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Vitamin Kinetics: Absorption, Distribution & Excretion

Vitamin kinetics describes how vitamins are absorbed, distributed, stored, and excreted by the body. Understanding these processes is key to optimal vitamin supply.

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Things worth knowing about "Vitamin kinetics"

Vitamin kinetics describes how vitamins are absorbed, distributed, stored, and excreted by the body. Understanding these processes is key to optimal vitamin supply.

What Is Vitamin Kinetics?

Vitamin kinetics refers to the complete set of processes that describe how vitamins are absorbed, transported, distributed, metabolized, and ultimately excreted by the human body. It is a sub-field of pharmacokinetics and nutritional science, providing essential foundations for the dosing of vitamin supplements and for understanding deficiency states and overdose risks.

The Four Phases of Vitamin Kinetics

Vitamin kinetics is commonly divided into four classical phases, summarized by the acronym ADME:

1. Absorption

Vitamins are primarily absorbed in the small intestine. The mechanism of absorption depends strongly on the solubility of the vitamin:

Fat-soluble vitamins (A, D, E, K) are incorporated into micelles along with dietary fats and absorbed through the intestinal mucosa, then transported via the lymphatic system into the bloodstream.
Water-soluble vitamins (B-group, vitamin C) are absorbed directly into the blood via active transport mechanisms or passive diffusion.

The bioavailability of a vitamin – that is, the proportion that actually enters the bloodstream – can be influenced by factors such as food composition, degree of food processing, gut health, and genetic variation.

2. Distribution

After absorption, vitamins are transported via the blood to various tissues and organs. Many vitamins bind to specific transport proteins. For example, vitamin D binds to vitamin D-binding protein (VDBP), while vitamin B12 is transported by transcobalamin II.

3. Metabolism

Many vitamins must be converted into their active form within the body, a process known as activation. A well-known example is vitamin D: it is converted in the liver to 25-hydroxyvitamin D, and subsequently in the kidneys to the biologically active form 1,25-dihydroxyvitamin D (calcitriol). Similarly, vitamin B6 (pyridoxine) is phosphorylated in the liver to the active coenzyme pyridoxal phosphate (PLP).

4. Elimination

The excretion of vitamins varies depending on their solubility:

Water-soluble vitamins are primarily excreted via the kidneys in urine. Excess amounts of most water-soluble vitamins are eliminated relatively quickly, which reduces the risk of toxicity.
Fat-soluble vitamins are stored in adipose tissue and the liver, and are excreted only slowly. This results in a longer half-life but can lead to hypervitaminosis (vitamin toxicity) if consumed in excessive amounts.

Factors Influencing Vitamin Kinetics

Numerous factors can affect how vitamins behave in the body:

Age: As people age, the absorption and utilization of certain vitamins changes. For example, the ability to absorb vitamin B12 from food decreases with age.
Genetics: Certain genetic variants (polymorphisms) can affect enzymes involved in vitamin activation, such as the MTHFR enzyme in folate metabolism.
Health status: Diseases of the liver, kidneys, or gastrointestinal tract can significantly alter vitamin metabolism.
Drug interactions: Certain medications can inhibit vitamin absorption or accelerate its breakdown. For example, metformin reduces vitamin B12 absorption.
Diet: Consuming fat-soluble vitamins alongside dietary fat significantly improves their absorption.

Clinical Relevance of Vitamin Kinetics

Understanding vitamin kinetics is of great practical importance for physicians, dietitians, and pharmacists. It helps to:

Establish meaningful dosing recommendations for vitamin supplements.
Detect and treat deficiency states at an early stage.
Avoid overdose and toxic effects, particularly with fat-soluble vitamins.
Anticipate interactions between vitamins and medications.

References

Combs, G.F. & McClung, J.P. (2017). The Vitamins: Fundamental Aspects in Nutrition and Health. 5th Edition. Academic Press.
World Health Organization (WHO) & Food and Agriculture Organization (FAO) (2004). Vitamin and Mineral Requirements in Human Nutrition. 2nd Edition. WHO Press, Geneva.
Elmadfa, I. & Leitzmann, C. (2019). Ernährung des Menschen (Human Nutrition). 6th Edition. Verlag Eugen Ulmer, Stuttgart.

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