Dietary Ca and Bone Health Flashcards
Ca hydroxyapatite
what mineralizes bone/teeth
99% of Ca
metabolic functions of extracellular Ca
- 1% of total Ca extracelluarly
- signal transmitter
- tertiary structure: activate catalytic and mechanical properties
- clotting, nerve impulse transmission/relaxation of muscle, mediation of hormones, growth factors
–VERY IMPORTANT!!
low Ca results
increases PTH:
- bone resorption, decrease Ca excretion, increased P excretion
activates
itself increases vitamin D hydroxylation in kidney, but increased PTH also does this — increased Ca intestinal absorption and decreased excretion in urine
results of increased serum Ca
- calcitonin increases: deposition of Ca into bone
- lower PTH
- lower Ca reabsorption in ascending loop of Henle
2 major ways Ca absorbed
1) Active: (3 steps)- vit D dependent
- apical membrane absorption – mostly mediated by TRPV6 (duodenum)
- transport to basal membrane via shuttling – Calbindins mainly
- get into blood: Na/Ca pump and ATP dependent Ca export
2) Passive:
1,25(OH)2 Vit D
active form of vitamin D; serves as gene transcription regulator–enters nucleus, and if cell expresses receptor (VDR) will bind and dimerizes with RXR. This complex binds to VDRE (vitamin D response elements) to upregulate genes
where does passive Ca absorption occur
duodenum, jejunum, ileum
- driven by concentration
- transcellular or paracellular via Ca absorption channels that only work when gradient present
where does active Ca absorption occur
duodenum
genes regulating vit D response elements
TRPV6, TRPV5, CaBP-9K & 28k, PMCA
increased transcription of all genes involved in getting Ca in
Ca absorption pattern
- generally more Ca intake, more absorbed (passive)
- lower intake the higher body will regulate active Ca absorption but not perfect, body can only do so much
- increased Chronic Ca– increased bone accretion
Ca enhanced by
- increased physiologic demand (prego, adolescence)
- vitamin D status
- lactose (maintains solubility)
- gastric acidity–need to release Ca from fod matrix
- Dietary protein – increase intake assoc with increased absorbed Ca
- NOT by bone mineral depletion
what drives Ca absorption
serum Ca NOT bone mineral depletion
Ca absorption impaired by
- vitamin D deficiency (northern latitudes, limited sun exposure dark pigmentation, elderly)
- Steatorrhea – unabsorbed fat binds Ca–soaps
- Gastric alkalinity– esp achlorhydria in elderly
- Oxalic acid: ex spinach
- Phytic acid: ex: legumes,
- Caffeine: increased urine Ca (easy to offset)
- dietary protein: increased urine Ca (net effect neutral/positive)
dietary protein effects on Ca
increases absorption and Ca release in urine; net effect neutral/positive
how does bone form
most formed through endochondral bone formation
- cartilage mineralizes and growth plate, which allows for longitudinal bone
types of bone
- trabecular–spongy, much more metabolically active; can absorb impact
- cortical– much more dense; what gives bone strength due to more hydroxyapatite
Bone turnover
happening all over bone; contnual balance between blasts making and clasts breaking down bone
- becomes bad when resorbtion > formation
how often do bones turn over
entire skeleton turns over every 10 years
Ca absorption through life cycle
- avg healthy adult about 25%
- fetus: 80% transfer in 3rd trimester–lots through placenta; doesn’t need much earlier since don’t have cartilage skeleton to mineralize
- infants: 40-60%
- early puberty- 34%
- pregnant women - 50%
- may decrease in elderly (vit D low, gastric acidity, exercise decreases)
Preterm infants and Ca
30g crosses placenta
- 80% transfer in 3rd trimester (endochondral bone formation)
- risk of osteomalacia of prematurely–bones at high risk of fracture
Ca in older children
worry about Ca deficiency– Rickets–very severe Ca deficiency, can’t mineralize bones and see bowing of bones supporting weight
- not often in US but more in African diets with less dairy
Does Ca intake matter in young children
YES
- probs not big effect with supplementation just acutely
- Ca intake matters! esp if chronic
when do you reach peak bone mass
about age 30
- a little you can do to modify
Ca absorption in puberty
super high early, and slower bone mineralization occurs throughout adolescence
