Let me, please, trying to share my opinion. The absorption function of digestive system depends on pathology or physiology condition of the body. Regarding of the body in physiology condition the absorption increases with decreasing intake, and vise versa - absorption decreases with increasing intake. The Body takes just what He needs.

calcium intake is mostly regulated by vitamin d. when thinking about calcium, one also has to pay attention to phosphate since these two are co-regulated in the body and are coupled in their functions sometimes acting in sinergy, but also in the opposite. in normal physiological conditions excess calcium wouldn't be absorbed in toxic concentrations due to tight regulation, but if the intake would be incredibly large and followed by ingestion of vitamin d, hypercalcemia could occur. For regulation, look also for hormones such as PTH and FGF23 - since phosphate is also involved here together with calcium. 

The real quesstion is the curve in a diagram with Calcium absorption in Ordinate andintake on abscissa. There is probably a curve that tends to a plateau-

I understand the question as a delicate balance between the amount of calcium absorbed by the intestine and the amount of calcium in food (ingestion), although not understand I would have to see the altitude of the Andes in the absorption of calcium. Are you under provision of this element in the diet, effect of altitude and less oxygenation ? It would be interesting to investigate the latter.

Returning to the simple balance of intestinal calcium absorption and ingestion thereof in the diet, there are many factors that are related to this basic equilibrium.

Essentially there is a direct relationship but not proportional, where higher calcium intake, increased absorption of it to the point at which the body has the quantity and then the upward curve stops, is a turning point and for a prolonged the curve flattens, and no more calcium is absorbed although this element exists much in the diet. This type of curve is obtained by placing the absorption axis "Y" (ordinate) and the amount of dietary calcium (ingestion) in the axis "X" (abscissa).

Now should consider all the factors involved in this delicate balance and not just calcium.

The active vitamin D [1,25 (OH) 2-D3 "calcitriol"] increases calcium absorption by the intestine and increases the sensitivity and responsiveness of the osteoid matrix bone when lack of this mineral (osteopenia, osteoporosis, osteomalacia )

Calcium absorption is also controlled by the amount of phosphates (mainly phosphorus "P") and a certain amount of this element is required to keep the ratio calcium x phosphorus (average 30), called index solubility of these elements in the blood and bone mineralized whose combination; In other words, the mineralization occurs not only by the presence of calcium but also its association with phosphorus.

In this balance "solubility and bone mineralization" with increasing blood calcium, decreases intestinal absorption of phosphorus; also with increasing blood phosphorus, calcium absorption decreases.

In addition, it has to consider the participation of Parathyroid hormone (PTH) and calcitonin (CT) at this delicate balance. Among these hormones there is an inverse relationship, increases PTH and decreases  CT.

All this is explained under normal physiological conditions. Of course the balance is broken when diseases occur, for example hyperparathyroidism or hypoparathyroidism. pseudo-hyperparathyroidism or pseudo-hypoparathyroidism 

Also liver failure (cirrhosis), chronic renal insufficiency and intestinal malabsorption (celiac disease) and poor digestion (pancreatic insufficiency) syndrome alter that balance.

regards

Sincerely

Dr. Jose Luis Garcia Vigil