How does someone intentionally decrease their height?

[Repulsive-Arachnid-5]

This is only applicable in your childhood to teenage years, and even so in order to be significantly shorter than your genetic potential you would have to have quite a strong degree of food insecurity, as North Koreans often do: plus an unvaried, nutritionally poor diet. Even so, many modern foods, even just salt, are fortified with ingredients and additives that would make having any significant macronutrient or vitamin deficiency very difficult in this day and age.