Phosphorus

Learning Objectives Dietary sources Daily Requirements Metabolism Important functions and Related health problems

Phosphorous P At. No. 15 Atomic Mass: 30.77

PHOSPHORUS

Distribution Phosphorus accounts for about 1% of TBW. Total Body Phosphorus is about 700 grams. ¼th of total body mineral matter 85 % joined with Ca++ in bones and teeth (Ratio 2:1) as a component of calcium phosphate and is being constantly deposited and liberated from bone structure. 10 – 15% present in all cells as phosphate ion (PO=4)

Inorganic phosphate (Pi) is required for cellular function and skeletal mineralization. Serum Pi level is maintained within a narrow range through a complex interplay between intestinal absorption, exchange with intracellular and bone storage pools, and renal tubular re-absorption.

Pi is abundant in the diet. Intestinal absorption of Pi is efficient and minimally regulated. The kidney is a major regulator of Pi. Kidney can increase or decrease its Pi re-absorptive capacity to accommodate Pi need. The crucial regulatory step in Pi homeostasis is the transport of Pi across the renal proximal tubule.

Survey in USA and Japan have shown that the amount of phosphorus from food is gradually increasing. It is thought that excess amounts of phosphorus intake for long periods are a strong factor in bone impairment and ageing. The restriction of phosphorus intake seems to be important under low calcium intake to keep QOL on high level.

DIETARY SOURCES Phosphorus can be found naturally in foods (organic phosphorus) . It is naturally found in protein-rich foods such as:  meats, poultry, fish, nuts, beans and dairy products. Phosphorus found in animal foods is absorbed more easily than phosphorus found in plant foods.

Dietary Source Present in all foods, Dietary deficiency is therefore unknown. Distribution is similar to Ca++ Adequate, intake of one ensures that of the other Best sources (Milk and milk products) Lean meat is also a good source.

Recommended Dietary Allowance Infants Up to 6 months 240 mg 6 m – 1.0 year 360 mg Children 1 – 3 800 mg 4 – 6 800 mg 7 – 10 800 mg

Recommended Dietary Allowance Adults (Male/Females) 11 – 14 1200 mg 15 – 18 1200 mg 19 – 22 800 mg 23 – 50 800 mg 51+ 800 mg Pregnant and lactating women should take an additional 400 mg.

Absorption and Metabolism Mid jejunum is the main site of absorption for free PO4 . In ileum absorption occurs via active process. Organic phosphate in food is hydrolyzed in GIT by Pancreatic & intestinal enzymes.

Absorption and Metabolism Absorption and Excretion depend upon Ca : P ratio in diet. Excess of either one causes, an increased excretion of other. Normally about 70% of dietary P is absorbed (For calcium 10 – 30%)

Absorption and Metabolism Most of phosphorus in food is in combined form, absorption requires splitting off of PO4 by intestinal enzymes (Phosphatases) Factors affecting Ca++ absorption also apply to Phosphorus, like enhancement of Vit D and inhibition by binding agents i.e. Iron and Aluminium.

Mechanism of Absorption Proper absorption of inorganic phosphate (Pi) from the lumen of the small intestine is of great importance for the achievement of Pi homeostasis. Although due to intra lumenal H+ and Pi concentrations, Pi probably can be absorbed as H2PO4- by passive means in the duodenum. Transepithelial transport of HPO4-2 requires uptake from the lumen by an active transport system.

Absorption and Metabolism Inorganic phosphorus is released and absorbed Its absorption decreases during period of increased utilization of carbohydrate

Metabolism Phosphate level is regulated by urinary excretion 2-3 mg / dl is the renal thresh hold 500 mg / day is excreted in urine

Metabolism General factors affecting intestinal absorption Plasma concentration and Hydrolysis of PO4 esters by phosphatases in the kidney. Decrease Ca++ intake will increase urinary PO4 excretion

Ratio of P : Ca, Ideal 1 : 1 Specially during the period of rapid growth (childhood), pregnancy and lactation. Ratio in other age groups if different, have no serious adverse effect

In Infants Care should be exercised as kidneys can’t handle high phosphate load. For Prevention of hypocalcemic tetany Ca : P ratio be 1.5 : 1 Ratio should be reduced to 1:1 by age 1 yr

Functions of Phosphate ions 1. Involved in formation of bones and teeth 2. Production of high energy PO4 compounds ATP, GTP,CTP, CP 3.Synthesis of nucleotide coenzymes NAD and NADP 4. DNA and RNA synthesis, where phospho diester linkage form the back bone of the structure.

FUNCTION OF PHOSPHATE IONS 5. phosphorylation of glucose in glycolysis. 6. Activation and inactivation of co-enzyme by phosphorylation, TPP, PLP,HMP shunt 7. Role in blood buffer system.

Functions Component of many compounds involved in many metabolic reactions. Sugar – PO4 linkage in DNA and RNA Phospholipids  Transport of fat in blood Phospholipid Bilayer  cell membrane transport of substances into and out of cell

Functions Phosphorylation Necessary for glucose absorption from intestine Glucose uptake by individual cells Resorption of glucose by kidney

Functions Monosaccharides are Phosphorylated several times during metabolic break down to yield energy. Involved in storage and release of Energy through high energy phosphate bond of ATP and ADP.

Functions Essential part of body’s delicate buffer system Many B vitamins becomes active only when combine with phosphate.

Hypo-Phosphatemia Diminished Supply: Starvation Malnutrition Vit D – Deficiency Increased excretion / loss Hyperparathyroidism Hyper thyroidism Renal defects

Hypo-Phosphatemia Intracellular shift of phosphorus Glucose induced Insulin induced Respiratory alkalosis Electrolytes administration Hypercalcaemia Hypomagnesimia

Hypophosphatemia Signs and Symptoms Anorexia Dizziness Bone pain Muscle weakness

Hyper-Phosphatemia Endocrine disease 2. Renal Diseases Increased growth hormones (acromegaly) Hypo parathyroidism low calcium Pseudo hypo parathyroidism 2. Renal Diseases Chronic renal insufficiency Acute renal failure

Hyper-Phosphatemia 3. Catabolic states Excess intake or Absorption Stress or injury Chemotherapy for malignant disease Excess intake or Absorption Laxatives or Enemas containing phosphate Hyper vitaminosis – D

Hyperphosphatemia signs and symptoms Excessive phosphorus can bind calcium Low calcium can cause nerve fibers to discharge repeatedly without provocation. This can lead to muscle spasms and convulsions. Excess phosphorus and not enough calcium can lead to increased bone loss. Increased phosphorus intake may lead to osteoporosis later in life.

Serum Phosphorous level Plasma level in adult -------- = 3-4 mg/dl In children-------------------------- = 5-6 mg/dl Occurs as free ion, almost 50% combined with proteins almost 40% and complexed with calcium and Magneessium about 10%. Fasting levels are higher than PP

Clinical Importance

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