Dietary Calcium Flashcards
Role of calcium in bone health:
A. Most abundant mineral in the body; average adult male contains ~1200-1500 g (1.7% of total body weight); 99% is in the skeleton;
- 0.7 g skeletal calcium turns over each day; bone Ca turns over ~ every 5-6 yr; extracellular Ca turns over 20-30x/day.
B. Metabolism and Homeostasis:
a. Key hormonal regulators that effect Ca Homeostasis are: PTH, 1,25(OH)2D, and Calcitonin; of course many others can play a role, especially at specific times in the life cycle
b. Serum Ca is maintained in a very tight range at all costs
c. Absorption occurs passively in duodenum, jejunum, ileum; Active transport (effected by habitual intake via vitamin D) occurs in the duodenum
a. Enhancers of Absorption: Vitamin D; physiological demand; lactose; Gastric Acidity; protein
b. Impair Absorption: Vitamin D deficiency, Steatorrhea, Oxalic and Phytic Acid, Gastric Alkalinity
d. “Deficiency” acutely is a rare occurrence in adults because of the body’s ability to maintain serum levels; with low Ca intakes, however, because maintenance of serum [Ca] is at the expense of bone Ca. Thus development of deficiency is a long-term, “silent” process.
C. Critical times in the life cycle for Ca++ intake:
a. Premature infants: 3rd trimester period of rapid bone mineral accretion; preterm infants at risk for “osteopenia of prematurity”
b. Adolescence: hormonal milieu during puberty favors calcium absorption and bone deposition; ~ 50% of total adult bone mineral mass is accrued during adolescence.
c. After skeletal “maturity,” esp. peri-menopause: associated with relatively high requirements, increased losses, and frequently with low intake
d. Pregnancy and lactation: physiologic responses compensate for increased Ca demand so dietary requirements are not increased
e. Post-Bariatric Surgery
Metabolism and Homeostasis of Calcium
a. Key hormonal regulators that effect Ca Homeostasis are: PTH, 1,25(OH)2D, and Calcitonin; of course many others can play a role, especially at specific times in the life cycle
b. Serum Ca is maintained in a very tight range at all costs
c. Absorption occurs passively in duodenum, jejunum, ileum; Active transport (effected by habitual intake via vitamin D) occurs in the duodenum
i. Enhancers of Absorption: Vitamin D; physiological demand; lactose; Gastric Acidity; protein
ii. Impair Absorption: Vitamin D deficiency, Steatorrhea, Oxalic and Phytic Acid, Gastric Alkalinity
d. “Deficiency” acutely is a rare occurrence in adults because of the body’s ability to maintain serum levels; with low Ca intakes, however, because maintenance of serum [Ca] is at the expense of bone Ca.
i. Thus development of deficiency is a long-term, “silent” process.
Critical times in the life cycle for Ca++ intake:
a. Premature infants: 3rd trimester period of rapid bone mineral accretion; preterm infants at risk for “osteopenia of prematurity”
b. Adolescence: hormonal milieu during puberty favors calcium absorption and bone deposition; ~ 50% of total adult bone mineral mass is accrued during adolescence.
c. After skeletal “maturity,” esp. peri-menopause: associated with relatively high requirements, increased losses, and frequently with low intake
d. Pregnancy and lactation: physiologic responses compensate for increased Ca demand so dietary requirements are not increased
e. Post-Bariatric Surgery
Dietary and lifestyle factors that affect bone health
Remember: Genetic/intrinsic factors dominate the determination of bone mineral density (BMD); 60-80% of variance in peak bone mass attributable to heritable factors.
A. Nutritional/dietary factors:
a) Lifetime calcium intake:
- Adolescent females average ~80% of recommended amount (1300 mg/d)
- Adolescence = key time for bone mineral accretion
- Limited ability to adapt to low Ca intake
- Lifetime low intake of dairy products associated with increased osteoporosis
- Optimal calcium intake allows bone density to reach genetic potential
b) Protein intake: increased urinary calcium excretion (in some studies low Ca:protein intake associated with lower bone density); net effect may be negligible, but controversial;
c) Phosphate intake: ? increased PTH, increased bone resorption
d) Vitamin D – critically linked to calcium absorption:
− Status frequently suboptimal, esp in elderly;
− 25(OH)D levels (+) correlated w/ BMD in middle-aged & elderly women
− Target serum 25(OH)D level >30 ng/ml to reap skeletal benefits
e) Vitamin K - co-factor w/ osteocalcin & other bone forming proteins
f) Sodium intake – high Na++ intake increased urine Ca++ excretion
g) Vegetarian diet, w/ high fruit & veg intake (& K+, Mg++ ) associated w/ bone mass preservation, but if diet is low in dairy & high in sodium, not likely to be positive effect
h) Caffeine – increases urine Ca++ excretion (small effect x lifetime)
i) Whole Diet pattern: DASH may have benefits beyond increased Ca intake
B. Behaviors/lifestyle
a) Exercise: weight-bearing;
- muscle mass directly related to bone mass
- Mechano-sensors in bone stimulate osteoblasts
b) Smoking
c) Alcohol - depresses osteoblasts
C. Medications/medical conditions
a) glucocorticoids, immobility, gastrectomy
b) Maintenance of regular menses
b) Chronic illnesses – associated w/ malabsorption, chronic systemic inflammation
Dietary and Supplement sources of Calcium
A. Dietary Sources: Dairy products contribute ~70% of Ca diet in US (slight ↓ past few decades - ↑availability of fortified products?) Example Food: mg Ca Milk, 1 cup ~ 300 Yogurt, 8 oz. 350-450 Tofu, 4 oz (processed w/ Ca++) 155 Salmon, canned, w/ bones, 3 oz ~200 Soy milk, fortified, 1 c 295 Collard greens, ckd , 1c 350 Beans, navy, ckd, 1 c 90
B. Supplement Sources:
Ca-Carbonate:
- max elem. Ca++/tablet: 1250 mg = 500 mg elemental Ca++;
- best absorbed w/ meals; has least lead, esp manufactured (vs bone meal, oyster shell, dolomite);
CaCitrate:
IV. Strategies to optimize bone density
− Achieve “peak bone mass” when you can – ie, adolescence
− Weight bearing activity
− Maintain good calcium intake over lifetime
− Avoid excessive alcohol and tobacco
− Minimize practices that enhance calcium loss &/or bone resorption
− Maintain healthy diet including other cofactors that support Bone Health
♣ DASH diet: reduction in Na+ not only associated with reduced blood pressure but also with reduced Ca++ excretion and bone turnover.
− Supplement (only) when necessary
♣ Results of meta-analysis supports benefits of calcium supplementation ± Vit D supplementation on fracture reduction and reduced bone loss in subjects > 50 yr.
• Mild caution re Ca supplementation: Ca supplementation (w/o Vit D) associated with increased risk of myocardial infarction
Major Minerals:
a. Require intake > 100 mg/day
b. Contribute > 0.01% of body weight
Include:
Calcium Phosphorus
Magnesium Sodium
Potassium Chloride
Sulfur
c. Most abundant mineral: 1200-1500 g
Calcium: General Functions
a. Structural functions of extracellular Ca
i. Ca Hydroxyapatite Ca10(PO4)6(OH)2
ii. Bone & teeth (99%)
b. Metabolic functions of extracellular Ca
i. ~1% total extracellular Ca++
ii. Tertiary structure: activate catalytic & mechanical properties
iii. Most common signal transmitter in biology:
- clotting, nerve impulse transmission/relaxation, mediation of hormones, growth factors
Ca Homeostasis:Hormones
*Good review slide
a. Decrease in Serum Calcium
↑ PTH :
i. Increase Bone resorption of Ca
ii. Increase Ca reabsorption in ascending loop of henle
-(decrease urinary [Ca])
-Increase Phosphate excretion via kidney
iii. ↑ Vitamin D: Increase Ca intestinal absorption
b. ↑ Serum [Ca]…
i. ↑ Calcitonin—> Deposition of Ca into Bone
ii. Decrease PTH
Ca Homeostasis at the GI
a. Calcium is taken by active and passive transport
i. Passive absorption is driven by high concentrations of Ca in the gut, simply diffuse passively though
b. Active Transport happens with low to medium calcium amounts
i. Will need to get Ca into the enterocyte with unique proteins
ii. need important pump proteins on the basolateral side to get Ca to the blood
iii. Active transport happens mostly at the duodenum
iv. Vitamin D is important for active transport
c. When Ca intakes low/moderate – active = major source of Ca – occurs in duodenum VDR highly expressed
d. When Ca intakes high, passive = major source – occurs length intestine mostly in duodenum, jejunum, ileum
VDR/VDRE regulation of:
- Intestinal Ca Absorption
- Renal Ca Excretion
a. For the active transport of Ca, Vitamin D is critical
i. Active form of Vitamin D (1,25 OH Vitamin D3) will enter cell and bind to receptor
ii. Active Vit D3 will bind to the VDR, will lead to transcription and building of BDRE
b. This increased transcription leads to:
1. Increased basolateral membrane transport
2. Cystolitic shuttling
3. Apical membrane Export
* These three set of proteins lead to increased duodenum Ca2+ active absorption, also effects the nephron of the kidney
Calcium Absorption & Habitual intake
a. Limited “up-regulation” to compensate for low intake
i. 2,000 to 300 mg/day
ii. Absorption Increased from 27 to 37% in 1-2 wks
iii. 540 —> 111 mg absorbed
b. Ca intake (chronic) is associated w/ Increased bone accretion
Calcium Absorption Enhanced by…
Physiological
a. Vitamin D –> synthesis of Ca-transport proteins
b. Increased physiologic demand (Pregnancy/adolescence)
Dietary
a. Gastric acidity: Release from food matrix
b. Lactose (maintains solubility)
c. Dietary protein*: Increased intake assoc w/ increased absorbed Ca
[NOT by bone mineral depletion]
Calcium Absorption Impaired by…
Physiological
a. Vitamin D deficiency: northern latitudes, limited skin exposure, dark pigmentation, elderly
b. Steatorrhea: unabsorbed fatty acids bind Ca2+ —> “soaps”
Dietary
a. Gastric alkalinity: especially gastric achlorhydria in elderly
b. Oxalic acid: ex: spinach
c. Phytic acid : ex: legumes, soy beans, corn, wheat
d. Caffeine: ↑ urine Ca2+ (Easy to offset)
e. Dietary Protein*: Increased urine Ca2+ (net effect neutral/positive)
Ca Absorption Through the life cycle
Rates of absoprtion throughout different points of life:
a. Average healthy American Adult ~ 25%
b. Fetus: 80% transfer in 3rd trimester
c. Infants: ~40 - 60% (lactose)
d. Early puberty ~34%
e. Pregnant women ~50%
f. May ↓ in elderly (VD, gastric acidity)
