That is why the calcium content in milk is even more less useful than plant sources of calcium. Next to bioavailabillity of the absorption.
If that is true, how is it possible that bone mineral density in western countries is greatest?
In western countries, the ratio of plant foods relative to animal foods is smallest.
Also, the intake of milk products is greatest in western countries.
So, its not about the calcium in milk not being useful,
after all, it needs to supply calves with sufficient calcium for rapid bone development.
No doubt that this is succesful.
Consuming milk products is effective in increasing bone mineral density in humans,
and thus its calcium is useful in humans.
What causes osteoporosis is a very different question.
Thats just speculation.
We dont know exactly what plants/fruits (oxalic acid, phytic acid etc) / animal foods we ate, and in what proportions,
which will have been very different for different periods and different places / regions of the world.
Bone density also appeared to be stable over time with an apparent absence of osteoporosis.
Osteoporosis is particularly a disease of the elderly.
Who says that these humans ever reached the age of 70 years old?
Bone densities decreased in the agricultural age and the incidence of osteoporosis increased in both the Near East and the Americas.
Osteoporosis is most prevalent in countries with the highest average bone mineral density.
Today, the intestinal tract does not have effective absorption mechanisms for calcium
Not true, the higher the calcium intake, the lower the absorption rate.
"The effect of supplementing a basal diet containing 697 mg calcium daily with an additional 900 mg Ca daily from milk, Ca chloride, or a Ca carbonate preparation was examined in eight adult males during a 56-d metabolic balance study. The ingestion of the milk or Ca supplements had no overall effect on Ca retention by these subjects because the milk and supplements depressed apparent absorption of Ca in the gut and fractional tubular reabsorption of Ca in the kidneys." Lweis NM et al
"During a low calcium intake, fractional calcium absorption was significantly greater (0.582 compared with 0.260) and urinary calcium excretion was significantly lower (1.30 compared with 3.08) than values obtained during a high calcium intake"
"These results suggest that during short periods of inadequate calcium intake, girls are able to significantly increase the efficiency of calcium absorption and decrease urinary calcium losses to conserve calcium required for bone mineral acquisition". O'Brien KO et al
typical calcium absorption averages 25-35 percent at median intakes, and only 50 percent at very low calcium intakes.
Not true, calcium absorption rates may be much higher.
"Fractional 45Ca absorption was approximately 61% in all 3 groups when the diet was unmodified and increased to 71%, 69%, and 68% in the osteoporotic subjects, age-matched control women, and young women, respectively, when dietary calcium was reduced to 300 mg/d."
"In contrast, supplementation with 1200 mg Ca resulted in decreased fractional 45Ca absorption, suggesting that the increased calcium intake was associated with a potent compensatory ability of the intestine and calcitropic hormones to adapt.
" Kung AW et al
Thats is also why high calcium supplemenation is useless:
"The mechanisms regulating fractional calcium absorption as a function of intake suggest that very high daily doses are probably useless" Celotti F et al
Even when supplementing with only 300 mg / day, the absorption rate will decrease:
"After 6 months, true fractional Ca absorption of the study group (55.6)%) was significantly lower than that of the controls (64.3%)." Lee WT et al
Contrasted with sodium, which is absorbed with 100-percent efficiency, typical calcium absorption averages 25-35 percent at median intakes, and only 50 percent at very low calcium intakes. Net calcium accumulation in present day humans is only 4-8 percent because of renal, fecal, and perspiration losses.
Maybe there is a reason why net calcium accumulation is only 4-8%.
That reason is the differences in requirements, excretion rates and in 'safe deposits'.
The adjustment of calcium absorption/excretion rates (in response to exposure) is less linear than sodium absorption/excretion rates,
which is due to the presence of a large calcium reservoir: the bones, serving as a buffering system against
too low and too high serum calcium.
In times of high-calcium exposure, calcium may be stored for times of less calcium-exposure.
The downside of this system is the current constant overexposure of calcium,
which induces a contineous storage and deportation of calcium into and from the bones,
eventually exhausting osteoblasts, causing osteoporosis.