The recent review in this journal serves as a good introduction to the controversies surrounding scientific findings and policy recommendations regarding vitamin D, its health benefits, requirements for optimal health, and how to obtain adequate vitamin D1. The literature is full of contradictory findings, opinions, and recommendations, so unless one has studied it carefully for an extended period making sense of it at face value is difficult. The crux of the problem is that, because of inherent or practical limitations, different types of studies can arrive at different conclusions. This commentary on the work of Gilaberte and colleagues offers more insight into how to assess and interpret the literature on vitamin D and UV irradiance.

There are several ways of determining the role of vitamin D deficiency in the risk of disease. Ecological studies—which investigate the relationship between geographical variation in disease outcomes and the risk-modifying factors for each geographical unit of population or seasonal variation—can be useful in identifying and quantifying relationships between solar UV-B doses and disease outcome. The ecological approach has linked low solar UV-B doses to about 20 types of cancer.2 The time-varying ecological study approach has linked solar UV-B to reduced risk of influenza.3

Observational studies can be either case–control studies, which measure serum 25-hydroxyvitamin D [25(OH)D] concentrations at time of diagnosis, or nested case–control studies derived from cohort studies. Cohort studies compare patients who have the disease with similar healthy controls, grouping the disease by quantiles of 25(OH)D concentration. Case–control studies find stronger inverse correlations between 25(OH)D concentration and disease incidence because nested case–control studies measure vitamin D status with a single serum 25(OH)D concentration at time of enrollment; over time, this sole measure becomes less reliable.4

Cross-sectional studies are snapshots of health conditions and biometric values made by random sampling of the population.

In randomized controlled trials (RCTs), some participants receive the agent and others a placebo. Participants are followed up for weeks to years, and health outcomes, both positive and negative, are noted. Using RCTs to study the effects of vitamin D on disease outcome poses many problems: the dose is often inadequate, compliance may be poor, other sources of vitamin D exist, and serum 25(OH)D response to oral vitamin D intake varies considerably from person to person.5

The health benefits of vitamin D extend from better pregnancy and birth outcomes14 to reduced risk of many types of cancer,2 cardiovascular disease,7,16,17 diabetes,16 respiratory infections,3,8 and many other conditions and diseases (for more information see http://www.vitamindcouncil.org/health-conditions/). The 25(OH)D concentration–disease outcome relations have been used to estimate how raising population mean serum 25(OH)D concentrations from 50–55nmol/L to 100–110nmol/L would reduce mortality rates for countries and continents. The study for the United States found that doing so could avoid approximately 400000 premature deaths per year,18 representing 17% of all deaths. A worldwide study found mortality rate reductions of 8%–17%, increasing worldwide life expectancy by 2 years.7

Solar UV-B irradiance is the primary source of vitamin D for most people on Earth. Skin pigmentation has adapted to the prevailing solar UV doses in places where humans have lived for thousands of years. As descendants of a long line of Spaniards, modern Spaniards’ skin is well adapted to solar radiation in Spain: dark enough to protect against the adverse effects without sunscreen yet light enough to permit adequate production of vitamin D.19 One adaptation to life in mid-latitudes is the ability to tan, which increases protection against UV irradiance by a factor of 2–4.19

Several studies offer evidence that Spaniards can and should experience solar UV-B with strong benefits and limited risks. In an ecological study of cancer mortality rates in Spain during 1978–1992, nonmelanoma skin cancer mortality rates were inversely correlated with 15 types of cancer, including melanoma for females.20 Mortality rates for nonmelanoma skin cancer were much lower than the all-cancer mortality rate.

Many people are concerned about the risk of melanoma from solar UV irradiance. However, those exposed to the sun occupationally have no greater risk of developing melanoma than those who are not so exposed.21 Risk of melanoma is associated with intermittent UV irradiance21 and sunburn.

Another risk factor associated with solar UV irradiance is development of basal cell carcinoma or squamous cell carcinoma. A study in Denmark found that those who developed basal cell carcinoma had a lower mortality rate than those who did not (10-year RR=0.91 [95% confidence interval, 0.89–0.92]).22

Producing vitamin D from solar U-VB irradiance in the summer is easy. The important considerations are solar zenith angle (for optimal production, the sun should be within 45 degrees of being overhead), the amount of surface area exposed, time in the sun, and age. With whole-body exposure, one can make at least 10,000IU of vitamin D3 in less time than it would take to develop erythema (redness).

This review offers considerable evidence that higher serum 25(OH)D concentrations would greatly reduce the risk of disease and increase life expectancy in Spain. It would be worthwhile for health policy makers in Spain to review the evidence, with input from UV irradiance and vitamin D researchers in Spain and elsewhere, and then make recommendations for public policy.

Dr. William Grant has received funding from the UV Foundation (McLean, VA), Bio-Tech Pharmacal (Fayetteville, AR), the Vitamin D Council (San Luis Obispo, CA), and the Vitamin D Society (Canada).