Bone Health Flashcards by Erin Black

Vitamin D’s active form is

Calcitriol = 1,25 (OH)2D

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Form of Vitamin D from animals

Cholecalciferol

Vitamin D3

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Form of vitamin D from plants

Ergocalciferol

Vitamin D2

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Main circulating form of Vitamin D, and biomarker of status

Calcidiol

25 (OH)D

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Food Sources of Vitamin D

Fatty Fish, Liver
Shiitake mushrooms
Fortified milk or orange juice

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Digestion of Vit D. From diet

Non required

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Absorption of Vitamin D

With fats- micelles, chylomicrons

Fat soluble

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Synthesis of vitamin D

In skin with exposure to UV light. Influenced by skin exposure, color, age, time of day, season

Cannot get toxic levels of Vit D through sun

Makes another product that gets sloughed off with dead skin cells

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Transport of vitamin D

Bound to vitamin D binding proteins (DBP)

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Activation of Vitamin D

Liver: calciferol converted to 25(OH)D -main circulating form = calcidiol = 25-hydroxyD

Kidney: 25(OH)D converted to 1,25(OH2)D - active form

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Functions of Vitamin D

Genomic and non-genomic
Calcium regulation
Cell differentiation + proliferation

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Vitamin D’s role in Calcium homeostasis

Low calcium = PTH increases = activates Vitamin D.

Vit D + PTH= 1. Increase Ca absorption 2. Increase Ca reabsorption form kidney 3. Promote cellular resorption from bone

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Vitamin D deficiency

In children- Rickets under mineralization - soft bones
In adults- Osteomalacia loss of bone minerals, soft bones
At risk: limited sun exposure (Vancouver winter)
Fat malabsorption
Elderly
Breastfed infants

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Calcium Sources

Milk/dairy
Tofu
Seafood
Some vegetables and legumes

Spinach is low bioavailable
Dairy is good bioavailble

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Digestion of Calcium

Convert Calcium salts to free Ca2+

Using Stomach acid - HCl

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Absorption of Calcium

Carrier mediated: TRPV6/ Calbindin / Ca-ATPase (enhanced by Vit D)
Paracellular transport

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Enhancers of Ca absorption

Protein
Sugars
Gastric activity

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Inhibitors of Ca absorption

Oxalic acid, phytic acid
Divalent cations
Fibre
Fatty acids

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Absorption of Calcium decreases with age due to

Less expression of calcium transporters
Less intake and synthesis of vitamin D
Lower stomach acidity

Excretion of calcium

Enhanced with protein, caffeine and sodium

Homeostasis of calcium

Extracellular Ca: (blood)

increased by PTH, calcitriol
decreased by calcitonin

Intracellular:
-regulated by cell uptake/secretion + release/sequestering in organelles

Functions of Calcium

Bone Mineralization
Muscle contraction 
Blood clotting
Signal transduction
Enzymes
Nerve transmission

Nutrient Interactions of Calcium

Ca influences absorption of Iron, Phosphorus & Fatty acids

Absorption:
Enhanced by -protein , sugars, Vit D

Inhibited by -phytic acid and oxalic acid, fibre, fatty acids, divalent cations

Excretion:
Enhanced by protein, caffeine, sodium

Calcium deficiency

Osteoporosis
Neuromuscular impairment
Risk: low intake, Vit D deficiency

Excess Calcium - Toxicity

Hypercalcemia: | Lethargy, vomiting, heart arrhythmias

phosphorus forms

Inorganic: Pi Organic: phosphates - phospholipids, phosphorylated proteins

Functions of phosphorus P

Structural component of Bone Component of biological molecules: DNA, ATP, phospholipids Regulation of enzyme function

Sources of Pi Phosphorous

Widespread | Meats, dairy, legumes, grains

Digestion of phosphates

Organic phos. Converted to inorganic P by phosphatases

Absorption of P

Main is Paracellular diffusion | Can also happen via active transport

Excretion of P

Urinary excretion is main

Bioavailablity of Phosphorous

Decreases with high intakes of Ca or Mg (antacids)

Homeostasis of P

Regulated by FGF23 & PTH - promotes urine excretion = lowers serum P Increased serum P by 1,25(oh)2 = increases absorption, reabsorption, done resorption

Phosphorous deficiency

Uncommon Risk if: severe malnutrition, hyperthyroidism Leads to rickets, osteomalacia

Excess phosphorous

HyPOthyroidism, renal disease, excess Vit D Leads to calcification of soft tissues & increases risk of heart disease UL set.

Magnesium Functions

Bone mineralization Enzymatic function - structural cofactor (stabilize enzyme) ->** hydroxylation of vitamin D (25 hydroxylase Ion channel flux - mg is a Ca channel blocker Insulin signalling

Digestion of Mg

None required

Absorption of MG

Mg-specific transporter Inhibited by high Mg concentration Paracellular diffusion

Bioavailability of Mg

Inhibited by: phytic acid, fibre, fatty acid, phosphorous

Urinary excretion of Mg

Increased at high Mg levels | Decreased by PTH

Homeostasis of Mg

Maintained through regulation of absorption and urinary excretion PTH increases renal reabsorption, bone resorption, and w/ V D increased intestinal absorption At high intakes, absorption if decreased and excretion is increased

Nutrient interactions with mg

Deficiency of Mg

Leads to low Ca, K, Vit D Neuromuscular, cardiovascular, and CNS effects Low intake is common, associated with increased risk of hypertension and type 2 diabetes

Mg supplements have benefits for

May have benefits for sleep, migraines, and mood disorders

Toxicity of magnesium

UL set based on harmful effects of excess intake from supplements