KIN 343 - 6.1 Phosphorus

Unit 6: Phosphorus

6.1 Overview of Phosphorus

  • Named after the Greek god personifying the evening star, known to bring light.

  • Early forms of phosphorus would glow, particularly white phosphorus.

  • White phosphorus reacts spontaneously with oxygen at temperatures above 30°C, and at 240°C it ignites, releasing heat and light, which is useful in matches.

6.2 Importance of Phosphate in Energy Metabolism

  • Phosphate is crucial in energy metabolism and is found in many organic molecules in cells.

  • Inorganic phosphate (PO4^3−) is reactive and used in cellular reactions.

  • Organic phosphate is incorporated into various biological molecules.

    • Phosphoproteins: Phosphorylation alters protein function and location.

    • Phospholipids: Contain phosphate in their polar head groups, forming bipolar membranes in plasma membranes and organelles.

    • Nucleic Acids: Have a phosphate backbone linking nucleotides in DNA's double helix.

    • Carbohydrates: Glucose often phosphorylated as glucose-6-phosphate when entering cells, enabling participation in metabolic pathways.

6.3 Phosphorus Distribution in the Body

  • Phosphorus is the second most abundant mineral, constituting about 1% of adult body weight (600-900 grams).

  • Most phosphorus is stored in bones, specifically in hydroxylapatite, where 6 phosphate molecules combine with calcium (6:10 ratio).

  • Only 1% of body phosphorus is found in blood and fluids, and 14% resides in soft tissues.

  • Blood inorganic phosphate (~1 millimolar) is less than 0.1% of total body phosphorus and serves as a labile, active form for metabolic functions.

  • Higher concentration of organic phosphorus in phospholipids within membranes (red/white blood cells and lipoproteins).

6.4 Dietary Reference Intakes (DRI) for Phosphorus

  • Intake levels vary across age groups:

    • AI for infants (0-12 months) based on breast milk phosphorus content.

    • RDA increases significantly for ages 9-18 due to bone growth.

    • RDA stabilizes at 700 mg/day for adults, with the same for lactation and pregnancy.

  • Tolerable Upper Intake Levels (UL): Generally 3-4 grams/day; UL decreases with age due to decreased kidney function affecting excretion ability.

6.5 Sources of Phosphorus in the Diet

  • Phosphorus is widely distributed in all biological molecules; therefore, it is present in most foods.

  • High levels found in: nuts, legumes, and cereal grains (often as phytate).

  • Notable phosphorus content in soft drinks; some can offer significant amounts (e.g., phosphoric acid).

  • Increased phosphorus consumption in processed diets over the last 50 years; phosphoric acid in sodas significantly contributes to dietary intake.

6.6 Digestion and Absorption of Phosphorus

  • Upcoming sections will discuss digestion, absorption, and bioavailability of phosphorus.