Vitamins Part 2

Part 2: Calcium and Bone Related Vitamins

Calcium

There is normally 1.2 kg in adult body; 99% of this is in bones and teeth – rest is used for regulatory functions (as Ca⁺⁺)

Regulation of blood Ca⁺⁺ occurs by means of three hormones:

(1) 1,25-dihydroxy vitamin D₃ (calcitrol) – facilitates intestinal calcium absorption (see "vitamin D," below)

absorption inhibited by excess fat/mineral oil/fiber in intestinal lumen, also glucocorticoids or diphenylhydantion

(2) parathyroid hormone (PTH) – affects kidney calcium conservation and bone resorption

(3) calcitonin – produced by thyroid gland – removes Ca⁺⁺ from blood, more bone formation/less bone loss

Four Sources of Calcium:

(1) dairy products – milk, yogurt, cheese, etc. – however not all dairy products contain vitamin D for calcium absorption

(2) canned fatty fish – Ca⁺⁺ diffuses from fish bones

(3) leafy greens – Ca⁺⁺ well absorbed from bok choy, kale, and broccoli; less so from spinach

(4) calcium supplements – Ca⁺⁺ administered as salt – attached to anion (carbonate, lactate, gluconate, malate, etc.)

CaCO₃ not well absorbed if not much stomach acid (achlorohydria, 10-30% elderly) – only with meals
natural supplements (bone meal, dolomite, oyster shell, etc.) often have high lead levels – not recommended

Calcium Deficiency:

tetany – low plasma Ca⁺⁺, leading to neurological problems – rare

rickets that does not respond to vitamin D – primarily infants/young children

osteoporosis – decreased bone mineral density – likely to fracture spontaneously, especially in spine, hips, wrist

more common in elderly, female, Asian and Caucasian
much less than 50% of Americans get RDA of calcium – come to maturity with less than optimal bone mass
many risk factors – most significant: insufficient dietary Ca⁺⁺/vitamin D – vitamin D deficiency associated with fractures

history: prolonged immobility, previous atraumatic fracture, smoking, alcohol
chemical: menopause (estrogen withdrawal), anticonvulsants (interfere with vit.D), glucocorticoids, thyroid hormone
physiological: thin and/or frail (less lean tissue surrounding skeleton)
associated conditions: chronic liver disease, malabsorption, gastrectomy, hyperparathyroidism/hyperthyroidism

diagnosis: DEXA screening – very expensive
treatment:

stimulating formation of mineralized bone
regulating bone loss
preventing falls

Calcium Excess – clinical manifestations:

nephrolithiasis (kidney stones)
hypercalcemia
renal insufficiency (milk-alkali syndrome)

Vitamin D – maintains Ca⁺⁺ levels by causing intestinal absorption/bone resorption

Metabolically active vitamin D binds specific intracellular nuclear receptors to stimulate transcription of absorptive genes

also some effect on proliferation of lymphocytes – not well understood

Common forms: vitamin D2 (ergocalciferol) and vitamin D3 (calciferol)

Synthesis of Vitamin D:

(1) 7-dehydroxycholesterol in skin Þ vitamin D – energy provided by UV B sunlight (290-315 nm)

10-15 minutes on hands and face 3 times per week is sufficient for most people
complications:

insufficient sun exposure – institutionalized/homebound(windows filter UV), winter, sunscreens
insufficient sunlight absorption – decreased skin thickness in elderly, melanin in darker skin

(2) vitamin D Þ 25-hydroxy vitamin D – takes place in the liver – forms active vitamin D (this form measured clinically)

anticonvulsants block this step

(3) 25-hydroxy vitamin D Þ 1,25 dihydroxy vitamin D – takes place in the kidney – another active form

lead poisoning blocks this step

Diseases of kidney or liver can cause deficiency; active form must be administered for therapeutic replacement

vitamin D replacement rickets – genetic defect – no improvement even with sufficient Ca⁺⁺ and vitamin D given

Dietary Vitamin D: Many foods high in calcium do not have vitamin D (yogurts, cheese, ice cream, greens, almonds)

vitamin D added to fluid milk in the manufacturing process; it also is found naturally in fatty fish (cod liver oil for rickets)
vitamin D is NOT required to be listed on labels (only calcium, iron, vitamin C, and vitamin A

Deficiency: rickets – poor bone/tooth mineralization, stunted growth, deformities (e.g., "rosary beads"); also osteomalacia

Toxicity: hypercalcemia, Ca⁺⁺ deposition in kidneys/ateries; loss of appetite/poor growth in infants

hypervitaminosis D during pregnancy: mandible hypoplasia/absence of certain teeth

Vitamin K (phytomenadione)

cofactor in carboxylation of glu to g -carboxyglutamic acid – mainly blood clotting (prothrombin, Factors VII, IX, and X)

also bone matrix-G/a protein and osteocalcin (secreted by osteoblasts, 10% of non-collagenous bone formation)

from diet (leafy vegetables) and intestinal bacterial flora

Boron

influences macromineral metabolism and steroid metabolism – beneficial to Ca⁺⁺ metabolism and estrogen abs.

from diet (leafy vegetables, non-citrus fruits, nuts, legumes)

Phosphorus

calcifies bones/teeth (with Ca⁺⁺); ATP; nucleic acid backbone; enzyme/vitamin activation; intracellular buffer

from animal sources rich in protein or phosphates (processed foods and soft drinks)