Calcium and phosphorus: phosphorus

Question 1

What are some sources of Phosphorus?

Answer 1

widely distributed in foods

meat, poultry, fish, eggs, dairy

Cola type soft drinks

Phosphate containing supp.

May be inorganic (PO42-) or organic (phospholipids)

Found in some foods as phytic acid (less bioavailable)

Question 2

Where can phosphorus be found?

Answer 2

> 80% P found in grains (wheat, rice, corn): phytic acid

~50% P absorbed

Mammals lack phytase to breakdown

Question 3

How is phosphorus being absorbed in the small intestine?

Answer 3

**i = P is absorbed ias inorganic P ions

1. Pi may be absorbed by diffusion, primarily in the proximal duodenum

or

2. when P intakes very low: Pi may be absorbed by an active sodium phosphate cotransporter (ATPase)–> enhanced by calcitriol

Question 4

What happens after Pi is absorbed into the enterocytes?

Answer 4

After hydrolyzing enzymes release organically bound-phosphorus–> Pi–> enterocytes:
exit basolateral membrane via facilitated diffusion
–> Blood:
1. Pi complexed w/ other minerals (Ca2+, Na+, or Mg2+)

2. Pi complexed and found as organic phosphate
3. PO43- free

Question 5

What substance enhance phosphorus absorption?

Answer 5

vitamin D

Question 6

What substances inhibit absorption?

Answer 6

phytate

excessive intakes of: Mg, Ca, Al

Question 7

How is phosphorus transported in the body?

Answer 7

phosphorus exits the basolateral membrane via facilitated diffusion

Uptake into cells is believed to occu passively (driven via a gradient)

found in all cells in body

Question 8

What is the plasma composition of P?

Answer 8

2.5-4.4 mg/dL; 0.81-1.45 mmol/L

70% phospholipids (organic)

26.5% iorganic phosphates (HPO42-, H2PO4)

3% protein

1.5% Ca, Mg

Question 9

What is the forms and functions of phosphorus in bone?

Answer 9

85% total body P :
Ca3(PO4)2
CaHPO4*2H2O
Ca3(PO4)*3H2O
Hydroxyapatite: Ca10(PO4)6(OH)2

Question 10

How are phosphorus regulated in our body?

Answer 10

Calcitonin: promotes use of P in bone mineralization
(lower P level)

PTH and calcitriol: promote bone resorption of P from bone
PTH stimulates P excretion in urine
Calcitriol stimulate P absorption

Question 11

How does P influence intermediary metabolism (function)?

Answer 11

high energy phosphate bonds (ATP, UDP)

phosphorylation of substrates (second messenger, cAMP)

Phosphoproteins: enzyme inactivation/activation (P or deP)

Question 12

How is P involve in DNA and RNA syn.?

Answer 12

P is in the backbone of RNA, DNA

alternates w/ pentose sugar

Question 13

How does P involve in acid-base buffer?

Answer 13

phosphate is the main intracellular biffer

Na2HPO4- + H+
–> NaH2PO4 + Na+

Question 14

How is P involve in oxygen delivery?

Answer 14

2,3-diphosphoglycerate

regulates release of O2 from hemoglobin

Question 15

What are some P’s interactions w/ other nutrients?

Answer 15

Mg, Al, Ca: decrease P absorption

Mg and P form precipitate/complex: Mg3(PO4)2

3 g aluminum hydroxide w/ meal: decrease P absorption from 70%-35%

Phytate P is only ~50% available: lack phytase

Question 16

How does vit.D (1,25(OH)2D) interacts w/ P?

Answer 16

increase P absorption when P intake low

not PTH mediated

increase resorption of P from bone (release of P from bone)

Question 17

How is P being excreted?

Answer 17

67%-90% in urine, inorganic P:

• high dietary P –> high serum P which leads to increased urinary excretion
• if P intake and serum conc. low: most filtered P absorbed across the kidney (facilitated by Na-P cotransporter)

10-33% feces

P balance largely regulated at kidneys:
high P intake = high P excretion

Question 18

How do we assess nutrient status and deficiency of P?

Answer 18

Serum P or urinary P excretion

adults: deficiency confined to those consuming large amounts of antacids containing Ca, Mg, Al

Re-feeding syndrome

Premature infants (requirement high, hard to deliver)

Question 19

What is the toxicity of P?

Answer 19

UL:
19-70 y: 4g/d
>71 y: 3g/d

Hyperphoshatemia: most often seen w/ intakes below UL in chronic kidney disease

• promotes formation and deposits of Ca-PO4 crystals in soft tissues and organs (including bvs)
• Treat w/ medication to bind and prevent dietary phosphorus absorption and cause excretion and a low phosphate diet