Dietary Calcium Flashcards
Ca functions
-Ca hydroxyapatite: bone, teeth (99%)
-Metabolically active extracellular Ca (1%)
signal transmission (most common)
tertiary structure: activate catalytic and mechanical properties
Clotting, nerve impulse transmission/relaxation, mediation of hormones, growth factors
Decreases Serum Ca
increase PTH: increase bone resoption of Ca, decrease Ca excretion, increase phosphate excretion
increase Vit D hydroxylation in kidney: increased Ca intestinal abs, decreased excretion in urine
increased serum Ca
-increases Calcitonin (deposition of Ca into bone)
decreased PTH
decreased Ca reabsorption in ascending loop of henle
active absorption of Ca
- 3 steps:
1. apical membrane absorption (TRPV6 protein in duodenum)
2. Ca in enterocyte transported to basolateral membrane (Calbindin)
3. out of enterocyte into blood: ATP dependent Ca export; PMCA
*Vit D regulated
Gene regulation by Vit D
1,25 OH Vit D3 serves as gene transcription regulater
- if cell expresses Vit D receptor, VDR dimerezes with RXR and bind to Vit D response element (VDRE)
- serves as upregulator of gene trancription
Genes regulated by VDRE
basolateral memb import: TRPV6 and TRPV5 Cytosolic trans: Ca BP-9K CaBP9K and 28K
Apical membrane export:
PMCA 1b
PMCA 1b
Top: duod
bottom:distal nephron
Ca absorption and habitual intake
-Ca upregulates absorption percentage with lower intake of Ca to compensate
Ca absorption enhanced by
Vit D: synthesis of Ca-transport proteins in proximal small intestine
Increased phsyiologic demand (preg/adolescence)
lactose
gastric acidity
dietary protein
Ca absorption impaired by
Vit D deficiency
Steatorrhea (fat malabsorption): unabsorbed FA binds Ca2+, forming Ca based “soaps”
Gastric alkalinity oxalic acid (spinach) phytic acid caffeine (increase urine Ca) dietary protein (increase urine Ca)
Ca hydroxyapatite stronger/increased in cortical or trabecular?
cortical
Bone formation and resorption
- PTH and 1,25 vit D increase bone resorption by activating osteoclasts
- formation: osteoblasts form a matrix to replace resorbed bone with new bone
Avg health adult absorbs how much Ca
25%
Fetus: 80% in 3rd tri
May decrease in elderly (decreased Vit D, decreased gastric acidity)
Why is preterm infant at risk
- hard to match high level of Ca transfer that would occur in utero (330 mg/d at 35 weeks)
- can get osteomalacia of prematurity (deficits of bone mineralizaiton)
Ca supplementation in young children
-increase in skeletal size and mineralization, but fail to show consistently that BMC is retained over the long term
When is 30-50% of bone mass accrued?
- adolescents (calcium absorption and retention highest in early puberty; slower bone mineralization throughout adolescence)
- peak bone mass at 30 y
- menopause: decrease
During pregnancy and lactation, what requirement of Ca increases?
-physiological but not dietary requirements increase.
Pregnancy: Ca abs increases to meet fetal demand
Lactation: PTH increases and bone mass lost, but recovered post weaning
Ca supplements
Calcium carbonate (best absorbed with meals) Calcium citrate (best absorbed between meals) 43% of americans take Ca supplements
Can Ca supplements be harmful?
- increased risk of MI ~30%
- Dietary Ca not associated w/ increased risk
- too much supplementation esp for those who do not need it may be harmful
Ca needs not being met in which pops? high risk groups?
- adolescent girls
- older men and women
High risk groups? premature infants adolescents peri-menopausal women [bariatric surgery pts]
Osteoporosis
- reduction in bone mass associated w/ impairment in bone structure; increased fracture risk
- loss of height and distorted body shape
25% of women over 50 develop osteoporosis
Fractures often lead to institutionalization
Non-nutritional factors assoc with bone mineral density
- genetics (70-80% of peak bone mass driven by genetics)
- initial bone mineral density
- hypogonadism (esp decreased estrogen)
- age
- meds (esp corticosteroids): glucocorticoids, chronic illness, magnitude of steroid use
- behaviors/lifestyle (tobacco, alcohol; opposite (positive) effect with weight bearing exercise
Nutritional Factors assoc with BMD
-lifetime Ca intake
-Vit D
-caffeine
-protein
-sodium intake (increased Ca in urine)
Vegetarian diet (decreased Ca in urine)
Other minerals
phosphorous
Mg (can give hypoparathyroid)
Vit C (collagen)
Vit K (cofactor with osteocalcin)
ideal diet for bone health
DASH -fruit/veg low fat dairy whole grains poultry, fish, nuts
Limit:
red meat
sweets, sugar
total and unsat fat
(decreased turnover of bone)
Prevention strategies to optimize bone density
- achieve peak bone mass when possible (childhood and esp adolescence)
- dietary focus: Ca, Vit D, Vit K, protein, decreased Na
- Maintain ovulation/regular menses
- weight bearing exercise
- avoid: smoking/alc/steroids
supplement: judiciously when necessary