Vitamin D Deficiency and Its Health Consequences in Africa
Total Page：16
File Type：pdf, Size：1020Kb
View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Springer - Publisher Connector Clinic Rev Bone Miner Metab (2009) 7:94–106 DOI 10.1007/s12018-009-9038-6 ORIGINAL PAPER Vitamin D Deficiency and Its Health Consequences in Africa Ann Prentice Æ Inez Schoenmakers Æ Kerry S. Jones Æ Landing M. A. Jarjou Æ Gail R. Goldberg Published online: 28 April 2009 Ó Humana Press Inc. 2009 Abstract Africa is heterogeneous in latitude, geography, and potential health consequences in Africa. The reader is climate, food availability, religious and cultural practices, referred to other articles that discuss in detail some of the and skin pigmentation. It is expected, therefore, that general background to vitamin D, including evolution, prevalence of vitamin D deficiency varies widely, in line biologic functions and dietary requirements; photobiology; with influences on skin exposure to UVB sunshine. Fur- assays; pregnancy and lactation; calcium and vitamin D thermore, low calcium intakes and heavy burden of deficient rickets and children’s bone health; dietary sour- infectious disease common in many countries may increase ces; sun deprivation and health consequences. Where vitamin D utilization and turnover. Studies of plasma appropriate these are also considered here because they are 25OHD concentration indicate a spectrum from clinical particularly relevant to, or may be different in, an African deficiency to values at the high end of the physiological context. range; however, data are limited. Representative studies of Vitamin D status and supply is influenced by sunshine status in different countries, using comparable analytical exposure, diet and underlying health conditions. People techniques, and of relationships between vitamin D status acquire vitamin D by cutaneous synthesis following and risk of infectious and chronic diseases relevant to the exposure to sunlight, and from the diet. It is not possible to African context are needed. Public health measures to generalize about the proportions contributed by different secure vitamin D adequacy cannot encompass the whole sources because this varies considerably around the world, continent and need to be developed locally. across population groups, and between individuals, mainly because of wide differences in skin exposure to ultraviolet Keywords Rickets Á Osteomalacia Á Vitamin D status Á B (UVB) radiation, the efficiency of cutaneous synthesis, 25-hydroxyvitamin D Á Vitamin D turnover Á dietary supplementation and food fortification practices [1]. Calcium intake Á Sunlight Á Children Reliance on dietary sources of vitamin D is greatest during the winter at latitudes outside the tropics ([30°N and [30°S) and among people with restricted skin sunshine Introduction exposure [1, 2]. The endogenous supply of vitamin D3 (cholecalciferol) depends on the skin’s exposure to UVB This article addresses the prevalence of vitamin D defi- radiation at wavelengths of 290–315 nm, the efficiency of ciency and issues that need to be considered when con- cutaneous vitamin D synthesis, and the extent to which sidering vitamin D status (supply, sufficiency, utilisation) vitamin D is degraded within the skin [3–5]. The quantity of UVB radiation of the relevant wavelengths that reaches the skin depends on many factors (see section ‘‘Ultraviolet A. Prentice (&) Á I. Schoenmakers Á G. R. Goldberg radiation and skin pigmentation’’) including time of day, Human Nutrition Research, MRC, Cambridge, UK and, at latitudes north and south from approximately 30°N e-mail: [email protected] and 30°S, on month of the year. In the summer months, A. Prentice Á K. S. Jones Á L. M. A. Jarjou Á G. R. Goldberg however, the amount of UVB containing wavelengths that MRC Keneba, MRC, Keneba, The Gambia can generate vitamin D synthesis is the same at all latitudes Clinic Rev Bone Miner Metab (2009) 7:94–106 95 [6]. In addition, even when there is abundant sunshine, the of the world’s population. Africa comprises more than 50 extent of UVB skin exposure depends on clothing, living countries ranging in landmass from 2.4 million km2 and working environments and sunscreen use [7]. (Sudan) to 455 km2 (The Seychelles) (www.fao.org). The status marker for vitamin D, plasma 25 hydroxy Individual counties are not only very diverse with respect vitamin D (25OHD) concentration, depends on factors that to size but also population ranging from 145,000,000 determine the balance between supply and turnover [8]. (Nigeria) to 86,000 (The Seychelles) and the proportion of Supply depends on factors including host, dietary and the population that live in urban areas (10% Burundi; 84% environmental factors. Turnover of 25OHD and 1,25 Gabon; average 38% in WHO Africa Region) [14]. Across dihydroxy vitamin D (1,25(OH)2D) in renal and extra-renal the continent as a whole *40% of Africans are Christian, tissues depends on many factors, including nutrient intake *40% are Muslim and *20% follow other religions (of which calcium is the most important), protein-energy (including indigenous African religions, Hinduism and malnutrition, body composition, high disease burden and Judaism). There is a great deal of variation within and innate characteristics such as polymorphisms in the VDR between countries and population groups. Some religious and DBP genes [8–12]. beliefs and traditional practices can impact on customary dress and dietary habits. The Continent of Africa Ultraviolet Radiation and Skin Pigmentation Geography, Topography, and Climate The factors that affect the amount of ambient UVB radia- tion include stratospheric ozone levels, cloud cover, lati- Africa is the world’s second largest continent after Asia, tude, season, and lower atmospheric pollution. The factors occupying an area of about 30 million km2 including adja- that affect an individual’s UVB exposure and biological cent islands. Thus, Africa represents about 20% of the earth’s effects of UVB include sun-seeking and sun protective total land area. It is the only continent that straddles the behaviours, skin pigmentation, and cultural dress and equator and has both northern and southern temperate zones. behaviour [15]. The concentration of atmospheric ozone The distance between the most northerly (*37°N in Tunisia) that is able to absorb UVB, and the amount and spectral and most southerly (*34°S in South Africa) points is structure of radiation reaching the body is dependent on the *8000 km. From the westernmost (*17°W, Cape Verde) angle at which the sun’s rays pass through the atmosphere; to the easternmost (*51°E in Somalia) points is *7400 km. at lower latitudes, closer to the equator there is more Some of the larger African countries are distributed across intense radiation. At higher altitudes, UVB radiation is more than one latitude band. The climate ranges from trop- increased because there is less mass of air for solar radia- ical to subarctic and the topography includes deserts, tion to pass through. Time of day, season and presence of mountains, grassland plateaus, lakes and rivers. Much of clouds, dust, haze and various organic compounds can alter northern Africa is predominantly desert or arid land, and the intensity of incident solar radiation. The UV Index, the central and southern areas contain deserts, savannah plains maximum UV level on a given day during the 4-h period and dense rainforest. All these features affect the available around solar noon, varies according to time and place, and water supply, food sources, population distribution and the with season and latitude. Levels of total annual UV radi- sorts of clothing worn (e.g. for warmth, protection against the ation (UVR) vary approximately fourfold globally, but sun, wind, rain and sand). The weather in Africa is also very within any area there can be a tenfold difference in per- variable, and temperatures vary greatly throughout the con- sonal exposure related to behavioural and cultural factors. tinent. Many countries have wet and dry seasons which in Individual exposure within population groups may vary turn affect food availability, farming activities and therefore from one tenth to ten times the mean exposure in a par- levels of physical activity and weight, and body composition ticular location. changes, morbidity and mortality, including incidence of low Different skin pigmentations are thought to have birthweight infants, and the prevalence and incidence of developed to protect against high ambient UVR whereby malaria. About 74% of people in Africa live in areas that are those inhabiting low latitudes with high UVR have darker highly endemic for malaria, 19% in epidemic-prone areas pigmentation to protect against deleterious effects, whilst and only 7% in low-risk or malaria-free areas [13]. those inhabiting higher latitudes developed fair skin to maximize vitamin D production from much lower ambient Demography UVR. However, in recent history, migration of people of different skin types means that skin pigmentation is not Africa is the second most populated continent after Asia necessarily so suited to the environment. For the purposes with more than 900 million people who account for *14% of assessing the effects of UVR on different skin 96 Clinic Rev Bone Miner Metab (2009) 7:94–106 pigmentations, a scale classifying skin types for UVR studies in the literature investigating cutaneous synthesis in sensitivity is used which comprises six skin types [16, 17]. indigenous African people. This is often broken down into three broader skin pig- On the African continent, seasonality in cutaneous mentation groups [15]: lightly pigmented (Caucasian fair, vitamin D synthesis would be expected in countries such as medium and darker skin, skin types I–IV), intermediate Morocco, Tunisia, Algeria, Libya, Egypt, and South Africa pigmentation (Asian or Indian skin, skin type V), and which lie at latitudes [30°N and [30°S[23]. This is sup- deeply pigmented (Afro-Caribbean or black skin, skin type ported by studies in hip fracture patients in South Africa VI).