Vitamins A K D – Functions, Sources & Deficiency

Overview: Vitamins A, K, and D

Vitamins A, K, and D belong to the group of fat-soluble vitamins. This means they are stored in body fat and the liver, and require dietary fat for optimal absorption. Together, these three vitamins play a central role in bone health, immune function, vision, and blood clotting. In preventive medicine and nutritional counselling, they are often considered together because they complement and interact with one another.

Vitamin D

Vitamin D (also called calciferol) is primarily produced by the skin in response to sunlight exposure and can be obtained in smaller amounts from food. It is essential for the absorption of calcium and phosphorus in the intestine, making it indispensable for bone formation. Vitamin D also supports immune function, muscle strength, and may have a positive effect on mood.

Biological Functions of Vitamin D

• Promotes calcium absorption in the intestine
• Regulates calcium and phosphate levels in the blood
• Supports bone mineralisation
• Activates immune cells
• Contributes to muscle coordination and strength

Sources and Recommended Intake of Vitamin D

According to the World Health Organization (WHO) and national health authorities, the recommended daily intake of Vitamin D for adults is 20 micrograms (800 IU) per day when adequate sun exposure is not available. Dietary sources include fatty fish (salmon, mackerel), egg yolk, liver, and fortified foods.

Deficiency Symptoms and Risk Groups

Vitamin D deficiency can lead to rickets in children and osteomalacia (softening of the bones) or osteoporosis in adults. Risk groups include older adults, people with darker skin tones, those with limited sun exposure, and individuals with malabsorption disorders.

Vitamin K

Vitamin K is a fat-soluble vitamin that occurs in two main forms: Vitamin K1 (phylloquinone), found mainly in green leafy vegetables, and Vitamin K2 (menaquinone), produced by gut bacteria and found in fermented foods and animal products.

Biological Functions of Vitamin K

• Activation of clotting factors (e.g. prothrombin) – essential for blood coagulation
• Activation of osteocalcin – important for bone mineralisation
• Activation of Matrix Gla Protein (MGP) – helps prevent arterial calcification
• Supports cardiovascular health

Sources and Recommended Intake of Vitamin K

Health authorities recommend a daily Vitamin K intake of approximately 60–80 micrograms for adults. Good dietary sources include spinach, kale, broccoli, parsley, and fermented products such as natto (for K2).

Deficiency and Interactions

Vitamin K deficiency is rare but can occur in newborns, individuals with intestinal disorders, or those on antibiotic therapy. Symptoms include increased bleeding tendency and delayed wound healing. Importantly, Vitamin K interacts with anticoagulant medications such as warfarin – patients on anticoagulant therapy should maintain consistent Vitamin K intake in their diet.

Vitamin A

Vitamin A (retinol and its precursors, the carotenoids) is essential for vision, immune function, and cell growth. It occurs in two forms: preformed Vitamin A (retinol) found in animal products, and provitamin A (beta-carotene) found in plant-based foods, which the body converts into Vitamin A as needed.

Biological Functions of Vitamin A

• Formation of rhodopsin – the visual pigment for dim-light and night vision
• Maintenance of the integrity of skin and mucous membrane cells
• Support of the immune system (T-cell differentiation)
• Regulation of cell growth and differentiation
• Support of reproduction and embryonic development

Sources and Recommended Intake of Vitamin A

The WHO recommends a daily intake of 700–900 micrograms of retinol equivalents for adults. Animal sources include liver, fish liver oil, dairy products, and eggs. Plant sources (beta-carotene) include carrots, sweet potatoes, spinach, and mango.

Deficiency, Risk Groups, and Toxicity

Vitamin A deficiency is common in developing countries and leads to night blindness, xerophthalmia (dryness of the conjunctiva and cornea), and increased susceptibility to infections. Unlike beta-carotene, preformed Vitamin A can be toxic in high doses (hypervitaminosis A), causing headaches, nausea, liver damage, and, during pregnancy, birth defects.

Synergistic Effects of Vitamins A, D, and K

These three fat-soluble vitamins complement each other in key metabolic pathways:

• Vitamins D and K work synergistically for bone health: Vitamin D stimulates the production of osteocalcin, while Vitamin K activates this protein so that calcium can be properly deposited into bones.
• Vitamins D and A share common receptor pathways (nuclear receptors) and mutually influence their effects on immune function and cell maturation.
• A balanced intake of all three vitamins is especially important for bone health, cardiovascular health, and immune function.

Supplementation

Care should be taken when using supplements containing fat-soluble vitamins, as overdosing is possible. Combination supplements with Vitamins A, D, and K should only be taken after medical consultation, particularly for:

• Pregnant individuals (high-dose Vitamin A is teratogenic)
• People on anticoagulant therapy (Vitamin K)
• People with liver or kidney disease

References

1. World Health Organization (WHO): Vitamin and Mineral Requirements in Human Nutrition. 2nd edition. Geneva: WHO Press, 2004.
2. Deutsche Gesellschaft für Ernährung (DGE): Reference Values for Nutrient Intake. 2nd edition, 2021. Available at: www.dge.de
3. Schurgers LJ, Vermeer C. Determination of phylloquinone and menaquinones in food. Effect of food matrix on circulating vitamin K concentrations. Haemostasis. 2000;30(6):298-307. PubMed PMID: 11356998.