Calcium 6 pp - Calcium Food Sources Calcium 2011 Major...

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Calcium 2011 Calcium Food Sources • Major source : dairy products – Milk or yogurt: 1 cup 300 mg • Ca-fortified juices – Some brands: 1 cup 300 mg Calcium Food Sources = 16 servings of spinach 1 cup milk Calcium in spinach not very bioavailable Other Sources 8 oz = 100 mg Ca 20 oz = 250 mg Ca 2 chews = 1000 mg Ca 2 slices = 250 mg Ca Amount o f Ca varies in different supplements: To provide 500 mg Ca must take 2.4 g Ca citrate vs 1.3 g Ca carbonate Bioavailability ~30% all supp forms of Ca salts Calcium: Absorption • Variable absorption – Adults: 25-60% – Infants: 60% • 2 mechanisms Current Model for Role of D-Dependent Ca Absorption Calcium extrusion protein “Basolateral Ca Pump” Calcium channel protein
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1) Active transport • Calcium binding proteins – unknown which proteins are responsible Calcium: Absorption Mechanisms • Calbindin no longer thought to be responsible for calcium uptake – Kutuzova et al. (2006) – calbindin knockout mouse have normal calcium status Calcium Absorption Mechanism • Calcium Channel Protein (TRPV6) –New research evidence indicates that this protein also is not required for calcium absorption • Kutuzova et al. PNAS 105: 19655, 2008 Calcium Absorption Mechanism • Basolateral Calcium Pump –Knockout mouse model lethal so unable to determine effect on blood calcium –Maybe the vitamin-D dependent protein essential for calcium absorption 2) Passive absorption • Paracellular route • Not under homeostatic control Calcium: Absorption Mechanisms Not under homeostatic control • Influenced by Ca content of meal – As Ca load increases, absorption decreases although net absorption increases Calcium: Absorption Tip Absorption is more efficient if Ca is consumed in divided doses throughout the day Calcium: Absorption Inhibition • Oxalic acid – green leafy vegetables – Strongest inhibitor of Ca Forms a salt with Ca – Forms a salt with Ca • Phytic acid (phytate) – Oatmeal & whole grain pita bread – More modest inhibitor than oxalate but consumed in higher amounts
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Calcium: Phytic acid Phytate is less detrimental when bread is leavened and phytate complexes are hydrolyzed by enzymes in yeast during fermentation Unleavened Leavened Calcium: Interaction with Nutrients • Dietary sodium is the key dietary factor influencing urinary calcium loss –Every additional 1 gram sodium results in an additional urinary loss of ~26 mg Ca in adults Na = Ca Calcium: Absorption • Factors influencing absorption – Vitamin D – Hormonal status (e.g. estrogen increases) – Dietary calcium – Dietary calcium • Low Ca intake – increases rate of absorption • Prolonged Ca deficiency – Not completely corrected by an increase in absorption efficiency – Bone loss ensues to maintain serum calcium Table 11-2, p. 433 Calcium: Absorption • Efficiency varies with calcium demand – Rapid growth: Increased absorption • Adolescence • Pregnancy & lactation –Age • Efficiency declines with age in men and women and is very rapid after age 60
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Calcium 6 pp - Calcium Food Sources Calcium 2011 Major...

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