Academic Reading — Test 96

Reading passage

Vitamin D occupies an unusual position among the nutrients required for human health, because the body is able to manufacture most of what it needs rather than relying on food. The process begins in the skin, where a cholesterol-like compound is converted into an inactive form of the vitamin when the skin is struck by ultraviolet B radiation from the sun. This molecule then travels through the bloodstream to the liver and afterwards to the kidneys, where it is transformed in two further steps into the active hormone that the body can actually use. For this reason, vitamin D is often described as being more like a hormone than a conventional dietary nutrient, and sunlight rather than diet is its principal source for most people. The active form of the vitamin plays a central part in regulating the amount of calcium and phosphate circulating in the blood. These two minerals are the raw materials from which bone is built, and without sufficient vitamin D the intestine cannot absorb them efficiently from food. In growing children, a prolonged shortage leads to a condition known as rickets, in which the bones fail to harden properly. The long bones of the legs, which must bear the weight of the body, may bend under the strain, producing the bowed limbs that were once a familiar sight in poorer districts. In adults, the equivalent disorder is called osteomalacia, a softening of bone that causes pain and weakness but does not deform the skeleton in the same dramatic way. The quantity of vitamin D that the skin can produce depends heavily on how much ultraviolet B radiation reaches the ground, and this in turn varies with latitude. Near the equator, the sun stands high in the sky throughout the year and strong ultraviolet light is available in every season. At high latitudes, however, the sun sits low on the horizon for much of the year, and during the winter months its rays pass through so much of the atmosphere that almost all the ultraviolet B is filtered out before it arrives. In northern Europe, for example, the skin may synthesise little or no vitamin D for several months, a period sometimes called the vitamin D winter. Populations living far from the equator therefore face a recurring seasonal shortage that those in tropical regions rarely encounter. Skin colour adds a further layer to this geographical pattern. The pigment melanin, which gives skin its darker shades, is highly effective at absorbing ultraviolet radiation and thereby protects the deeper layers of the skin from damage. Yet the same pigment also reduces the amount of ultraviolet B available for vitamin D synthesis, so that darker skin requires considerably longer exposure to produce the same quantity of the vitamin as paler skin. Many scientists argue that the lighter skin tones found among peoples who settled at high latitudes evolved partly as an adaptation that allowed adequate vitamin D production under weaker sunlight. This hypothesis remains debated, but it offers a plausible link between the migration of early humans away from the tropics and the gradual lightening of their skin. The dangers of vitamin D deficiency became especially visible during the rapid growth of industrial cities in the nineteenth century. In smoke-filled towns at high latitudes, thick palls of coal smoke blocked much of the sunlight, and many children spent long hours indoors or in narrow streets where little direct light penetrated. Rickets became so common in such places that it was sometimes called the English disease, although it was by no means confined to one country. The eventual discovery that the disorder could be prevented either by sunlight or by certain foods, notably cod liver oil, marked an important turning point. During the twentieth century, the deliberate addition of vitamin D to everyday foods such as margarine and milk dramatically reduced the incidence of rickets in many wealthy nations. Despite these advances, vitamin D deficiency has not been eliminated, and in some communities it appears to be returning. Modern lifestyles often keep people indoors, while widespread advice to avoid the sun, motivated by concern about skin cancer, may unintentionally limit vitamin D production. Groups thought to be at particular risk include those with darker skin living at high latitudes, people who cover most of their skin for cultural reasons, the elderly, whose skin synthesises the vitamin less efficiently, and infants who are exclusively breastfed without supplements. Public health authorities in several countries now recommend that such groups take vitamin D supplements, especially during the darker months, recognising that sunlight alone cannot be relied upon to meet the body's needs everywhere.