Secrets of the Human Body. Andrew Cohen

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СКАЧАТЬ Britain and have only just been knocked off the global top spot, outranked by the lofty Chinese couple Sun Ming, 7 ft 8 in (233 cm), and Xu Yan, 6 ft 1 in (185 cm), in 2016.

      While Keisha and Wilco are outliers at the far end of the distribution of human height, collectively as a species we have all gone through an incredible growth spurt in the last 150 years or so. Since the middle of the nineteenth century records show that the average height in industrialised countries has increased by about 10 cm. That’s a serious increase in such a short space of time, and as far as we know it is unprecedented. In fact the study of early human skeletons strongly suggests that human height stayed pretty much the same from the Stone Age until the mid-part of the nineteenth century. So what happened around the 1820s? Well, all of the available evidence suggests that this swift increase in height was not driven by any rapid-fire evolutionary selective pressures. The time frame is far too short for evolution by natural selection to play out and there is no reason to think that height has been under particular selective pressure in the last 100 years or so. The environmental influences, on the other hand, seem to track very tightly with the increase in height. We know that if a child is malnourished or suffering from disease at particularly critical moments in childhood, they will never reach their full potential adult height. But since boys stop growing around their late teens and girls in their mid-teens, proper nutrition before puberty is essential to fulfil genetic potential for height. Protein, calcium, vitamins D and A all have an effect on height, and deficiency in all of these nutrients in the early nineteenth century was commonplace. But starting around the mid-1800s the punishing lives of populations through the early years of the industrial revolution began to give way to more widespread benefits, including better sanitation, clean running water and improved nutrition. Slowly this allowed the populations of countries like the UK to start fulfilling the genetic potential of human height. The truth is (as many parents instinctively know) that eating up your greens and drinking your milk really will make you grow up strong and tall.

      EAT YOUR GREENS PROTEINS

      During childhood the most important food that influences your final height is protein. Meat, fish, eggs, nuts, legumes and dairy products are all good sources of protein (which is why it is a considerable nutritional challenge to bring children up on a vegan diet). Other minerals, in particular calcium, and vitamins A and D also have a direct influence on height. For this reason malnutrition during the key stages of childhood can have a direct and significant effect on growth. This means good nutrition is particularly important before and around the growth spurts of puberty. For girls this begins around 10 years old and continues until their mid-teens when maximum height is reached. For boys it’s later, with maximum height not being reached until the late teens.

      The most startling journey from low to high across this time has been made by the Dutch. For the data shows that the average nineteenth-century Dutchman waslooking up enviously at almost all of their European neighbours, but then a dramatic climb fuelled by increased living standards has taken them slowly but surely to the top of the global height charts. Although not without a few blips – both World Wars triggered a reversal in the upward trend in many countries as the availability of resources tightened dramatically. In recent years widespread increase in height has slowed down, stopped or even reversed. This is the case in the United States, where a lack of free health care, and a diet high in calories but low in nutrients, may be the major contributing factors. It is likely that the Dutch are approaching the maximum height their genes will allow. Supplementation with extra vitamins, calcium and protein beyond the recommended daily amounts will not increase gains (and in fact there are large studies showing that excess vitamin supplementation shortens life).

      BOY HEIGHT PREDICTOR

      (Father’s Height [cm] + Mother’s Height [cm] + 13 cm) / 2

      GIRL HEIGHT PREDICTOR

      (Father’s Height [cm] + Mother’s Height [cm] − 13 cm) / 2

      But tall people aren’t just tall because they have eaten better as children. Human height is determined by both genetics and environment. Your genes are a hand of cards you are dealt. Your environment is the way you play them. It’s a case of nature via nurture. The major environmental influence on height is nutrition, affected by both diet and disease. Around 80 per cent of the variation in height between people is determined by their genes, and around 20 per cent can be attributed to the environment, although these numbers vary with different populations around the world.

      You can work out how much of your height has actually been influenced by your parents demanding you clear your plate, and how much was set in stone from the moment of conception. The average height of a man in the UK is around 5 ft 9 in (175 cm). Take me for example. I’m 6 ft 1 in (185 cm) so I’m 10 cm taller than the average. Eight of those 10 cm are determined by my genes (my dad is a 6 ft 4 in [200 cm] Dutchman) and 2 cm by my diet (my mother is, in the words of P. G. Wodehouse, ‘God’s gift to the gastric juices’). I tower over Xand by a full centimetre simply because I listened to mum a bit more.

      You don’t need to be a population scientist to see that tall parents beget tall children by passing on genes for tallness.

      In the case of Keisha and Wilco van Kleef-Bolton, this certainly seems to be playing out predictably. They are the proud parents of five children, Lucas, the oldest at 11, is already 5 ft 4 in (c. 163 cm) and towering over his classmates; Eva, 8, is the average height of an 11-year-old; and 4-year-old Jonah is standing shoulder-to-shoulder with boys twice his age. While it’s still a little too soon to judge the newest arrivals to the family, early indications point them to the skies as well: Ezra, the tallest of the 1-year-old twins, is in the 91st percentile, and Gabriel is not far behind.

      Map showing variation in average adult male height in various nations across the world.

      But we don’t really need to wait to see roughly how tall any of their children will be. Since the 1970s we’ve been using a rough and ready formula to predict the eventual height of offspring with nothing more than just the parents’ measurements. By simply adding the height of two parents together, adding 13 cm to the sum of the two numbers for boys and subtracting 13 cm for girls and then dividing the result by two (see here), you end up with a pretty good estimation for the height of the children. So in the case of the van Kleef-Boltons, the boys would be expected to be 210.5 cm, and the girls 195.5 cm.

      This is not, of course, a precise calculation, but its rough reliability does indicate that height is a trait that is significantly inherited. That doesn’t mean there is a single gene for height; very few traits have a direct one-to-one relationship. Instead your height, like many other characteristics, is controlled by a multitude of genes interacting with a multitude of environmental factors.

      Average female growth chart from birth to 20 years old: showing from the 3rd to the 97th percentiles.

      We now know that in the case of height your genes are about 80 per cent of the story in determining how tall you and your children will be. The reason we know this with such accuracy is because there have been a wide variety of studies that have explored the heritability of human height using a long-established method of teasing out the influence of nature vs. nurture.

      The principle of these studies is simple. Take a group of identical or monozygotic twins, to use the technical term, like Xand and myself, twins who have developed from a single fertilised egg and so share 100 per cent of the same genes. You then compare a trait such as height difference between each of the identical twins in the group with a group of dizygotic twins, or non-identical twins (or even just siblings) who all share only about 50 per cent of their genes.

      It is assumed that identical and non-identical СКАЧАТЬ