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Water-Soluble Vitamins

For medical students2 min readUpdated 2026-10-10

Water-soluble vitamins are a group of organic compounds that are primarily converted into active coenzymes or act as cofactors within the human body. They are essential for normal biochemical reactions, and their deficiencies lead to specific pathologies such as pellagra, beriberi, or scurvy.

The "3 Ds" SyndromeVitamin PP (niacin) deficiency causes pellagra, which presents with dermatitis, diarrhea, and dementia.
The ExceptionAscorbic acid (vitamin C) does not form a true coenzyme; instead, it acts as a reducing cofactor.
Endogenous SynthesisBiotin is partially synthesized by intestinal microflora, and lipoic acid can be synthesized in tissues.
Hematological EffectsFolic acid deficiency leads to macrocytic anemia, whereas vitamin $B_6$ deficiency leads to microcytic anemia.

B Vitamins: Key Players in Energy Metabolism

Most water-soluble vitamins serve as precursors for coenzymes, without which enzyme systems cannot function.

Thiamine (Vitamin $B_1$) The daily requirement is 1.5–2.5 mg. Main sources include pork, whole grains, legumes, and whole-wheat bread. In the body, thiamine is phosphorylated to form the active coenzyme thiamine diphosphate (TDP), also known as thiamine pyrophosphate (TPP). Its deficiency leads to beriberi (severe polyneuritis) as well as Wernicke-Korsakoff syndrome.

Riboflavin (Vitamin $B_2$) Required at 1.5–2.5 mg per day. Found in liver, cheese, milk, and eggs. It serves as the basis for flavin coenzymes—FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide). Hypovitaminosis leads to ariboflavinosis, which manifests as angular cheilitis, glossitis, and corneal vascularization.

Nicotinic Acid (Vitamin PP, $B_3$, Niacin) The body requires 15–25 mg daily, obtained from meat, fish, liver, and legumes. Niacin is incorporated into critical electron carriers—$NAD^+$ and $NADP^+$. Acute deficiency causes pellagra, a disease characterized by the classic triad of symptoms: dermatitis, diarrhea, and dementia.

Vitamins Regulating Protein Metabolism and Hematopoiesis

A distinct group of water-soluble compounds is critical for amino acid and nucleic acid synthesis, as well as proper blood cell maturation.

Pyridoxine (Vitamin $B_6$) The recommended intake is 2–3 mg. Sources are similar to other B vitamins: grains, legumes, meat, and fish. The active form is pyridoxal phosphate (PLP). Decreased levels lead to peripheral neuropathy, dermatitis, microcytic anemia, and can provoke seizures in infants.

Folic Acid (Vitamin $B_9$, $B_c$, Folacin) The daily requirement is small, just 400 µg (0.4 mg). Leafy green vegetables, yeast, and liver are rich in this vitamin. In cells, it is converted into tetrahydrofolate (THF). Hypovitaminosis is extremely dangerous for dividing cells, leading to leukopenia and megaloblastic (macrocytic) anemia.

Pantothenic Acid (Vitamin $B_5$) Found almost everywhere (egg yolk, yeast, liver); the daily requirement is 10–15 mg. It is a structural component of Coenzyme A (HS-CoA) and 4-phosphopantetheine. Deficiency results in paresthesias ("burning feet syndrome"), hair depigmentation, and pellagra-like dermatitis.

Specific Cofactors: Biotin, Vitamin C, and Lipoamide

Some water-soluble compounds feature unique mechanisms of action and pathways of entry into the body.

Biotin (Vitamin $H$, $B_7$) Required in microdoses (150–300 µg). Aside from liver and egg yolk, a significant amount is synthesized by the gut microbiota. It forms the coenzyme biocytin (carboxybiotin). Deficiency presents with alopecia, muscle pain, and seborrheic dermatitis (often occurring as "raw egg white injury").

Lipoic Acid (Vitamin N, Lipoamide) The requirement is about 30 mg, and the substance can be synthesized endogenously. Sources include heart, kidneys, and liver. It functions as the coenzyme lipoamide. A specific deficiency syndrome in humans is not well-defined, though a shortage may impair cellular energy metabolism.

Ascorbic Acid (Vitamin C) The recommended intake is 75–100 mg daily. Main sources include rose hips, bell peppers, black currants, and citrus fruits. Unlike other vitamins in this group, ascorbic acid does not form a true coenzyme, instead acting as a powerful reducing cofactor. Profound deficiency leads to scurvy, characterized by petechiae, loose teeth, and prominent gingival bleeding.

Mnemonic

To remember the classic manifestations of pellagra (niacin/vitamin PP deficiency), use the "3 Ds" rule: Dermatitis, Diarrhea, Dementia.

Frequently asked questions

Which specific protein in raw egg white binds biotin and causes its deficiency?

The raw egg white protein that binds biotin is called avidin.

  • Avidin is a glycoprotein that binds biotin into an insoluble, unabsorbable complex. It is destroyed (denatured) by heat treatment, which is why the negative effect only occurs when consuming raw egg whites.
In which biochemical reactions does thiamine diphosphate (TDP) participate?

Thiamine diphosphate (TDP) participates in the following biochemical reactions:

  • Oxidative decarboxylation of pyruvate: TDP is the coenzyme for pyruvate decarboxylase within the pyruvate dehydrogenase complex, participating in the transfer of the acetyl (hydroxyethyl) group to lipoic acid.
  • Non-oxidative phase of the pentose phosphate pathway: TDP is the coenzyme for transketolase, which transfers a two-carbon fragment.
In which amino acid metabolism reactions does pyridoxal phosphate participate?

Pyridoxal phosphate participates in the following amino acid reactions:

  • Transamination — transfer of an amino group from an amino acid to an $\alpha$-keto acid. Examples include the synthesis of alanine from pyruvate, aspartate from oxaloacetate, and glutamate from $\alpha$-ketoglutarate.
  • Decarboxylation — removal of $CO_2$ from the $\alpha$-carboxyl group of amino acids to form biogenic amines (serotonin, histamine, GABA, etc.).
What reaction does vitamin C catalyze as a cofactor in collagen synthesis?

Vitamin C acts as a reducing cofactor in the hydroxylation of proline and lysine residues (forming hydroxyproline and hydroxylysine) during post-translational modification of collagen.

Specific hydroxylases (prolyl hydroxylase and lysyl hydroxylase) catalyze these reactions, requiring ascorbic acid to maintain the active-center iron in its reduced ferrous form ($Fe^{2+}$). This reaction is essential for forming the hydrogen bonds that stabilize the triple helix of the tropocollagen molecule.

Which water-soluble vitamin does not form a coenzyme?

Ascorbic acid (vitamin C). It does not form a true coenzyme, instead functioning in biochemical reactions as a reducing cofactor.

What is the difference between the anemias caused by vitamin B6 and vitamin B9 deficiencies?

Pyridoxine ($B_6$) deficiency leads to microcytic anemia. Folic acid ($B_9$) deficiency disrupts cell maturation, resulting in megaloblastic (macrocytic) anemia.

Why does biotin deficiency occur if it is synthesized in the body?

Biotin hypovitaminosis can develop due to suppression of the intestinal microflora or the frequent consumption of raw egg whites, whose proteins bind tightly to the vitamin, preventing its absorption.

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