What is it about aging kidneys that reduces their ability to convert Vitamin D to its active form?
Do people with one kidney or CKD also have much greater issues converting it?
How is it treated?
The kidneys contribute to vitamin D (calcitriol) synthesis in the following way.
The epidermis of the skin converts a cholesterol derivative, 7-dehydrocholesterol, into cholecalciferol (vitamin D3), using the energy of ultraviolet light to do so—hence the importance of getting enough sun on the skin.
The liver converts this cholecalciferol to calcidiol and the kidneys then convert that to the final, most active form, calcitriol. Calcitriol regulates the body’s calcium balance by acting on the bones, small intestine, and kidneys themselves, in ways stated at the right side of this diagram.
Here’s a summary of these points so far:
Next, where in the kidneys do they carry out that step? Each kidney contains about 1.2 million blood-filtering, urine-forming units called nephrons. A nephron begins with a spheroidal blood-filtering structure called a glomerulus. The fluid it filters then enters a long, very coiled duct called the proximal convoluted tubule (PCT), which begins its transformation into urine. We don’t have to concern ourselves with nephron structure beyond that point, but here it is—stretching out the tubules from their normal tangled state to see the nephron components better. As a necessity of textbook illustration, this diagram greatly understates the length and convolutions of the PCT.
(Preceding figures are from my own textbook.)
Here’s what those PCT cells look like microscopically.
These are the cells that convert calcidiol to calcitriol, completing the vitamin D synthesis process. They contain the enzyme for this conversion, 25-hydroxyvitamin D3 1-alpha-hydroxylase (CYP27B1).
Why does the process decline with age? Because as we grow older, the kidneys atrophy (shrink). If you live to be 85, you’ll have 30% to 40% fewer nephrons than when you were 25—that much less renal functionality. Furthermore, even of the ones that remain at that age, up to one-third are bloodless and nonfunctional. At age 90, the kidneys are 20% to 40% smaller than those of a 30-year-old and receive only half as much blood.
(Source: www.healthlibrary.com, via Pinterest)
With fewer nephrons and PCT epithelial cells, we produce less calcitriol. This is one factor, among others, in loss of bone density in old age. It is also a factor in the common problem of overmedication of the elderly, as our kidneys become less capable of processing the drug doses that we take at younger ages.
This is an issue, too, in various diseases that cause premature and more extreme renal atrophy, as in the kidney shown on the right side below. Atherosclerosis (fatty plaque) of the renal artery is one of the most common causes of this. Degeneration of the kidneys then tends to promote hypertension, and hypertension causes even further kidney damage, in an insidious positive feedback loop of worsening kidney function.
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