Vitamin D: Introduction and Overview of Functions

Vitamin D: Introduction and Overview of Functions

Vitamin D Content in Fish

  • Vitamin D (IU/oz): The table below summarizes the Vitamin D content from various fish types:

    • Blue Fish: 280 ± 68 IU/oz

    • Cod: 104 ± 24 IU/oz

    • Grey Sole: 56 ± 36 IU/oz

    • Farmed Salmon: 240 ± 108 IU/oz

    • Wild Salmon: 988 ± 524 IU/oz

    • Farmed Trout: 388 ± 212 IU/oz

    • Tuna: 404 ± 440 IU/oz

Characteristics of Vitamin D

  • Classification:

    • Fat-soluble and conditionally essential micronutrient.

  • Requirements: 270-320 nm (UV-B) sunlight exposure on skin.

Limitations Impacting Vitamin D Status

  • Dietary Intake: Amount of precursor in diet and supplements.

  • Lactose Intolerance: Affects consumption of dairy, which is a common source.

  • Age: As age increases, the skin's ability to synthesize Vitamin D decreases; specifically, individuals over a certain age have reduced capacity to produce 7-dehydrocholesterol in skin.

  • Skin Pigmentation: Individuals with darker skin require 5 to 10 times greater sun exposure to synthesize the same amount of Vitamin D as those with lighter skin.

  • Clothing: Coverage can limit UV exposure to the skin.

  • Environmental Factors: High temperatures and location significantly influence UV exposure; the farther a location is from the equator, the less UV exposure is typically available. Factors like prolonged UV exposure, ozone levels, smoke, and sunscreen use can also limit synthesis.

  • Time of Day: Highest UV exposure occurs at noon.

  • Seasonality: The amount of UV light available varies by season.

Forms of Vitamin D

  • Vitamin D3 (Cholecalciferol):

    • Derived from animal sources, has hormone-like functions, and is more effective in raising serum 25(OH)D concentrations compared to Vitamin D2.

  • Vitamin D2 (Ergocalciferol):

    • Derived from plant sources and yeast.

Lactose Intolerance Statistics Worldwide

  • A map indicates varying rates of lactose intolerance globally. Rates by race or ethnicity include:

    • Southeast Asian: 98%

    • Native Americans: 62-100%

    • Asian Americans: 90%

    • Alaskan Eskimo: 80%

    • African-American Adults: 79%

    • Rural Mexicans: 74%

    • North American Jews: 69%

    • Greek Cypriots: 66%

    • Cretans: 56%

    • Mexican American Males: 55%

    • Indian Adults: 50%

    • African-American Children: 45%

    • Indian Children: 20%

    • Descendants of Northern Europeans: 5%

Fortification and Bioavailability of Vitamin D

  • Cow’s Milk: Has been fortified with either Vitamin D2 or D3 (100 IU/8 oz) since the 1930s due to a lack of standards.

  • Fortified Foods: Include breakfast cereals and orange juice; however, their bioavailability may vary.

Synthesis of Vitamin D

  • Metabolic Pathway

    • Synthesis of 25(OH)D (calcidiol) from Vitamin D3 occurs in the liver through the action of 25-hydroxylase.

  • Key Enzymes in Vitamin D Metabolism:

    • BBP (Vitamin D Binding Protein): Binds Vitamin D.

    • 25-Hydroxylase: Converts Vitamin D to 25-hydroxyvitamin D (25(OH)D or calcidiol), the circulating form measured for Vitamin D status.

    • 1-Hydroxylase: Converts 25(OH)D to the active form 1,25-dihydroxyvitamin D (1,25(OH)2D), important for biological activity.

Hydroxylation Process and Enzymes

  • First Hydroxylation:

    • Enzyme: P450C25

    • Location: Liver; also present in skin, kidney, intestine.

  • Second Hydroxylation:

    • Enzyme: P450C1

    • Location: Kidney; also found in skin, intestine, macrophage, and bone.

  • Third Hydroxylation:

    • Enzyme: P450C24

    • Location: Kidney; found in nearly all cells.

Overview of Hydroxylases

  • Activation Hydroxylases: Part of the cytochrome P450 superfamily, responsible for the conversion processes of Vitamin D into active metabolites.

  • Excretion: The metabolic products of Vitamin D undergo excretion processes.