Sunbathing can make you taller, so why are people in the south generally not as tall as people in the north?

It is said that sunbathing helps to grow taller, so why are people in tropical regions generally shorter than those in cold regions? Is this concept wrong or are there other factors?

Let's put it this way, background is the first, nutrition is the second. After all, height is not only related to sun exposure, there are deeper reasons, let's talk about it today.

01. Why are people in tropical areas short ?

Body size is a complex trait that is subject to many controls. The reason why tropical people are short is mainly due to Bergmann's law - for the same species (you can't compare polar bears with Arctic foxes)

Bergman's law was proposed by Bergman , and its original definition is: "Under equal environmental conditions, all isothermal animals dissipate the same amount of heat per unit surface area."

Various interpretations of Bergmann's law When explaining the geographical variation in body size of modern homeotherms, Bergmann's law has another form of expression: " Among the same species, populations living in colder climates are larger than those living in warmer climates." The general interpretation of this principle is that large animals, due to their small surface area to volume ratio, consume less energy in thermoregulation than small animals, so large animals living in cold climates are more economical than small animals. Further expansion of this is that increased body weight is an adaptation to cold environments.

This is why people in cold zones are taller than people in tropical zones.

02 How does heat dissipation area affect height?

The physical principle of Bergmann's law is the problem of heat dissipation area . That is, the larger the body, the larger the average heat dissipation area. For an extreme example, let's take a ball as an example:

The surface area of ​​a sphere is calculated by the formula: the surface area of ​​a sphere = 4πr^2 , where r is the radius of the sphere.

The volume calculation formula of a sphere : Vsphere = (4/3)πr^3 , r is the radius of the sphere;

The relative surface area of ​​the sphere is 3/r.

That is, the relative surface area is inversely proportional to the radius .

Applying the above principle to living things, the larger the height, the smaller the relative heat dissipation area. As shown in the figure below, as the radius increases, the rate of increase in volume is significantly higher than the rate of increase in surface area.

The ratio of the two, that is, the trend of relative surface area change

This is because as the radius increases, the relative heat dissipation area decreases significantly. An intuitive example is that people in the north are taller than people in the south (there is statistical data to support this, but in modern society, nutrition is more important for height).

03 When it comes to height, it all comes down to genetics

The underlying reason is that the genetic background is different . The genetic background of tropical people and temperate people is too different (for example, yellow people and black people belong to two subspecies), which leads to the fact that even if you are tanned, you will not grow tall. This can be done by a simple experiment. You can choose a pair of temperate twins, keep one in the temperate zone and take the other to the tropics, give them the same diet, and their heights will definitely be roughly the same.

In other words, if you move from Sanya to Harbin, it will not affect your child's height. If you want to affect it, you have to wait at least three or four generations, and you have to do it by marrying a local.

After solving the above core problems, you will find that sunbathing is actually a simple variable to promote vitamin D. The premise for its establishment must be that you have a similar genetic background (Pan Changjiang can't catch up with Yao Ming even if he is sunburned into charcoal).

In a word, sunbathing is just the icing on the cake. The icing is the key.

Disclaimer: The above belongs to Li Lei's popular science series, so to a certain extent, it lacks rigor and should not be used for academic purposes.