I hope I haven't discussed calcium absorption enough times for the cows to come home. Sometimes I don't want the cows to come home.
There are two primary ways, mechanisms if you will, for our body to absorb calcium. One is by active transport which utilizes chemical energy stored in the bonds of ATP; the other is by passive diffusion. Active transport is orders of magnitude more effective than passive diffusion. In that little diagram you can see it will even move a nutrient into a cell against a gradient!
Below is from my textbook, Advanced Nutrition and Human Metabolism, Groff and Gropper, 3rd edition:
"One of the transport processes, operative primarily in the duodenum and proximal jejunum, is saturable, requires energy, involves a calcium binding protein, and is regulated by calcitriol (1,25-(OH)2 D3). The calcitriol-dependent calcium transport system is stimulated with ingestion of low-calcium diets, especially intakes less than 400 mg, as well as in conditions of growth, pregnancy, and lactation in which calcium requirements are increased. Growing children, for example, absorb up to 75% of dietary calcium in contrast with adults, who average about 30% absorption.There's a third way:
The second of the two processes for calcium absorption occurs throughout the small intestine but mostly in the jejunum and ileum, is nonsaturable and passive, and appears to be paracellular (i.e. absorbed between cells rather than through them). The amount of calcium absorbed via the nonsaturable, paracellular mechanism depends on the supply of calcium in the intestinal lumen up to a threshold level. Increased absorption via this mechanism becomes possible when there is an increased intake of the mineral."
"The large intestine also appears to play a role in calcium absorption. Bacteria in the colon may release calcium bound to some fermentable fibers such as pectins. Up to 4% of dietary calcium is absorbed by the colon per day; this amount may be higher in people who are absorbing less calcium in the small intestine."So, someone who is eating less than 400 mg - which is half the recommended amount (the DRIs are 800-1000 mg/day, some groups recommend up to 1300 mg) - may, all else being equal, end up with a similar calcium status as someone eating 1000 mg or more because an active transport mechanism kicks in at lower intakes. As well, more calcium may be absorbed from the colon.
Calcium is not unusual in this regard. Absorption of nutrients is often higher when intake is low, and vice versa. Zooming in on one nutrient, in this case calcium, and fretting over whether we're "getting enough" has a downside if it leads to taking supplements. Many nutrients compete for intestinal absorption, e.g. zinc supplements have been shown to substantially reduce calcium absorption. And, it should be said, what the body doesn't absorb goes out with the feces.
It may be better to focus on eating a variety of minimally processed foods than to focus on getting enough of a discrete nutrient, and let the body take care of itself. I'm reminded of this quote by Lewis Thomas:
"If I were informed tomorrow that I was in direct communication with my liver, and could take over now, I would become deeply depressed. ... I am, to face the facts squarely, considerably less intelligent than my liver. I am, moreover, constitutionally unable to make hepatic decisions, and I prefer not be obliged to, ever."
Calcium is a divalent cation, evident by that little 2+ that often accompanies it. Zinc is also a divalent cation, as is magnesium.
Divalent cations as a group compete for intestinal absorption ... so if you're taking just magnesium, or just calcium, or just any other divalent cation, you will be absorbing less of the others.
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