How do your lifestyle habits effect your vitamin D levels? Here are some studies on it:

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.


Vitamin D Status Indicators in Indigenous Populations in East Africa (2012)

Conclusion: The vitamin D levels of five different traditionally living indigenous African populations were measured, all ages and both sexes. These are people who spend a lot of time out in the tropical sun. Despite their very dark skin (which can block 90+% of UVB necessary for vitamin D production) the average vitamin D level was about 46 ng/ml, which is substantially higher than levels recorded in western populations. Troubling though, is that despite being at the equator and receiving so much sun exposure, their levels were still just under what most vitamin D experts now consider to be an optimal level. Besides their dark type VI skin, another stated explanation for their sub-optimal vitamin D levels is that they, as with the Hadzabe and Maasai tribes, “avoid direct exposure to the fierce sun whenever possible, and most of their activities are planned in the early morning and late afternoon, while spending the middle part of the day sleeping, eating, or talking in a cooler place under a tree or rock.” By avoiding sunlight and the UVB window at the hottest part of the day, even at the equator vitamin D levels are suboptimal. One woman, however, did have a very high level of 105 ng/ml.


The Relation Between Sunscreen Layer Thickness and Vitamin D Production After Ultraviolet B Exposure: A Randomized Clinical Trial (2012)

Conclusion: 37 light-skinned people applied varying thicknesses of SPF 8 sunscreen to their upper body and were exposed to a standard dose of UVB irradiation four times over a two week period. Vitamin D levels were measured before and after. The study found that those who were applying the thickest application of the sunscreen (which is also the recommended dose) experienced an insignificant increase in their vitamin D levels. Thinner applications of sunscreen resulted in better vitamin D production, with vitamin D levels increasing exponentially for the lowest doses of applied sunscreen.


Prevalence of Vitamin D Deficiency Among Adult Population of Isfahan City, Iran (2011)

Conclusion: This study analyzed the vitamin D levels of 1,111 healthy people in Isfahan, Iran. 50.8% were deficient, with 26.9% being severely deficient. Women were especially found to have low vitamin D levels due to the increased amount of clothing they’re required to wear under Islamic law. “Exposure to sun is limited due to the type of clothing required by current law.”


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.


Vitamin D, Adiposity, and Calcified Atherosclerotic Plaque in African-Americans (2010)

Conclusion: A study of 340 black adults with type-2 diabetes found that those with the highest percentage of body fat also had the lowest vitamin D levels and the highest amount of carotid artery and aorta plaque. There is a correlation between high body fat and low vitamin D levels and a correlation between low vitamin D levels and increased risk for atherosclerosis.


Prevalence and Associations of 25-Hydroxyvitamin D Deficiency in US Children: NHANES 2001-2004 (2009)

Conclusion: A 3-year study of 9,757 U.S. children and young people found that: 61% had low vitamin D levels, those who spent more time watching/using TVs and computers had lower vitamin D levels, obese children had lower vitamin D levels, and those with lower vitamin D levels had higher blood pressure.


Vitamin D Status and Parathyroid Hormone in Obese Children Before and After Weight Loss (2007)

Conclusion: Obese children were found to have lower vitamin D levels than non-obese children. In the obese children, losing weight resulted in an increase of their vitamin D levels.


Vitamin D From Dietary Intake and Sunlight Exposure and the Risk of Hormone-Receptor-Defined Breast Cancer (2007)

Conclusion: Canada study where researchers compared vitamin D intake and lifetime sun exposure habits between 759 breast cancer patients and 1,135 healthy, matched controls. The study found that women with a higher vitamin D intake and increased lifetime sun exposure enjoy a significantly lower risk of estrogen-receptor positive and progesterone-receptor positive breast cancers, and a slightly lower risk for estrogen and progesterone negative breast cancers. For sun exposure specifically, they found that increased frequency of outdoor activities and ever having an outdoor job between the ages of 10 and 29 was associated with lower breast cancer risk, suggesting that breast cancer could possibly be due to developmental defects caused by early life vitamin D deficiency. The study also found that women who reported exposing more naked skin to the sun enjoyed a lower breast cancer risk. Early life sunburns, while not good for skin cancer risk, are actually associated with lower breast cancer risk.


Vitamin D Status and Glucose Homeostasis in the 1958 British Birth Cohort: The Role of Obesity (2006)

Conclusion: Vitamin D levels and BMI were measured in 7,198 adult caucasian subjects in the 45-year British birth cohort survey. The vitamin D levels of the obese subjects were found to be lower than the non-obese subjects.


Tanning is Associated with Optimal Vitamin D Status (Serum 25-Hydroxyvitamin D Concentration) and Higher Bone Mineral Density (2004)

Conclusion: A study of 50 people who use tanning beds at least once a week were found to have vitamin D levels almost twice as high, significantly higher bone density, and lower parathyroid hormone levels compared to 106 control subjects.


The Relationship between Obesity and Serum 1,25-Dihydroxy Vitamin D Concentrations in Healthy Adults (2004)

Conclusion: This study of 152 obese people and 148 non-obese people found that the obese people had lower vitamin D levels.


Up-Regulatory Impact of Boron on Vitamin D Function – Does it Reflect Inhibition of 24-Hydroxylase? (2004)

Conclusion: There is evidence that the mineral boron improves vitamin D levels by slowing down catabolization of vitamin D, improving it’s metabolism, and making it last longer in your body. Multiple studies have found that increasing boron intake increases vitamin D levels.


Vitamin D3 from Sunlight May Improve the Prognosis of Breast-, Colon- and Prostate Cancer (2004)

Conclusion: 115,096 cases of breast, colon, and prostate cancer diagnosed over a 28-year period were observed. Researchers found that patients diagnosed in the summer and fall, when population vitamin D levels are highest, had the lowest cancer fatality rate. “The results suggest that a high level of vitamin D3 at the time of diagnosis, and thus, during cancer treatment, may improve prognosis of the three cancer types studied.” The study also found that men and women who received medium to high levels of occupational sun exposure had an extraordinarily lower cancer fatality rate compared to people with low occupational sun exposure.


Body Fat Content and 25-Hydroxyvitamin D Levels in Healthy Women (2003)

Conclusion: In a study involving 410 women, women with higher total body fat percentages had lower vitamin D levels.


Sunlight and Mortality from Breast, Ovarian, Colon, Prostate, and Non-Melanoma Skin Cancer: A Composite Death Certificate Based Case-Control Study (2002)

Conclusion: American cancer deaths and matched non-cancer control deaths in 24 American states were evaluated for lifetime sun exposure by evaluating the subjects based on the solar region they lived their lifetime in (state) and their occupation (how much time their job had them in the sun versus indoors) – all this info was collected from their death certificates. Researchers found that the people who received the most lifetime sun exposure enjoyed significantly lower rates of internal cancers, and these associations were stronger in the states with more intense sunlight. These people also had the highest rates of non-melanoma skin cancer, but that’s a good trade. These associations were the strongest for female breast and colon cancer, but also significant in ovarian and prostate cancer.


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.


Decreased Bioavailability of Vitamin D in Obesity (2000)

Conclusion: In vitamin D experiments carried out on a group of 19 obese people and a group of 19 normal weight people, the obese group experienced 57% lower vitamin D level increases following UVB irradiation compared to the non-obese group.


The Influence of Smoking on Vitamin D Status and Calcium Metabolism. (1999) (Download)

Conclusion: This study evaluated 510 middle-aged women, 50% of whom were smokers, comparing the vitamin D levels and bone mineral densities between smokers and non-smokers. The smokers had lower vitamin D levels and lower bone mineral density compared to the non-smokers. “Smoking has a significant effect on calcium and vitamin D metabolism.”


Chronic Sunscreen Use Decreases the Concentration of 25-Hydroxyvitamin D: A Preliminary Study (1998)

Conclusion: Vitamin D levels were measured in 20 long-term users of PABA-containing sunscreen and compared with the vitamin D levels of 20 age-matched controls who don’t use sunscreen. The average vitamin D level of the sunscreen group was a very poor 16 ng/ml, whereas the control group had a much better (and more than double) level of 36 ng/ml. The sunscreen group also had two cases of severe deficiency, but the control group had no cases of severe deficiency.


Clothing Prevents Ultraviolet-B Radiation-Dependent Photosynthesis of Vitamin D3 (1992)

Conclusion: This experiment had people wear full-body outfits made from various types of clothing fabric (wool, cotton, and polyester) to see whether or not a person can synthesize vitamin D from sunlight while wearing clothes. The results were that ALL of the fabrics completely prevented any vitamin D production following 40 minutes of sun exposure. More clothing = less vitamin D. Get naked!


Sunscreens Suppress Cutaneous Vitamin D3 Synthesis (1987)

Conclusion: Sunscreens reduce vitamin D production.


Evidence for Alteration of the Vitamin D-Endocrine System in Obese Subjects (1985)

Conclusion: The study compared the serum pre-vitamin D levels of a group of obese individuals to the levels of a group of normal weight individuals and found the obese group to have significantly lower pre-vitamin D levels.


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