Calcium and Phosphorus: Calcium Flashcards
osteoporosis and aging
What are some dietary sources of calcium?
rich source: milk. soy, cheese, ice cream, yogurt, salmon, juice
Poor sources: meats, grains, and nuts
What are some forms of calcium supplements?
Calcium carbonate: found in tums, inexpensive
-need to consume with food
Calcium citrate: not dependent on stomach acid for absorption
-good for older individuals w/ limited gastric acid production -doesnt need to be consumed w/ food
What are sths to keep in mind when consuming dietary/supp. sources of calcium?
- look for amount of elemental calcium in each dose
e. g. 1260 mg tablet of calcium carbonate may contain 500 mg of elemental calcium
**DIN, NPN, GP, or USP on supp.
Calcium sources like bone meal, fossils, oyster shell and dolomite (rock mineral) may contain high amounts of lead/aluminum
How do we digest calcium?
Calcium in food/supp. is present as relatively insoluble salts
-need ~1 hour at acidic pH in stomach to solubilize calcium salts –> free calcium (Ca2+)
Where is calcium absorbed in the body?
primarily duodenum, proximal jejunum (90%)
- saturable, carrier mediated active transport: requiring energy
- accounts for 50-60% of Ca absorption in the SI at intakes of 400-500 mg/meal and 1000 mg/day
How is calcium absorbed in the body?
absorb via membrane channel protein: transient receptor potential (TRP) vanilloid (V) 6 [TRPV6] and calbindin/calcium binding protein
-Calcium leaves enterocyte basolateral membrane via Ca2+-ATPase pump
- mediated by 1,25-vit.D
- tend to absorb more in doses <400-500 mg
Where is calcium absorbed secondarily?
Mostly in jejunum and ileum
non-saturable, paracellular
vit-D dependent claudins
-higher Ca intakes
Colon: bacterial release of Ca from fermentable fibers (4-10% total dietary Ca/day)
What are the steps of calcium absorption?
- Ca2+ crosses the brush border membrane of the enterocyte through a calcium channel TRPV6
- Ca2+ binds to calbindin D, which carries the calcium across the cytosol of the enterocyte
- Ca2+-ATPase pumps calcium across the basolateral membrane for entrance into the blood
- some Ca2+ is absorbed b/w cells–typically w/ high lumenal Ca2+ conc.
–> Protein-bound calcium (albumin or prealbumin)
or
non-protein complexed calcium (citrate, sulfate, or phosphate)
What are some factors influencing calcium absorption?
Amount of Ca consumed (increased absorption <400 mg)
Requirements (i.e. preg. and infancy)
-up to 60% absorption compared to 30%
Age: efficiency of absorption declines w/ age (15-20%)
vit.D status
Food, lactose-improve solubility
non or slowly fermentable fibers and polysaccharides such as inulin can increase colonic calcium absorption
What are some other factors decreasing calcium absorption?
Oxalate: chelates Ca (beet, celery)
fiber, phytate bind Ca–inhibit absorption
Competition w/ other divalent cations (Mg2+, Zn2+)
Formation of insoluble “fatty saps”: Ca and FA >7g fecal fat/day
individuals on proton pump inhibitors (ulcers) due to insuff. production of gastric acid
proton pump inhibitors (drugs) may increase the risk of bone fractures?
Yes
How are calcium transported in our body?
Bound to proteins (~40% albumin and prealbumin)
complexed w/ sulfate, phosphate, citrate (~10%)
Free in blood (ionized calcium ~50%)
from blood most calcium is deposited in bone (reservoir for blood levels)
How are calcium regulated in our body?
Calcium concentration tightly regulated
EC calcium conc. regulation: pTH, Calcitriol, Calcitonin (from thyroid, suppress PTH action)
IC calcium Ca2+ conc. regulation: Ca2+/2H+ (proton) ATPase pump, CA2+/3Na+ exchanger
What are the steps of calcium regulation?
- Low blood calcium signals the parathyroid gland to release PTH into blood
- PTH binds to bone cell receptors –> triggers resorption/breakdown of bone mineral for the release of calcium into the blood
- PTH acts on the kidneys to synthesize the active form of vit.D and calcitriol
- PTH and calcitriol promote the reabsorption of calcium from the kidney and into the blood
- Calcitriol leaves the kidneys
- ->travels to intestine, promotes calcium absorption across the brush border membrane, its transport in cell cytosol, and egress into the blood - Calcium enters the blood:
a. after release from bone
b. after release from kidneys
c. after absorption from intestinal cells (enterocytes)
What is the effect of parathyroid hormone (PTH)?
- increase serum calcium
- decrease bone calcium
- increase renal calcium reabsorption
- increase intestinal calcium absorption
What is the effect of calcitriol?
- Increase serum calcium
- works w/ pTH to decrease bone calcium
- increase renal calcium reabsorption
- increase intestinal calcium absorption
What is the effect of Calcitonin?
opposite to PTH secrete when blood calcium level high 1. Decrease serum calcium 2. Increase bone calcium 3. Decrease renal calcium reabsorption 4. NO effect on intestinal calcium absorption
What are the functions of calcium?
bone mineralization
blood coagulation
nerve conduction
muscle contraction
enzyme regulation
membrane permeability
What are the main composition of bone?
Bones 1-1.2 kg Ca
Bones and teeth 99% Ca
Cortical (outer layer) 75%
Trabecular (more metabolically active 25%)
Minerals: Ca, P, Mg
Matrix
Proteins (e.g. Type 1 collagen (85-90% of proteins)
vit.K dependent proteins: osteonectin, ostepontin, osteocalcin
What are osteocytes and osteoblast (OB)?
Osteocytes = bone stability, Mature OB
OB = bone formation
- secrete bone matrix proteins
- respond to estrogen, testosterone, GH, 2,25(OH)2D
- become embedded in protein and ground substance matrix
- some undergo apoptosis and others differentiation and morphological changes to become osteocytes
What is osteoclast?
OC = bone resorption
- lysosomes, proteases, acid
- respond to PTH, 1,25-vit.D
What is the process of bone modelling?
Osteoblast > osteoclast = deposition of bone mass
OB = OC = maintenance of bone mass
OB < OC = loss of bone mass
OB secretes collagen + proteins = ECM
Ca, P, Mg from blood attach to proteins–>hydroxyapatite crystals = mineralized
OB in matrix morph = osteocytes (stability)
OC (stimulated by OB through RANKL) degrades bone
How does calcium influence muscle contraction?
Increased calcium will –>
calcium will bind to troponin–> conformational change
muosin binding site on actin exposed
How does calcium play role in enzyme regulation?
Calcium ions + calmodulin –> Calmodulin-Ca2+ complex (now active and able to stimulate/interact w/ other compounds/enzymes)
ex. AC, Ca/Mg-ATPase, Myosin kinase, Ca-dependent protein kinase
How is calcium being excreted from the human body?
average 60 mg, up to 182 mg
Feces ~80%
Fecal excretion: fiber, phytate, oxalate, FAs
Urine ~20%
-high protein intakes, caffeine, and sodium thought to increase urine excretion
Skin (sweat)
How does our body react to calcium as we age?
decrease absorption
increase bone reabsorption (cessation in sex steroid production)
kidney function: hydroxylation activity of 1,25(OH)2D
increase excretion
steatorrhea
osteoporosis, fractures
colon cancer? hypertension?
How do we assess the nutritional status of calcium?
Serum calcium: tightly regulated by hormones
Serum ionized calcium
Protein bound calcium:
mg/dL = 0.44 + [0.76albumin (g/dL)]
or
[0.8(normal albumin - actual albumin)] + measured calcium (mg/dL)
Dual-energy X-ray absorptiometry (DXA)
What are some consequences of calcium deficiency?
Rickets
Increased risk for osteoporosis
Codeficiency of vit.D
Hypocalcemia–> tetany
Hypertension, pre-eclampsia
What is the prevalence of inadequacy of calcium?
children: (4-8, 9-13) high
> 14: high prevalence
Can calcium supp. (w or w/o vit.D) help prevent cancer and fractures?
reduce risk of fracture
Does calcium help with weight loss?
may help with weight loss
