Fat malabsorption from ileal Crohn's disease impairs absorption of all fat-soluble vitamins (A, D, E, K) even when dietary intake is adequate. Vitamin K is required for the final activation of clotting factors II, VII, IX, and X, so deficiency causes prolonged clotting times. B12 malabsorption is also possible with ileal disease, but it causes megaloblastic anemia, not coagulation problems. This example shows how the fat-soluble/water-soluble distinction predicts clinical consequences.
Question 2 Multiple Choice
A strict vegan who eats no animal products develops neurological symptoms and megaloblastic anemia after 8 years despite never supplementing. Why does deficiency take so long to manifest?
APlant foods contain enough B12 to maintain adequate levels for several years
BThe body synthesizes B12 from other B vitamins when dietary intake is absent
CThe liver stores several years' worth of B12, so deficiency develops slowly despite zero intake
DB12 is fat-soluble and stored in adipose tissue for long-term use
B12 is water-soluble but is the major exception to the rule that water-soluble vitamins cannot be stored significantly: the liver stores 2–5 mg, enough for 3–6 years. This is why strict vegans can take years to develop deficiency. B12 is not found in bioavailable form in plant foods, and neurological symptoms (subacute combined degeneration) reflect B12's role in myelin synthesis, while megaloblastic anemia reflects impaired DNA synthesis.
Question 3 True / False
Taking large doses of fat-soluble vitamins is safe because the kidneys will simply excrete the excess.
TTrue
FFalse
Answer: False
This is the critical misconception. Fat-soluble vitamins (A, D, E, K) are stored in adipose tissue and liver and cannot be excreted readily by the kidneys. Excess vitamin A causes hypervitaminosis A (liver damage, bone pain, teratogenic effects in pregnancy). Excess vitamin D causes hypercalcemia. The kidneys efficiently excrete water-soluble vitamins — vitamin C megadoses cause diarrhea but not systemic toxicity — but fat-soluble vitamins accumulate to dangerous levels.
Question 4 True / False
Thiamine (B1) deficiency primarily affects the nervous system because neurons are unusually fragile and generally sensitive to nutritional insults.
TTrue
FFalse
Answer: False
The real reason is mechanistic, not a general sensitivity. Thiamine is a coenzyme for the pyruvate dehydrogenase complex, which converts pyruvate to acetyl-CoA — a critical step in glucose metabolism. Neurons are almost entirely dependent on glucose for energy (unlike muscle, which can use fatty acids). Without thiamine, neurons cannot metabolize glucose effectively, causing energy failure in the brain. The neurological devastation is a direct, predictable consequence of disrupting that specific pathway.
Question 5 Short Answer
Why do deficiency symptoms differ so dramatically between vitamins (e.g., scurvy for C, night blindness for A, Wernicke's encephalopathy for B1)? Explain the underlying principle.
Think about your answer, then reveal below.
Model answer: Each vitamin functions as a coenzyme (or coenzyme precursor) for specific metabolic reactions. Deficiency disrupts exactly those reactions, and the symptoms reflect what that particular pathway does. Vitamin C is required for collagen synthesis (proline hydroxylation), so deficiency causes bleeding gums and poor wound healing. Vitamin A forms retinal, the chromophore in rod photoreceptors, so deficiency impairs night vision first. Thiamine is needed for the pyruvate dehydrogenase complex, so deficiency blocks glucose metabolism in neurons.
Vitamins are not interchangeable generic nutrients — each performs a specific biochemical role. Knowing which pathway each vitamin enables lets you predict the deficiency syndrome from mechanism rather than memorization. This is why understanding vitamins as coenzymes is far more powerful than memorizing lists of symptoms.