What is the biochemistry of vitamins?

Biochemistry of Vitamins

Vitamins are essential organic micronutrients required in small quantities for proper metabolic functioning, with distinct biochemical pathways and mechanisms that significantly impact human health. 1

Classification and General Properties

Vitamins are broadly classified into two major categories based on their solubility:

Fat-Soluble Vitamins

• Include vitamins A, D, E, and K 1
• Require special mechanisms for transport in the aqueous bloodstream
• Can be stored in the body's fatty tissues
• Excessive intake may lead to toxicity due to accumulation

Water-Soluble Vitamins

• Include B-complex vitamins and vitamin C
• Not stored significantly in the body, requiring regular dietary intake 2
• Generally excreted in urine when in excess
• Found in various food sources including animal proteins, dairy products, leafy green vegetables, and beans

Vitamin D Biochemistry

Vitamin D represents a unique class of nutrients that functions as a steroid hormone rather than a traditional vitamin 1:

• Forms and Synthesis:

  • Exists in two main forms: vitamin D2 (ergocalciferol, plant-derived) and vitamin D3 (cholecalciferol, animal-derived) 3
  • Endogenously synthesized in skin through UVB radiation exposure of 7-dehydrocholesterol, converting it to cholecalciferol (D3) 1
  • For light-skinned individuals, just 15 minutes of weekly sun exposure can produce thousands of units of vitamin D 1, 3

• Transport and Metabolism:

  • Endogenous D3 is carried in blood by vitamin D-binding protein (DBP) 1
  • Dietary/supplemental D3 is absorbed via chylomicrons with approximately 35% carried by lipoproteins 1
  • Undergoes two-step hydroxylation:
    1. First in liver to 25(OH)D3 (calcidiol) - the form measured for vitamin D status
    2. Then in kidneys to 1,25(OH)2D3 (calcitriol) - the biologically active form 4
  • 10-24 hour time lag between administration and physiological action due to required metabolic activation 4

• Physiological Functions:

  • Promotes active absorption of calcium and phosphorus in small intestine
  • Elevates serum calcium and phosphate to permit bone mineralization
  • Mobilizes calcium and phosphate from bone
  • May increase reabsorption of calcium and phosphate by renal tubules 4

B Vitamins Biochemistry

B vitamins function as essential cofactors in numerous metabolic pathways:

• General Functions:

  • Critical cofactors for axonal transport and neurotransmitter synthesis
  • Essential for cellular metabolic pathways
  • Involved in both catabolic metabolism (energy production) and anabolic metabolism (bioactive molecule synthesis) 2
  • Function as cofactors for enzymes involved in RNA and DNA biosynthesis

• Vitamin B1 (Thiamine):

  • Functions primarily as thiamine pyrophosphate (TPP), a coenzyme in carbohydrate metabolism
  • Has both coenzyme and non-coenzyme mechanisms of action
  • Exhibits neurotropic properties through specific protein-acceptors 5
  • UK RDA (RNI) is 0.9 mg/day for men and 0.8 mg/day for women aged ≥50 years 1

• Vitamin B3 (Niacin):

  • Functions in NAD+ and NADP+ formation, critical for energy metabolism
  • Age-related decline in cellular NAD+ concentration observed in both men and women 1
  • UK RDA (RNI) is 16 mg/day for men and 12 mg/day for women aged 50+ years 1
  • Can be synthesized endogenously from tryptophan in a 60:1 ratio 1

• Vitamin B6 (Pyridoxine):

  • Functions primarily as pyridoxal 5′-phosphate (PLP) in amino acid metabolism
  • Associated with synthesis of heme, neurotransmitters, purines, hormones, and fatty acids
  • Naturally abundant in meat, poultry, nuts, and legumes 1

• Vitamin B12 (Cobalamin):

  • Participates in two critical biochemical reactions:
    1. Conversion of L-methylmalonyl coenzyme A to succinyl coenzyme A
    2. Formation of methionine by methylation of homocysteine
  • Not synthesized by eukaryotes (including humans) - must be consumed in diet
  • Primary sources include meat, milk, dairy products, fish, shellfish, and eggs
  • More stable than other B vitamins with specific absorption mechanisms and large storage capacity 6

Other Vitamins in Human Milk

Human milk contains a wide range of vitamins with varying concentrations:

• Significant variations observed in vitamin concentrations according to:

  • Stage of lactation
  • Time of day (circadian variation)
  • Geographic location
  • Maternal diet 1

• Circadian variation observed for most vitamins except vitamins A and E when adjusted for milk fat

• Significant variations within the same feed for thiamin, niacin, and vitamins A and E 1

Clinical Implications

• Vitamin deficiencies can lead to various pathological states, including neurologic disorders 2
• Elderly populations may require particular attention to B vitamin intake due to reduced food intake and absorption efficiency 2
• Vitamin D deficiency remains a global health concern, often requiring supplementation and/or adequate sun exposure 3
• Vegetarians are at particular risk for vitamin B12 deficiency and should consume fortified foods 6

Common Pitfalls and Considerations

• Dietary intake assessments often don't correlate with biochemical markers of vitamin status 1
• Storage, cooking methods, and co-ingestion of certain foods (alcohol, sulfites, tannins) can reduce vitamin content and bioavailability 1
• Excessive vitamin D supplementation can lead to toxicity and calcific vasculopathy 1
• The primary source of vitamin B3 (niacin) is mature staple cereal grains, which contain glycosides that can significantly reduce bioavailability 1
• Most B vitamins are generally safe even at intake levels reached with fortified foods or supplements 2