Childhood and maternal undernutrition
Many people in the developing world, particularly women and children, continue to suffer from undernutrition. The poor especially often suffer from a basic lack of protein and energy, the adverse health effects of which are frequently compounded by deficiencies in micronutrients, particularly iodine, iron, vitamin A and zinc. Another important risk factor is lack of breastfeeding.
The theoretical minimum exposure and measured adverse outcomes for this group of risk factors are shown in Table 4.2. Each of these factors is discussed separately below and some summary results are shown graphically in Figure 4.2a and Figure 4.2b .
Undernutrition, defined in public health by poor anthropometric status, is mainly a consequence of inadequate diet and frequent infection, leading to deficiencies in calories, protein, vitamins and minerals. Underweight remains a pervasive problem in developing countries, where poverty is a strong underlying determinant, contributing to household food insecurity, poor child care, maternal undernutrition, unhealthy environments, and poor health care. All ages are at risk, but underweight is most prevalent among children under five years of age, especially in the weaning and post-weaning period of 6--24 months. WHO has estimated that approximately 27% (168 million) of children under five years of age are underweight (2). Underweight is also common among women of reproductive age, especially in Africa and South Asia, where some prevalence estimates of undernutrition are as high as 27--51% (3).
Underweight children are at increased risk of mortality from infectious illnesses such as diarrhoea and pneumonia (4). The effects of undernutrition on the immune system are wide-ranging, and infectious illnesses also tend to be more frequent and severe in underweight children. A child's risk of dying from undernutrition is not limited to those children with the most severe undernutrition. There is a continuum of risk such that even mild undernutrition places a child at increased risk. Since mild and moderate undernutrition are more prevalent than severe undernutrition, much of the burden of deaths resulting from undernutrition is associated with less severe undernutrition. These analyses indicate that 50--70% of the burden of diarrhoeal diseases, measles, malaria and lower respiratory infections in childhood is attributable to undernutrition. Chronic undernutrition in the first two to three years of life can also lead to long-term developmental deficits (5). Among adolescents and adults, undernutrition is also associated with adverse pregnancy outcomes and reduced work capacity.
Underweight was estimated to cause 3.7 million deaths in 2000. This accounted for about 1 in 15 deaths globally. About 1.8 million deaths occurred in Africa, 1.2 million in SEAR-D and 0.5 million in EMR-D, accounting for 10--20% of deaths in these regions. The disease burden occurred about equally among males and females. Since deaths from undernutrition almost all occur among young children, the loss of healthy life years is even more substantial: about 138 million DALYs, 9.5% of the global total, were attributed to underweight. These estimates of burden resulting from underweight, together with those given below for micronutrient deficiences, are consistent with previous estimates of over half of childhood deaths in developing countries being caused by undernutrition (6).
Iodine deficiency is likely to be the single most common preventable cause of mental retardation and brain damage. "Endemic cretinism", the form of profound mental retardation most closely identified with iodine deficiency, represents the severe end of a broad spectrum of abnormalities collectively referred to as iodine deficiency disorders. Iodine deficiency has also been associated with lower mean birth weight and increased infant mortality, hearing impairment, impaired motor skills, and neurological dysfunction. Iodine deficiency is controlled through direct supplementation with oral or intramuscular iodized oil, addition of iodine to a vehicle such as irrigation water, or most commonly iodization of salt. Over 2.2 billion people in the world may be at risk for iodine deficiency, and recent estimates suggest over one billion experience some degree of goitre (7,9). Globally, iodine deficiency disorders were estimated to result in 2.5 million DALYs (0.2% of total). Approximately 25% of this burden occurred in AFR-E, 17% in SEAR-D and 16% in EMR-D.
Iron is required in all tissues of the body for basic cellular functions, and is critically important in muscle, brain and red blood cells. Anaemia is simple to measure and has been used as the hallmark of iron deficiency severe enough to affect tissue functions. However, iron deficiency is not the sole cause of anaemia in most populations. Even in an individual, anaemia may be caused by multiple factors.
Iron deficiency is one of the most prevalent nutrient deficiencies in the world, affecting an estimated two billion people (10). Young children and pregnant and postpartum women are the most commonly and severely affected because of the high iron demands of infant growth and pregnancy. Iron deficiency may, however, occur throughout the life span where diets are based mostly on staple foods with little meat intake or people are exposed to infections that cause blood loss (primarily hookworm disease and urinary schistosomiasis).
About one-fifth of perinatal mortality and one-tenth of maternal mortality in developing countries is attributable to iron deficiency. There is also a growing body of evidence indicating that iron deficiency anaemia in early childhood reduces intelligence in mid-childhood. In its most severe form, this will cause mild mental retardation. There is also evidence that iron deficiency decreases fitness and aerobic work capacity through mechanisms that include oxygen transport and respiratory efficiency within the muscle.
In total, 0.8 million (1.5%) of deaths worldwide are attributable to iron deficiency, 1.3% of all male deaths and 1.8% of all female deaths. Attributable DALYs are even greater, amounting to the loss of about 35 million healthy life years (2.4% of global DALYs). Of these DALYs, 12.5 million (36%) occurred in SEAR-D, 4.3 million (12.4%) in WPR-B, and 10.1 million (29%) in Africa.
Vitamin A deficiency
Vitamin A is an essential nutrient required for maintaining eye health and vision, growth, immune function, and survival (11). Several factors, often acting together, can cause Vitamin A deficiency: low dietary intake, malabsorption, and increased excretion associated with common illnesses. Severe vitamin A deficiency can be identified by the classic eye signs of xerophthalmia, such as corneal lesions. Milder vitamin A deficiency is far more common. While its assessment is more problematic, it can be gauged by serum retinol levels and reports of night blindness.
Vitamin A deficiency causes visual impairment in many parts of the developing world and is the leading cause of acquired blindness in children. Children under five years of age and women of reproductive age are at highest risk of this nutritional deficiency and its adverse health consequences. Globally, approximately 21% of all children suffer from vitamin A deficiency (defined as low serum retinol concentrations), with the highest prevalence of deficiency, and the largest number affected, in parts of Asia (30% in SEAR-D and 48% in SEAR-B) and in Africa (28% in AFR-D and 35% in AFR-E). There is a similar pattern for women affected by night blindness during pregnancy, with a global prevalence of approximately 5% and the highest prevalence among women living in Asia and Africa where maternal mortality rates are also high.
This analysis estimated that Vitamin A deficiency also caused about 16% of worldwide burden resulting from malaria and 18% resulting from diarrhoeal diseases. Attributable fractions for both diseases were 16--20% in Africa. In South-East Asia, about 11% of malaria was attributed to vitamin A deficiency. About 10% of maternal DALYs worldwide were attributed to vitamin A deficiency, again with the proportion highest in South-East Asia and Africa. Other outcomes potentially associated with vitamin A deficiency are fetal loss, low birth weight, preterm birth and infant mortality.
In total, about 0.8 million (1.4%) of deaths worldwide result from vitamin A deficiency, 1.1% in males and 1.7% in females. Attributable DALYs are higher: 1.8% of global disease burden. Over 4--6% of all disease burden in Africa was estimated to result from vitamin A deficiency.
Zinc deficiency is largely related to inadequate intake or absorption of zinc from the diet, although excess losses of zinc during diarrhoea may also contribute. The distinction between intake and absorption is important: high levels of inhibitors (such as fibre and phytates) in the diet may result in low absorption of zinc, even though intake of zinc may be acceptable. For this reason, zinc requirements for dietary intake are adjusted upward for populations in which animal products -- the best sources of zinc -- are limited, and in which plant sources of zinc are high in phytates.
Severe zinc deficiency was defined in the early 1900s as a condition characterized by short stature, hypogonadism, impaired immune function, skin disorders, cognitive dysfunction, and anorexia (12). Using food availability data, it is estimated that zinc deficiency affects about one-third of the world's population, with estimates ranging from 4% to 73% across subregions. Although severe zinc deficiency is rare, mild-to-moderate zinc deficiency is quite common throughout the world (13).
Worldwide, zinc deficiency is responsible for approximately 16% of lower respiratory tract infections, 18% of malaria and 10% of diarrhoeal disease. The highest attributable fractions for lower respiratory tract infection occurred in AFR-E, AMR-D, EMR-D and SEAR-D (18--22%); likewise, the attributable fractions for diarrhoeal diseases were high in these four subregions (11--13%). Attributable fractions for malaria were highest in AFR-D, AFR-E and EMR-D (10--22%).
In total, 1.4% (0.8 million) of deaths worldwide were attributable to zinc deficiency: 1.4% in males and 1.5% in females. Attributable DALYs were higher, with zinc deficiency accounting for about 2.9% of worldwide loss of healthy life years. Of this disease burden, amounting to 28 million DALYs worldwide, 34.2% occurred in SEAR-D, 31.1% in AFR-E and 18.0% in AFR-D.
Lack of breastfeeding
Breast milk provides optimal nutrition for a growing infant, with compositional changes that are adapted to the changing needs of the infant. Human milk contains adequate minerals and nutrients for the first six months of life. Breast milk also contains immune components, cellular elements and other host-defence factors that provide various antibacterial, antiviral and antiparasitic protection. Breast-milk components stimulate the appropriate development of the infant's own immune system. On the basis of the current evidence, WHO's public health recommendation is that infants should be exclusively breastfed during the first six months of life and that they should continue to receive breast milk throughout the remainder of the first year and during the second year of life (14). "Exclusive breastfeeding" means that no water or other fluids (or foods) should be administered. In almost all situations, breastfeeding remains the simplest, healthiest and least expensive method of infant feeding, which is also adapted to the nutritional needs of the infant.
In general, exclusive breastfeeding rates are low. The proportion of infants less than 6 months of age that are exclusively breastfed ranges from about 9% in EUR-C and AFR-D, respectively, to 55% in WPR-B (excluding EUR-A and WPR-A for which sufficient information was not available). On the other hand, the proportion of infants less than six months old that are not breastfed at all ranges from 35% in EUR-C to 2% in SEAR-D (again, excluding all A subregions). In Africa, however, where breastfeeding is nearly universal, exclusive breastfeeding remains rare. For infants aged 6--11 months, the proportion not breastfed ranges from 5% in SEAR-D to 69% in EUR-C. In all the subregions in Africa and South-East Asia, over 90% of infants aged 6--11 months are still breastfed.
Lack of breastfeeding -- and especially lack of exclusive breastfeeding during the first months of life -- are important risk factors for infant and childhood morbidity and mortality, especially resulting from diarrhoeal disease and acute respiratory infections in developing countries. For example, in a study in Brazil (15), infants less than 12 months of age who received only powdered milk or cow's milk had approximately 14 times the risk of death from diarrhoeal disease and about 4 times the risk of death from acute respiratory infection compared with those who were exclusively breastfed. Furthermore, those who received powdered milk or cow's milk in addition to breast milk were found to be at 4.2 times the risk of diarrhoeal death and 1.6 times the risk of death from acute respiratory infection, compared with infants exclusively breastfed. Breastfeeding has also been demonstrated to be important for neurodevelopment, especially in premature, low-birth-weight infants and infants born small for gestational age.