U01.01.072 Vitamin C

Antioxidant Role: Vitamin C acts as an antioxidant, protecting cells from oxidative damage.
Iron Absorption: Facilitates the absorption of iron by reducing iron to its ferrous (Fe²⁺) state.
Collagen Synthesis: Necessary for the hydroxylation of proline and lysine, key steps in collagen synthesis.
Neurotransmitter Synthesis: Required for dopamine β-hydroxylase, which converts dopamine to norepinephrine (NE).
Treatment for Methemoglobinemia: Serves as an ancillary treatment by reducing Fe³⁺ to Fe²⁺, aiding in the conversion of methemoglobin to normal hemoglobin.

Dietary Sources: Found in various fruits and vegetables (e.g., citrus fruits, strawberries, bell peppers, broccoli).
Pronunciation: Commonly pronounced as “absorbic acid.”

Scurvy: Symptoms include:
- Swollen gums
- Easy bruising
- Petechiae (small red or purple dots)
- Hemarthrosis (bleeding in the joints)
- Anemia
- Poor wound healing
- Perifollicular and subperiosteal hemorrhages
- “Corkscrew” hair appearance
Weakened Immune Response: Vitamin C deficiency can impair the immune system.
Cause: Scurvy is primarily caused by a defect in collagen synthesis due to vitamin C deficiency.

Toxicity Symptoms:
- Nausea
- Vomiting
- Diarrhea
- Fatigue
- Calcium oxalate nephrolithiasis (kidney stones)
Iron Toxicity Risk: Excess vitamin C can increase dietary iron absorption, which may worsen iron overload conditions like hereditary hemochromatosis or transfusion-related iron overload.

Methionine and Homocysteine Metabolism:
- Vitamin C plays a role in the methylation cycle, helping convert homocysteine to methionine.
- Key Steps:
  - Methionine → SAM (S-adenosyl methionine)
  - SAM → Methyl groups for protein and fatty acid synthesis, as well as for odd-chain fatty acids and branched-chain amino acids.
Methylmalonyl-CoA Conversion:
- Methylmalonyl-CoA → Succinyl-CoA via methylmalonyl-CoA mutase.
- Vitamin C aids in the production of heme and supports the TCA (tricarboxylic acid) cycle function.

Functions: Antioxidant, iron absorption, collagen synthesis, neurotransmitter synthesis.
Deficiency: Leads to scurvy—characterized by swollen gums, bruising, anemia, and poor wound healing.
Excess: Causes gastrointestinal disturbances and increases the risk of kidney stones.
Therapeutic Use: Helps with methemoglobinemia by reducing Fe³⁺ to Fe²⁺.

Vitamin C Deficiency Symptoms	Excess Vitamin C Symptoms
Swollen gums	Nausea, vomiting, diarrhea
Easy bruising	Fatigue
Petechiae and hemarthrosis	Calcium oxalate nephrolithiasis
Poor wound healing	Iron overload risk
Corkscrew hair

Prepare thoroughly for the USMLE Step 1 by understanding these physiological and clinical aspects of vitamin C.

USMLE Step 1 Review Series