(2) phosphorus and vitamin D

phosphorus- function

bone mineralization, component of nucleic acid (DNA/RNA), phospholipids, and ATP, activation/deactivation of enzyme, buffer to maintain blood pH, and oxygen availability

phosphorus RDA

9-18: 1250mg, 19+: 700mg

phosphorus- sources

milk, yogurt, cheese, salmon, beef, poultry, nuts, processed foods

phosphorus deficiency- hypophosphatemia

rare, a serum phosphorus concentration that is less than the lower end of the normal range of about 2.5 mg/dL

as serum phosphate concentrations drop below about 1.5 mg/dL

anorexia, confusion, muscle tissue damage, bone is also impacted, rickets occurs in infants and children, in adults osteomalacia occurs due to bones that are soft due to inadequate mineralization of the bone matrix

a severe phosphorus deficiency concentrations less than 1.0 mg/dL

reduced oxygen transport and delivery, reduced cardiac output, arrhythmias, decreased diaphragmatic contractility, respiratory failure, skeletal muscle and cardiac myopathy, and neurological problems (ataxia and paresthesia), death

populations at risk of phosphorus deficiency

premature infants, people being re-fed, people undergoing diabetes treatment, critical illness and chronic alcoholic consumption

phosphorus- toxicity

UL= 4000 mg/day, mineralization of soft tissues, bone demineralization

Vitamin D- forms

Vitamin D2 is produced by yeast and fungi when exposed to UV light- vegan, and Vitamin D3 is produced by animals and humans

Vitamin D- activation in the body 

it needs to be hydroxylated twice to form 1,25 dihydroxy vitamin D, first hydroxylated in the liver, then a second hydroxylation occurs in the kidneys, after the second hydroxylation, 1,25(OH)2D is released into circulation

Vitamin D function- calcium homeostasis

calcitriol functions within the kidneys, the vitamin is also released into the blood bound to VDBP and then acts alone on its other target tissues (the intestine and bone), causing serum calcium concentrations to rise to within the normal range

Vitamin D function in the kidneys

PTH stimulates 1-hydroxylase to convert 25-OH D to calcitriol, which increases calcium reabsorption in the kidneys by interacting with nuclear VDRs to directly regulate specific genes encoding for proteins involved in calcium reabsorption and to raise serum calcium concentrations to within a normal range 

Vitamin D function in the intestine

up-regulate transcription for proteins involved in calcium absorption

Vitamin D function in the bone

calcitriol interacts within the nucleus of mature osteoblasts leading to the expression of a cytokine called receptor activator of NFκb ligand (RANKL) which interacts with the receptor protein found on the cell surface of immature osteoclasts, to stimulate osteoclasts

phosphorus homeostasis

in the intestine, calcitriol increases the activity of brush border alkaline phosphatase, which frees phosphorus bound to food to allow for absorption and up-regulates carrier proteins responsible for absorption

Vitamin D- cell differentiation, proliferation, and growth 

maintain cell growth, promotes terminal differentiation, and inhibits proliferation, it guides stem cells to become specific immune cells (macrophages) and bone cells (osteoclasts) 

Vitamin D- immune function

increase production of anti-inflammatory cytokines and down-regulates pro-inflammatory cytokines, involved in differentiation of immune cells

Vitamin D- muscle structure and function

function through genomic mechanisms to enhance calcium uptake and intracellular calcium concentrations, changes in intracellular calcium concentrations are needed for contraction and relaxation of muscle

Vitamin D’s non-genomic actions through cell signaling

calcitriol binds to cell membrane receptors in certain tissues to trigger a series of events to evoke cellular response

The genomic mechanisms refer to vitamin D’s role in regulating gene expression in target cells

calcitriol moves from the cytosol into the nucleus, where it binds to nuclear vitamin D receptors (VDRs)

Vitamin D- RDA

14-70: 15mcg, 70+: 20mcg

Vitamin D- sources

sun, fatty fish (trout/salmon), mushrooms

Vitamin D- deficiency

impairs bone mineralization at all ages, but how it manifests depends on whether the growth plates are open or closed 

Vitamin D deficiency- children

may lead to rickets due to a failure to mineralize the growth plates of developing bones which leads to bowed legs and widened wrists

Vitamin D deficiency- adults

results in osteomalacia which is a failure to mineralize existing bone during remodeling, this "softening" of the bones leads to bone pain, muscle weakness, and increased fracture risk

populations at risk for vitamin D deficiency 

insufficient sun exposure, older adults, malabsorptive disorders, bariatric surgery, disorders affecting liver/kidney, and people on anticonvulsant drug therapy 

Vitamin D- toxicity 

TUL= 100mg, can lead to high serum calcium, high serum phosphorus, and hypercalciuria, calcinosis, the calcification damages the blood vessels and tissues, resulting in hypertension, headache, renal dysfunction