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

Diet, Sunlight, and 25-Hydroxy Vitamin D in Healthy Children and Adults. (2017)

Conclusion: 1970’s study of 110 white, British school children found that their vitamin D levels peaked at the end of summer, hit their nadir at the end of winter, and children who went on summer beach vacations had higher vitamin D levels months after.


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.


Vitamin D Status, Adiposity, and Lipids in Black American and Caucasian Children (2011)

Conclusion: 237 children had their BMI and vitamin D levels measured. 98% of the black and 90% of the white kids had low vitamin D levels.The study also found that the FATTER kids had lower vitamin D levels than leaner kids. Vitamin D-deficiency is known to be both a contributing factor TO and a symptom OF obesity. Vitamin D levels rose slightly in the summer.


A Prospective Analysis of Plasma 25-Hydroxyvitamin D Concentrations in White and Black Prepubertal Females in the Southeastern United States (2007)

Conclusion: This study of the vitamin D levels of young girls living in the southeastern U.S. found that 75% had levels below 32 ng/ml (experts recommend maintaining 50 ng/ml), 18% had levels below 20 ng/ml, the white girls had higher vitamin D levels than the black girls, and vitamin D levels fluctuate with the seasons. The study also found that younger girls have higher vitamin D levels than older girls, which may be due to the increased ability of younger people to cutaneously synthesize vitamin D.


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.


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.


Vitamin D Deficiency in Adolescent Females (2005)

Conclusion: The vitamin D levels of 370 adolescent girls living in the north eastern U.S. were analyzed. The average vitamin D level of all the girls was only a low 21 ng/ml, 17% were found to be severely deficient, and 54% had levels below 20 ng/ml. The average level of the black girls was only 17 ng/ml, with non-black girls having a higher average level of 28 ng/ml (which is still bad). The vitamin D levels were higher in the spring and summer and lower in the fall and winter.


Vitamin D is Associated with Improved Survival in Early-Stage Non Small Cell Lung Cancer (2005)

Conclusion: Study of 456 early-stage lung cancer patients found that those who underwent their removal surgery in the summer, when population vitamin D levels are highest, had less than half of the cancer recurrence rate as those who had their surgeries in the winter, when vitamin D levels are lowest. This study was conducted in Boston, where it is physically impossible to produce vitamin D from sun exposure for nearly half the year during winter. Those who had their surgeries in the summer also had 4x the overall survival rate compared to the winter group.


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 In Internal Medicine Residents (2004)

Conclusion: This study measured the vitamin D levels of 35 medical residents before and after winter. 74% of them had low vitamin D levels in the spring following winter versus only 26% in the fall following summer. 20% had low levels for both testing periods and 51% had low levels for at least one of the testing periods. Once again the deficiency rates in this 2004 study are actually much worse because the definition of vitamin D inadequacy here is only <20 ng/ml, which has since been updated by vitamin D experts to <50 ng/ml. The conclusion of the researchers is that the medical residents have such bad levels because they work long hours indoors and don’t get much sun exposure.


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.


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.


Vitamin D Insufficiency in a Population of Healthy Western Canadians (2002)

Conclusion: 60 Canadian adult men and 128 Canadian adult women living in Calgary had their vitamin D blood levels measured four times in one year during each season. 97% of them were found to have inadequate vitamin D levels in at least one of the four testings, with 34% being severely deficient in at least one of the four testings. The vitamin D levels were highest in the spring and summer and lowest in the fall and winter.


Relation Between Vitamin D, Physical Performance, and Disability in Elderly Persons (2002)

Conclusion: 269 elderly people living in retirement homes were evaluated for their vitamin D levels their physical performance in isometric strength tests and a 6-minute walking test. The rate of vitamin D-deficiency was significantly higher in the women than in the men and vitamin D status was significantly correlated with physical performance in the female subjects. Among the women, those with the lower vitamin D levels demonstrated lower arm and leg muscle strength and reported more disability than those with higher vitamin D levels. The study also found that vitamin D levels were significantly lower in the winter for everyone.


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.


Vitamin D Status and Its Adequacy in Healthy Danish Perimenopausal Women: Relationships to Dietary Intake, Sun Exposure and Serum Parathyroid Hormone (2001)

Conclusion: 2.5 year-long study of 2,016 healthy, white menopausal women in Denmark concludes that menopausal women who avoid summer sun exposure and do not supplement with vitamin D are prone to vitamin D insufficiency. Those who intentionally exposed themselves to regular sun exposure had the highest levels, those who reported occasional sun exposure had lower levels, and those reported actively avoiding sun exposure had the lowest levels. Active sunbathing was correlated more highly with vitamin D status than supplementation.The study also found that their vitamin D levels spiked significantly in the summer.


Vitamin D Intake is Low and Hypovitaminosis D Common in Healthy 9- to 15-Year-Old Finnish Girls (1999)

Conclusion: This one-year study followed 186 young caucasian Finnish girls aged 9-15, tracking their dietary habits, physical activity habits, measured their vitamin D levels multiples times throughout the year, and tested the effectiveness of supplementing with 400 IU of vitamin D daily for preventing deficiency. The vitamin D level of the group at the beginning of the study in winter was a horribly low 13.5 ng/ml.The group vitamin D level rose significantly during summer to 25 ng/ml, which is still inadequate (and in spite of taking 400 IU per day). At the 12-month testing in the next winter, after taking 400 IU of vitamin D most days, the vitamin D level of the group had barely increased and was still at the deficient level. The findings of this study are: vitamin D levels rise in the summer, vitamin D levels fall in the winter, and not only is 400 IU of vitamin D incapable of preventing vitamin D-deficiency in young girls, this dose barely increases vitamin D levels in young girls at all. Also, even during summer they failed to achieve adequate vitamin D levels. This is probably due to Finland’s very high latitude.


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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