Calcium intake in a normal diet is about 200mgs per day. That is fine but what matters most is absorption. You can consume all the calcium you want but you will not take it into your body without vitamin D. Vitamin D comes in several forms.
It needs to be pre-activated by healthy kidneys and then it needs to be activated by the skin and the eyes. While we are rightfully concerned about skin cancer, we overdo it with sunglasses, hats, and sunscreen which precipitates widespread vitamin D deficiency. For instance, if you spend about 20 minutes in the sun without a shirt on, you will absorb 10,000 international units of vitamin D. Up until recently the recommended daily intake of vitamin D was 200 international units and has recently been increased to 400 international units.
I would propose that if our skin is capable of giving us 10,000 international units of vitamin D in 20 minutes, there is a very high likelihood that we need more than 400 international units of vitamin D to be healthy. There is increasing evidence that vitamin D deficiency is increasing rapidly in our population.
In the early 20th century, we artificially added vitamin D to milk in order to treat a skeletal disorder known as rickets that was occurring disproportionately in the central part of the country where there was little or no fresh fish consumption. Since that part of the country was dairy country, the best way to get vitamin D in the population to treat rickets was to add it to milk.
Currently we should probably consider adding vitamin D to soft drinks as its consumption eclipses milk. Vitamin D is essential to absorbing calcium