Sunlight Deficiency: A Review of the Literature

Cynthia Good Mojab
Issue 117, March/April 2003

Making informed decisions about complex and controversial health issues, such as vitamin D supplementation of breastfed infants, is inherently challenging. When evaluating information, mothers may wish to consider the goals, potential biases, and sources of funding of health organizations, researchers, healthcare providers, and vitamin manufacturers; the depth, breadth, and limitations of the information on which public health policies are based; whether a recommendation might be out of date or applicable in only some situations; whether any conflicts of interest might be involved; and whether the organization or individual making the recommendation is in full compliance with the letter-and spirit-of the WHO/UNICEF International Code of Marketing of Breast-Milk Substitutes. 1, 2 A review of the related scientific literature, though essential, is just the beginning.

What Is Vitamin D?
Vitamin D is actually not a vitamin at all, but a steroid hormone produced in the body after direct exposure of the skin to ultraviolet B (UVB) radiation in sunlight. Vitamin D plays a critical role in the maintenance of proper blood calcium and phosphorous concentrations, and in bone mineralization by stimulating the absorption of calcium and phosphorous in the small intestine. It also acts as a chemical messenger in a wide variety of other biological responses.3

What Is Vitamin D Deficiency and What Are Its Consequences? In the absence of underlying organic causes, such as prematurity or liver or kidney disease, vitamin D deficiency is sunlight deficiency. Vitamin D deficiency can lead to bone disease: osteomalacia in adults, rickets in infants and children. Research has shown that higher latitude and lower vitamin D levels are related to several cancers, type 1 diabetes, and other diseases.4-6

How Do People Get Enough Vitamin D?
The direct, casual exposure of skin to sunlight is the most common and the biologically normal way that human beings attain sufficient levels of vitamin D. However, sunlight exposure for many people around the world has been reduced by industrialization, urbanization, migration, concern about skin cancer, and social inequities. Because only a few foods naturally contain significant levels of vitamin D (e.g., the oils and livers of some fatty fish), it would be unusual for people to obtain adequate vitamin D from their diet alone without supplementation or enrichment.7

The skin has a large capacity to produce vitamin D. Exposure of the entire adult body to the smallest amount of UVB radiation that produces transient, just perceptible skin reddening is comparable to taking an oral dose of 10,000 to 25,000 IU of vitamin D.8, 9 Therefore, sufficient levels of vitamin D can be developed from partial exposure of the body to sunlight well before sunburn occurs.

Levels of vitamin D vary seasonally among people exposed to sunlight at higher latitudes, where UVB radiation is higher in the summer and lower in the winter.10, 11 With inadequate summer exposure, vitamin D deficiency and insufficiency can result, particularly during the winter.12, 13 However, with adequate exposure to sunlight in the summer, vitamin D can be stored in the body for winter use.14 The lower vitamin D stores of the spring can be replenished with exposure to the higher UVB radiation of summer sunlight.

How Do Nurslings Get Enough Vitamin D?
The natural sources of vitamin D for nurslings are primarily the stores they developed prenatally (for newborns) and the vitamin D they produce with exposure of their skin to sunlight; a smaller additional contribution is from human milk.15, 16 The concentration of fat-soluble vitamin D in human milk varies from 5 to 136 IU/L, depending on how its activity is measured and on maternal vitamin D status during lactation.17-19 This concentration provides less than the 200 to 400 IU/day commonly recommended for infants under one year of age.20 However, human milk should not be considered "deficient" in vitamin D, because the biologically normal means of obtaining sufficient vitamin D in humans is via sunlight exposure, not diet.21-23

The neonate's stores of vitamin D depend on maternal vitamin D status during pregnancy.24, 25 A study of exclusively breastfed infants in Tampere, Finland (61° N) in winter showed that, without UVB exposure or vitamin D supplementation, vitamin D stores of fetal origin were depleted by eight weeks of age.26 Although these vitamin D-depleted infants had serum levels of vitamin D at which rickets can occur, none had active or biochemical rickets. The concentration of vitamin D in human milk increases significantly with what are currently considered pharmacological doses of vitamin D supplements.27, 28 Administration of 2,000 IU-but not 1,000 IU-to lactating mothers in another study normalized the 25-hydroxyvitamin D levels of their infants in winter.29 Supplementation with over 1,000 IU/d is currently considered to greatly exceed normal maternal vitamin D needs (200 IU/d).30