Study reveals how UV exposure and ethnicity impact vitamin D levels, calling for tailored supplementation guidelines

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.May 8 2024

A recent study published in the journal Clinical Nutrition discusses the need to adjust for ultraviolet ray exposure and other factors that change desirable vitamin D levels depending on an individual’s ethnicity.

Study: Ambient ultraviolet-B radiation, supplements and other factors interact to impact vitamin D status differently depending on ethnicity: A cross-sectional study. Image Credit: Anna Martyanova / Shutterstock.com

Vitamin D

Vitamin D deficiency occurs in 13-40% of Europeans; however, it is more prevalent among Whites by up to 70 times.

Vitamin D is produced in the skin following exposure to ultraviolet B (UVB); however, the amount of vitamin D produced through this process depends on the individual’s skin color and age, as well as the intensity of incident solar radiation. Vitamin D supplementation and consuming fortified food products can also increase circulating vitamin D levels within the body.

UVB-induced dermal synthesis can meet 80–100% of vitamin D requirements, making sunshine the most important natural source of vitamin D for many.”

Once vitamin D enters the blood, it is metabolized to its active form, 25-hydroxyvitamin D (25(OH)D, the most accurate measure of vitamin D status in the body.

About the study

The United Kingdom Biobank cohort, which includes nearly 440,000 White, Black, and Asian individuals, was included in the current study. The median age of the study cohort was 58 years.

UVB exposure was estimated by linking data from the Tropospheric Emission Monitoring Internet Service to the participants’ addresses. The cumulative exposure to UVB over 135 days before blood samples were obtained was calculated. These values were analyzed for their association with circulating vitamin D levels, body mass index (BMI), and other variables in varying combinations for different ethnic groups.

The median UVB exposure was 90 kJ/m², whereas over 40% of the study cohort reported taking vitamin D supplements. Over 66% of the study participants were overweight or obese.

What did the study show?

Among Asians, median vitamin D levels were about 25 ng/mL compared to 31 and 48 ng/mL for Black and White participants, respectively.

Irrespective of ethnicity, UVB exposure was strongly associated with vitamin D levels. The increase was most obvious among Whites, at 35%, compared to 20% and 15% for Black and Asian participants, respectively. Males, younger individuals, those who spent more time outdoors, and those who were underweight or had a normal BMI increased the likelihood of higher vitamin D levels.

Among Whites, UVB exposure accounted for 14% of the variability in vitamin D levels, with an increase of one nmol/L for every 10 kJ/m² increase in exposure. These findings emphasize the importance of recommending healthy amounts of sun exposure, particularly to White individuals.

Black and Asian study participants had two to four times smaller increases in vitamin D with UVB exposure compared to Whites, likely due to increased skin pigmentation that blocks UVB availability. This observation might explain why vitamin D supplementation was the most important predictor of vitamin D levels among Asian participants.

Other predictors of increased vitamin D levels in non-Whites included age. Conversely, with increasing BMI and higher cholesterol levels, the vitamin D levels in Whites were often lower. The inverse association with BMI was observed across all groups, especially those using vitamin D supplements.

Interactions between the factors

Whites exhibited lower increases in vitamin D levels with increasing UVB exposure as they aged. Among all groups, males responded with more significant increases to UVB exposure, with Black males exhibiting twice as high of increases in vitamin D levels as Black females.

Among Whites, non-supplementation led to a greater increase in UVB exposure than supplementation, which was in direct contrast to the pattern observed among Black individuals. The reasons for this association remain unclear; however, it may suggest a feedback pathway, perhaps through liver synthesis, that limits the increase in vitamin D levels according to the baseline level.

Conclusions

Vitamin D deficiency was widespread in the U.K. Biobank cohort, particularly among non-White people. More specifically, about 50% of Asians, 33% of Black individuals, and 12% of Whites were vitamin D deficient.

The body’s ability to produce vitamin D appears to decline with age; therefore, higher vitamin D dosages are required for older people. Nevertheless, it is crucial also to prescribe adequate doses to people under 60. The dosage may require adjustment among heavier individuals and those with higher cholesterol levels, perhaps because vitamin D is stored in fatty tissues.

The findings are practically relevant, because improved prediction can enable a more nuanced approach to vitamin D supplementation and prevention of deficiency in diverse populations.”