease Rates


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