Solar UV Doses of Adult Americans and Vitamin D₃ Production

Abstract

Background: Sunlight contains UV radiation that affects human health in both detrimental (skin cancers) and beneficial (vitamin D3) ways. An evaluation of the vitamin D status of adult Americans (22-40, 41-59, 60+ yr) show many have deficient or insufficient serum levels of 25-hydroxyvitamin D, indicating they are not getting enough from sunlight or dietary sources. Those findings are in conflict with calculated values that insist people make “ample” vitamin D3 (≥1,000 IU/day) from their “casual” or everyday outdoor exposures.

Objective: We investigated this situation using the outdoor UV doses of indoor-working adult Americans (~7,000) in the north (45°N) and south (35°N) to calculate how much vitamin D3 they produce each season of the year with and without vacationing.

Method: To do vitamin D3 calculations properly, we used action spectrum and geometric conversion factors, not previously incorporated into other calculations.

Results: Few indoor-working adults over 21 yr meet their daily vitamin D needs from everyday outdoor exposures anytime during the year, including summer. Only a few “outdoorsy” Caucasians (22-60 yr) can meet their vitamin D3 needs (~2,000 IU/day) during the summer, but only if they do not wear sunscreens except on beach vacations.

Conclusions: Most indoor-working adult Americans do not go outside enough to meet their vitamin D3 needs all year. Only some outdoor-working adults can make enough vitamin D3 without using sunscreen but other adults cannot, even with a vacation during the summer. Indoor-working African Americans can never make enough vitamin D3 during the year even with a 3-week vacation.

Disclosure of Potential Conflicts of Interest

D.E.G. and S.J.P. have nothing to disclose. W.B.G. gets funding from the UV foundation (McLean, VA), the Sunlight Research Forum (Veldhoven), Bio-Tech-Pharmacal (Fayetteville, AR), the Vitamin D Council (San Luis Obispo, CA), and the Danish Sunbed Federation. M.F.H. gets funding from the UV Foundation (McClean, VA).

Figures and Tables

Figure 1 The percent drop in the ability to make vitamin D₃ in human skin with age. We normalized values to 100% using the children's values (0–20 yr). The data plotted is the average of vitamin D₃ formed in the epidermis and dermis from 0–20 yr (100%), 22–40 yr (83%), 41–59 yr (66%) and 60+ yr (49%); calculated from the data in Figure 1 of MacLaughlin and Holick (1985). Note that some seniors over 70 yr make less than 25% of the vitamin D that children <21 yr can make.67

Figure 2 The average amount of vitamin D₃ (IU/day) indoor-working adult men and women of reproductive age (22–40 yr) with skin types II–V make from everyday outdoor UV exposures each season in the (A) Northern (45°N) and (B) Southern (35°N) US.

Figure 3 The average amount of vitamin D₃ (IU/day) indoor-working adult men and women of middle age (41–59 yr) with skin types II–V make from everyday outdoor UV exposures each season in the (A) Northern (45°N) and (B) Southern (35°N) US.

Figure 4 The average amount of vitamin D₃ (IU/day) indoor-working adult men and women seniors (60+ yr) with skin types II–V make from everyday outdoor UV exposures each season in the (A) Northern (45°N) and (B) Southern (35°N) US. Note that senior's ≥70 yr might only make about half the amount shown in the figure.

Figure 5 The average amount of vitamin D₃ made by indoor-working adults every day during the summer without taking a summer vacation compared to the amount made by indoor-working adults of different ages and skin types if they take a 1-, 2- or 3-week “mixed” summer vacation at latitude ∼40°N in the continental US. The dashed lines indicate the recommended IU/day of vitamin D for people under (600 IU/day) and people over 70 yr (800 IU/day). A mixed summer vacation is the average of four types of vacations: beach, country, sightseeing and home; (A) skin types II and III and (B) skin types IV and V.