Nutrition has profound effects on health across the life course. In this lecture, we will explore the health implications of under nutrition in adolescence, and also consider the implications of under nutrition in adolescence for the health of the next generation. Nutritional status at the time of conception and during pregnancy has critical effects on foetal growth and development. These changes have variable and persistent effects into adolescents, as we will explore subsequently. Micronutrients are required for physical as well as cognitive growth and development but it can be hard to differentiate the effects of micronutrient from macronutrient deficiency, as the presence of one is commonly accompanied by another. What we know is that stunted and underweight children have an increased risk of death from diarrhoea, pneumonia, tuberculosis, measles, and other infectious diseases. Good data is lacking for how these affect adolescents. But as we discussed in the earlier lecture on infectious diseases, diarrhoea and lower respiratory tract infections are also surprisingly significant causes of death in younger adolescents, which we can assume would be less of a problem, were adolescents to have better nutritional status. Many children in low and middle income countries enter adolescence thin, stunted, and anaemic, and often display other micronutrient deficiencies as well. What do you think of the effects in adolescence and beyond? Firstly, there are important effects on learning with stunted and malnourished adolescents having poorer school performance. Malnourished adolescents are also estimated to have a 22% reduction in future adult income. There are also effects on pubertal timing and final adult stature. Undernutrition in earlier childhood and undernutrition in early adolescence can alter the timing and trajectory of the normal pubertal growth spurt with the potential for stunting of final adult height. This is an area that has been remarkably under researched. Stunting itself is associated with increased reproductive health risks as women with short stature have smaller pelvises with greater risk of obstructive labor, which is accompanied by higher rates of complications for both the mother and child. There are also effects on adult health, as stunted adolescents grow up which an increased risk of obesity, diabetes and cardiovascular disease in adulthood. And, of course, there are implications for pregnancy. Pregnancy during adolescents will compound these concerns, as the growing foetus is preferentially nourished to the detriment of the mother. The concern is that the younger the adolescent is at the time of pregnancy, the greater the risk. Growth is a feature of childhood and adolescence. Humans reach their final adult height during adolescence. Girls reaching it well before boys. On the left here is a Center for Disease Control chart that shows the increase with age of linear growth in centimetres at the top, and weight in kilograms at the bottom. The multiple lines for both height and weight refer to different percentiles, which are a statistical approach to indicate the value below which a percentage of observations in a normative reference population lie. For example, the 30th percentile refers to the percentage below which 30% of the population would lie. If someone's height lies on the 86th percentile, they are taller than 86% of the population. So, if we think about that for someone who is growing from childhood through to adolescence, this is someone, say, growing on the 86th percentile, and this growth pattern here might be someone growing on about the 30th percentile. Both of these are completely normal patterns. In addition to poor nutrition, ill health from whatever means and emotional disturbance such as neglect, can interfere with normal growth in earlier childhood and affect the timing of puberty. In these situations, children can enter into adolescence and their adolescence growth spurt much shorter than expected for age, which then, typically results in reduced final adult height. While growth occurs across childhood, it is during early adolescence in girls and mid-adolescence in boys that we experience another period of very rapid growth, second only to the first year in life as we had referred to in earlier lectures. And you'll remember that we refer to this as the pubertal growth spurt. And as shown on the chart on the right, the pubertal growth spurt occurs at a younger age in girls than boys with this graph showing the age of peak growth velocity. About 15% of final adult height is achieved during the pubertal growth spurt, and about 45% of the density of the human skeleton is achieved during the second decade which, as we've described earlier, requires significant calcium in the diet. For those children who enter adolescence already stunted or malnourished, one question is how we might optimise their growth in adolescence. Clearly, good health and nutrition is critical for this. However, once the growth plates the growing ends of the long bones are closed, then, no further growth is possible. Which renders it then too late for interventions to promote an increase in linear height. And a concern about overly ambitious nutritional interventions for infants who are born small for gestational age, is whether this might inadvertently prime the child into puberty earlier, which would then reduce their final adult height. And, we have little knowledge of the effects on puberty of refeeding children in the years immediately prior to puberty to know whether this might be a good thing or a bad thing in terms of potentially unintended consequences for final adult height. The timing of puberty has undergone important secular changes, that is, changes over time. On the left, we can see the secular trend toward increased stature in Italian men, whose average height increased by 12 centimetres between 1854 and 1990. On the right is the secular trend toward earlier puberty using the age of menarche, or first menstruation, as a proxy marker of the timing of puberty. And, in high income countries, the age of menarche has fallen as child health has improved. In these countries, older age at onset of puberty is generally associated with taller stature. However, among low income settings, earlier menarche, earlier entry into puberty, predicts taller stature, presumably, because those who are better nourished within that population are entering puberty at a relatively more normal time in contrast to those who are malnourished, whose pubertal timing is delayed. However, the association between age at menarche and stature varies in relation to many different factors, and in developed countries, earlier puberty predicts shorter, not taller, stature. The timing of puberty is another example where we can see the importance of the links between maternal nutrition, and foetal growth. Aggressive refeeding of malnourished infants born small for gestational age has had unintended consequences with greater risk of childhood obesity, early puberty, and then adult stunting. This suggests we need to think up carefully about the timing of interventions across the life course, whether that's in pregnant women, whether that's in early infancy, later childhood, pre-puberty in order to avoid later, unintended consequences. Arguably, nutritionally focused interventions will have the most benefit when truly delivered pre-conceptually, rather than once the mother is already pregnant. This is an example, again, of the value of taking a life course perspective, suggesting there are important opportunities for the health of the next generation by better appreciating the importance of adolescents and their health. This is also what is referred to as preconception interventions. And if we are wanting to improve the health of the next generation, many preconception interventions will be delivered to adolescent girls, as interventions delivered to pregnant women will be delivered too late to achieve many of the potential gains for that generation. Let's then take a minute to consider maternal micronutrient deficiency in light of the timing of early embryological development or embryogenesis. Early embryological life is a critical window for healthy development, as we talked about earlier in those lectures on micronutrient deficiency. Yet, as the first antenatal visit is generally not before eight weeks of pregnancy, even in high income countries, by the time that women seek healthcare, a critical phase of foetal development has already passed as we talked about in relationship, for example, the folate deficiency and the risk of spina bifida. It is already too late for some interventions to have their greatest effect. Particularly in low income countries, where a significant proportion of first pregnancies are in the adolescent years and where there is less ready access to antenatal care. Arguably, any micronutrient intervention that has the preconception window as its focus would have greatest effect by improving adolescent nutrition across the board. And the same argument can be made for a wide range of health concerns beyond nutrition, including infection, infectious diseases, like rubella where immunisation strategies will be most effective well before conception. And also for health risk behaviours such as smoking and alcohol. Where again, the timing of risks to the foetus means that much of the damage would already have been done, before women are aware they are pregnant, or certainly before their first antenatal visit as an opportunity for preventive interventions. So to sum up, there is growing interest in adolescence as an important time to promote health in adolescence itself, as well as, appreciating the benefits for the next generation. We know that many women enter pregnancy with depleted nutritional reserves, being underweight, having micronutrient deficiencies or both. We know that obesity increases pregnancy risks with greater risk of gestational diabetes, hypertension, preeclampsia, increased need for obstetric interventions and worse neonatal outcomes. And, once women are pregnant, it is too late to intervene around many risks, such as neural tube defects and some nutritional interventions in infancy may have unintended consequences in adolescence. All good reasons to think about how we can improve the nutritional status of adolescents.
