I learned this today. The tallest a human could potentially grow is about 9 feet, 274cm.
Humans grow because of a growth hormone called somatropin. It stimulates growth by telling cells to reproduce. It is released by the pituitary gland. It reacts with a specific receptor on the surface of cells, and it stimulates cell division. It also changes metabolism to make more energy available for the growing cells.
Most people grow until their late teens or early twenties and then they stop. Their DNA controls how much they will grow. When they have reached their optimum height, the reproductive glands in both males and females release estrogen. The estrogen makes the growth plates of bones fuse. This means that they can no longer respond to the growth hormone, and the bones stop growing. The rest of your body stops growing as well.
Growth needs a constant source of energy, calcium, and protein. Modern diets supply that very well which means average heights are slowly increasing. Nearly any country you choose will show an increase in average heights over the last 100 years. American men have gone from 172cm in 1810 to 178cm now. French men have gone from 164cm in 1810 to 176cm now. And Dutch men have gone from 166cm in 1810 to 184cm now. They have the tallest average height of any country. If you look at the graphs at ourworldindata.org/human-height you can see this trend for most countries. The lines are all going up and it is tempting to think that they will keep going up. However, there is a maximum height that humans can reach. This can be seen by looking at the people who have almost reached it.
Our bodies release enough growth hormone for us to reach the height that our genes have planned for us. Unfortunately, this doesn’t always go to plan. Occasionally, a disease such as a tumor in the pituitary glands can alter the production of estrogen which means that the growth plates of the bones don’t fuse, and they continue to react to growth hormone. This can cause something called gigantism. Gigantism can affect only parts of the body, or it can affect the whole body. Most of the world’s tallest people suffer from this condition. The tallest man of all time, Robert Wadlow, reached 8 feet 11 inches, 271cm. The tallest man alive today is Sultan Kosen, who is 8 feet 3 inches, 251cm.
There are two things that will limit the constant growth of average human height. The first is our bones and organs, and the second is the energy requirement.
The largest land animals that have ever lived would be the Dreadnoughtus. It was 26m long and weighed 65 tons. It was able to grow so big because its bones were basically hollow. This meant that as its volume increased, its weight didn’t increase in tandem. Animals that grow larger have to cope with the square-cube law. As an object gets bigger, if it keeps the same proportions, its volume increases much more quickly. As humans get larger, their volume increases and so does their weight. Robert Wadlow weighed 200kg. His bones were not designed to hold such a weight. He had to wear leg braces throughout his life to support his body. His organs caused problems as well. People who reach these sizes usually have high blood pressure because their heart has to work extremely hard to pump blood up to their head. Our hearts have not evolved for that. High blood pressure causes a thickening of heart muscles and can lead to diastolic heart failure. If you look at a giraffe, for example, they have evolved to deal with it. Giraffes also have high blood pressure, but their hearts don’t thicken because they have evolved genes that prevent the thickening. Their hearts also pump with a different rhythm to ours, with each stroke pumping more blood.
A larger body obviously requires more fuel. The average male needs about 2,500 calories a day. Robert Wadlow needed 8000 calories. As his size increased, his volume increased much more rapidly, and with it his need for energy.
Taller and larger animals often have an evolutionary advantage. They are stronger and they can reach higher food. Extreme height in humans doesn’t confer the same advantages and is more of a health risk. Humans have reached their evolutionary height advantage and there is no need to go much taller. The average height may increase a little more, but not indefinitely.
So, humans grow because of a growth hormone. When we are tall enough, estrogen is released which seals up the growth plates in our bones and stops us growing. Sometimes, a tumor interferes with this process and people keep growing. These giant people show why 9 feet is probably the upper limit of human height. Taller people have to support far more weight and their bones are not strong enough to do it. They also suffer from high blood pressure because their hearts are not designed to pump blood that high. And this is what I learned today.
Sources
https://coloradoprimaryhealthcare.com/2014/06/when-do-you-stop-growing
https://www.yourhormones.info/hormones/growth-hormone-releasing-hormone/
https://en.wikipedia.org/wiki/Growth_hormone
https://ourworldindata.org/human-height
https://en.wikipedia.org/wiki/Robert_Wadlow
https://www.nbcnews.com/id/wbna3076703
https://www.smithsonianmag.com/science-nature/cardiovascular-secrets-giraffes-180977785/
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