Nutrition has profound effects on health across the life course, including physical growth, cognitive maturation, mental health and behaviour. The focus of this lecture is on micronutrients. Micronutrients are chemical elements or substances required in trace amounts for the normal growth and development of living organism. The microminerals or trace elements include iron, cobalt, chromium, copper, iodine, manganese, selenium, zinc and molybdenum. Micronutrients also include vitamins, which are organic compounds required as nutrients in tiny amounts by an organism. In this lecture, I've chosen to focus on the following six of these. Iron, Zinc, Iodine, Vitamin A, Folate, and Calcium. Iron deficiency is the most prevalent nutritional disorder worldwide, especially in developing countries. Around 2 billion people are anaemic. And that is mainly due to iron deficiency, amounting to over 30% of the world's population. Iron deficiency occurs when iron absorption cannot compensate for iron requirements and losses. And requirements are especially high in pregnant women, infants, young children, and adolescents, which is why these groups run a higher risk of being iron deficient. In developing countries, the main cause of iron deficiency is low bioavailability of iron in their diet. And there are serious consequences of iron deficiency in adolescents including anaemia and fatigue. Which can have profound effects on learning at school and concentration in the workforce with flow-on effects then of lost productivity. Of concern is the prevalence of iron deficiency in some countries, as shown here for the adolescent age group of 15 to 24. There are also serious consequences in pregnancy, with up to 80% of pregnant women in some countries, especially in South Asia, affected with anaemia that is mostly due to iron deficiency. Due to the association of iron deficiency with poor diet, it is the poorest, most vulnerable and least educated who are disproportionately affected by iron deficiency. And it is obviously the same group that also stands to gain the most by its reduction. This study from Shandong Province in China, describes the change in nutrition status of 7 to 17 year olds, over the ten year period from 1995 to 2005 as derived from two surveys on student health. Increments of height, body weight, and body mass index were reported for students aged 7, 9, 12, 14, and 17 years of age. And the prevalence of underweight, overweight, and obesity were categorised according to body mass index with anaemia also being measured. The study showed that at each age, there was an increase in mean height. In other words, Chinese children in adolescence in this province have gotten taller over this ten year period. And, in the past ten years, underweight was not nearly as evident. Pleasingly, as we can see on the figure in the right, there was also a dramatic reduction in the proportion of adolescents with anaemia. With the rate effectively halving over the ten-year period, from around 20% to 10% in males, and from around 23% down to 13% in females. The downside is that at the same time, the proportion of the population that was overweight and obese increased, as we will discuss in the subsequent lecture on macronutrients. Another important micronutrient is zinc. This plays an incredibly important role in cellular growth, in cellular differentiation, and in metabolism. It is estimated that 17% of the world's population has inadequate zinc intake. Zinc supplementation, on the other hand, has been shown to reduce all-cause mortality in children less than five, to reduce the instance of diarrhoea and pneumonia and to slightly increase linear height. Studies have focused on zinc deficiency in pregnant women, and in young children. But, there are also opportunities to treat earlier in the life course, prior to pregnancy, which in so many low income countries, is especially relevant for adolescents. The main role of iodine is in the synthesis of thyroid hormone, made by the thyroid gland that sits at the base of the neck, as shown in the line drawing here on the left. In extreme cases, iodine deficiency is linked to a thyroid goiter. The name we give to a visibly enlarged thyroid gland, as seen in a fairly extreme case in the photo below. Iodine deficiency is a major cause of preventable perinatal mortality and importantly of intellectual disability. Iodine deficiency has a devastating impact on the brain of the developing foetus and young children in the first years of life. Iodine deficiency also increases the risks of miscarriage and stillbirth and infant mortality, while iodine deficient mothers can bear children who suffer from extreme mental retardation. This level of severity is unusual. More commonly, iodine deficiency causes moderate loss of intelligence, of an average loss of about 12 IQ points. However, these early effects reverberate through adolescence across the life-course as these seemingly normal children have difficulty learning in school. With persisting effects on educational engagement, learning, and indeed, income potential. Globally in 2007, 31.5% of school aged children had inadequate iodine intake. While this is reduced from 36% in 2003, it is still an unacceptably high deficiency rate. Especially given how simply it can be fixed. Indeed, iodine deficiency can be prevented with just one teaspoon of iodine over a lifetime consumed in tiny amounts on a regular basis at very little cost. The addition of iodine to salt is now used throughout the world as a universal approach to iodine prevention. Over the past ten years there, has been steady progress in Europe, the eastern Mediterranean region, southeast Asia and the western Pacific regions in reducing iodine deficiency rates. Largely due to strengthened salt iodisation programs and improved monitoring. Sadly, there has been less recent progress in Africa. Interestingly, in the United States, the proliferation of iodised salt has been shown to have increased IQ by as much 15 points in some rural areas that were particularly iodine deficient. Let's now turn to Vitamin A, which is an important vitamin for vision. Over time, clinical features of Vitamin A deficiencies have greatly reduced. Largely due to extensive Vitamin A supplementation programs in many countries. However, nearly one in three people across the globe continue to have subclinical Vitamin A deficiency, especially in Africa and in Southeast Asia. Nearly 1% of preschool children are estimated to have night blindness due to Vitamin A deficiency, with double that rate in Africa. Randomised control trials of Vitamin A supplementation have shown reduced mortality in children age six months to five years. There has, however, been very remarkably little research on Vitamin A deficiency and night blindness in the adolescent age group. Folate or folic acid, the terms are used interchangeably, is a B vitamin provided in the diet. Folate is found naturally in a wide variety of foods including vegetables and fruits, nuts, pulses and grains, dairy products and eggs and meat and seafood. A gold star to those if you know the foods with the highest levels of folate, spinach, liver, yeast, asparagus, and indeed, Brussels sprouts. Folate is essential for many bodily functions, including the synthesis, repair, and methylation of DNA. What this means is, without folate, cells cannot divide, so folate is especially important at times of rapid cell divisions, such as in pregnancy, infancy and adolescence. There are many different effects of folate deficiency. It's an important cause of anaemia, of chronic diarrhoea, of peripheral neuropathy, a cause of numbness of the fingers and toes. And can be also associated with cognitive and emotional effects, such as depression and confusion. However, one of the most concerning effects of folate deficiency is neural tube defects in the developing embryo, what we call spina bifida. Spina bifida is caused by the failure of the neural tube to close during the first month of embryonic development. Generally, before a woman even knows she is pregnant. Fortification of food with folate is a population-wide strategy to reduce folate deficiency. In many countries, legislation now requires folate to be added to flour, breads, cereals, pasta, and rice. In addition, there is an effort to provide supplementation to women prior to pregnancy, to reduce the risk of neural tube defects in their children. In low and middle income countries where many pregnancies occur in adolescents, the most reliable way to ensure adequate folate is supplementation to all girls and women of child bearing age. And finally, calcium. Calcium is the most abundant mineral in the human body, with about 99% of the body's calcium found in our bones and teeth. Calcium concentrations in the blood and fluid that surround cells are tightly controlled. In order to preserve normal physiological function, with complex feedback loops involving parathyroid hormone and Vitamin D. An important vitamin, but in its activated form, increases intestinal absorption of both calcium and phosphorus. This is a complicated area of feedback loops. Those with darker skins risk inadequate Vitamin D levels due to its reduced absorption through the skin, which subsequently then effects calcium. Calcium-rich foods include dairy products such as milk, yogurt and cheese as shown here, as well as bony fish, such as sardines, and certain vegetables, such as broccoli and beans. While relative to adults, adolescents absorb more calcium from food, a higher calcium intake is required during adolescence than an adult would to meet their increased calcium needs. And the recommended daily intake of calcium is three to four serves of dairy foods a day. Across the world, however, many, many adolescents do not eat enough calcium-rich foods to meet these demands in terms of their growing bodies. The dramatic increase in linear height that incurs in early puberty combined with the maturation of the bony skeleton in adolescents. Involves a doubling of bone mineral density across the second decade. As we can see in this diagram on the left. Maximum bone growth occurs around six months after the peak height velocity. And critically, in the two years of peak skeletal growth, one year either side of that peak, adolescents accumulate over 25% of adult bone. A challenge is that there is a relatively fixed window during the second decade of life to accrue sufficient bone density to meet needs across the life course. It is estimated that for every 5% increase in adolescent bone mass, there is a 40% decrease in the risk of bony fractures and osteoporosis as a reflection of that later in life. Pregnancy places additional demands on the body's calcium. Inadequate intake during pregnancy will result in resorption of bone from the mother's skeleton with a greater risk of maternal osteoporosis, or overly thin bones, as the mother ages. There are also significant concerns about the health impacts of inadequate calcium on the growing foetus itself. Clearly, pregnancy during adolescence compounds the requirements for calcium. In summing up, there are clear links between maternal nutrition and foetal growth, which suggests that far more emphasis is warranted on the health and nutrition of young women prior to conception. And as many interventions delivered to pregnant women will be delivered too late to achieve the most effect, a stronger focus on improving the health of adolescents, in general, is required. Again, reminding us of the importance of this life course perspective.
