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Folate Kinetics – Absorption, Distribution & Metabolism

Folate kinetics describes how the body absorbs, distributes, metabolises, and excretes folic acid (vitamin B9), which is essential for DNA synthesis and cell division.

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Things worth knowing about "Folate kinetics"

Folate kinetics describes how the body absorbs, distributes, metabolises, and excretes folic acid (vitamin B9), which is essential for DNA synthesis and cell division.

What is Folate Kinetics?

Folate kinetics (also referred to as folic acid kinetics) describes the complete journey of folic acid (vitamin B9) through the human body – from intestinal absorption and distribution in tissues to metabolic transformation and excretion. Understanding these kinetic processes is crucial in medicine, nutritional science, and pharmacology, as folate plays a central role in DNA synthesis, cell division, and the formation of red blood cells.

Absorption (Intestinal Uptake)

Dietary folates are mostly present as polyglutamates and must first be converted to monoglutamates in the small intestine by the enzyme glutamate carboxypeptidase II (also known as folylpolyglutamate hydrolase) before they can be absorbed. Synthetic folic acid, as found in dietary supplements, already exists as a monoglutamate and is therefore absorbed more efficiently.

Site of absorption: primarily in the upper small intestine (jejunum)
Uptake mechanism: active transport via the proton-coupled folate transporter (PCFT) at low pH, and via the reduced folate carrier (RFC)
Bioavailability: dietary folates ~50–80 %, synthetic folic acid ~85–100 % (nearly complete when taken on an empty stomach)

Distribution in the Body

After absorption, folate is transported via the portal circulation to the liver. The liver is the primary storage organ for folate; total body stores in adults amount to approximately 10–30 mg, with around 50 % stored in the liver. In the blood, folate is transported mainly as 5-methyltetrahydrofolate (5-MTHF), partly bound to plasma proteins.

Plasma half-life: approximately 3–5 hours
Tissue distribution: liver, kidneys, erythrocytes, placenta
Erythrocyte folate is considered a more reliable long-term marker than plasma folate

Metabolism

In tissues, folic acid is converted into its biologically active forms. The central step is reduction to tetrahydrofolate (THF) by the enzyme dihydrofolate reductase (DHFR). THF acts as a single-carbon carrier and is essential for:

DNA synthesis (purine and pyrimidine biosynthesis)
Conversion of homocysteine to methionine (via methionine synthase, cofactor: vitamin B12)
Amino acid biosynthesis and epigenetic methylation processes

A genetic polymorphism in the enzyme MTHFR (methylenetetrahydrofolate reductase) (e.g., C677T) can impair the conversion of folate to 5-MTHF, significantly altering individual folate kinetics.

Excretion

Folate is excreted primarily via the kidneys. At low folate levels, renal reabsorption is increased to minimise losses. At high intake, urinary excretion exceeds reabsorption capacity. Additionally, folate is excreted via bile and partly reabsorbed through the enterohepatic circulation. Average daily losses in adults are approximately 1–2 µg/kg body weight.

Clinical Relevance of Folate Kinetics

Understanding folate kinetics is relevant in various clinical situations:

Pregnancy: Increased folate demand due to accelerated cell metabolism; supplementation reduces the risk of neural tube defects in the child.
Drug interactions: Methotrexate inhibits DHFR, deliberately disrupting folate kinetics as a therapeutic mechanism.
MTHFR polymorphism: Carriers of certain gene variants may require higher folate intake or benefit from direct supplementation with 5-MTHF.
Malabsorption syndromes: Coeliac disease, Crohn disease, and other intestinal disorders can significantly impair folate absorption.
Renal insufficiency: Altered renal clearance affects blood folate levels.

Recommended Intake

The European Food Safety Authority (EFSA) recommends a daily dietary folate equivalent intake of 330 µg for adults, 600 µg for pregnant women, and 500 µg for breastfeeding women. The WHO recommends periconceptional supplementation of 400 µg folic acid per day for women of childbearing age.

References

Bailey, L. B. et al. (2015): Biomarkers of Nutrition for Development – Folate Review. Journal of Nutrition, 145(7):1636S–1680S. PubMed PMID: 26451605.
European Food Safety Authority (EFSA): Dietary Reference Values for folate. EFSA Journal, 2017;15(8):4746.
World Health Organization (WHO): Guideline – Daily iron and folic acid supplementation in pregnant women. WHO Press, Geneva, 2012.

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