Secrets of the Human Body. Andrew Cohen

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 twins grow up in equally similar or different environments, so this method of comparing groups of identical and non-identical twins enables you relatively easily to quantify the heritability of a trait. So, for example, in the case of a height study, if it shows that the identical twins are considerably closer in height than the non-identical twins then this strongly indicates that genes play an important role. The actual analysis that can be applied to a study like this, both statistically and genetically, is far reaching and complex but the principle remains the same – the greater the similarity between identical twins compared to non-identical twins, the greater the heritability of the trait.

      One of the most recent of these large studies conducted by Peter Visscher of the Queensland Institute of Medical Research in Australia looked at 3,375 pairs of Australian twins and siblings and found that the heritability of height is around 80 per cent. Other studies have come up with similar findings, including one that looked at 8,798 pairs of Finnish twins, in which the heritability was found to be 78 per cent for men and 75 per cent for women. Interestingly, similar studies in Asia and Africa have found the per cent heritability to be around 65 per cent or lower, because these regions tend to have populations that are less mobile and so more ethnically and genetically defined compared to the greater genetic homogeneity we see in the west.

      Regardless of the height you reach as an adult, the journey to get there is not steady, and only now are we beginning to truly understand the extraordinary process behind this rapid growth, a process that is dependent on an intricate interplay between your genes, brain, a cascade of chemicals and every bone in your body.

      GROWING PAINS

      In the first six months of your life, you grew more than at any other time since. It’s a growth spurt unlike anything else our bodies experience, with most of us growing a massive 30 cm in that first year. As a new parent this is particularly evident. It seems some days as if my daughter is growing in front of me. If we continued to grow at this rate, we would be 10 feet (~3 m) tall by the time we were 10 years old, but by the end of that first year that frenzied growth rate has slowed down and will continue at a far more subtle speed until the madness begins again at puberty.

      The secrets behind the process of growth reveal how the body works as an integrated system, not just separate organs and limbs functioning in isolation. Starting at the business end of the process, the bones that really define your growth are the long bones of your body, in particular the femurs in your thighs, the fibulas and tibias in your lower legs, and the humerus, ulna and radius in your arms. These are the site of the major longitudinal growth during that first year of development. These bones don’t just uniformly increase in size as they lengthen; the growth is focused around a particular part of the bone called the metaphysis found at the end of each long bone. If you looked at an X-ray of the metaphysis region of the fibula and tibia of a 10-year-old, you might conclude that the child has a broken leg. But what you are actually seeing in the ‘fracture’ across the bone is the location of growth, a line that is called the epiphyseal plate or growth