Questions: Mineral Absorption, Regulation, and Homeostasis

Question 1

A patient has iron deficiency anemia and begins taking oral iron supplements, but also has severe chronic inflammation from rheumatoid arthritis. Why might oral supplementation fail to correct her anemia?

• A. Iron deficiency anemia and chronic inflammation cannot occur at the same time
• B. Inflammation raises hepcidin, which degrades ferroportin and traps iron inside enterocytes rather than releasing it to the bloodstream
• C. Oral iron is always absorbed at a fixed 20% rate, which is too low to overcome deficiency
• D. Non-heme iron in supplements requires vitamin C, which is depleted by chronic inflammation

Answer: (B)

Inflammation triggers hepcidin secretion by the liver. Hepcidin binds and degrades ferroportin — the only known iron export protein on enterocyte basolateral membranes. Without ferroportin, absorbed iron is sequestered inside cells and never reaches the bloodstream. This is 'anemia of chronic inflammation,' and it does not respond to oral iron for exactly this reason; IV iron or treating the underlying inflammation are the appropriate interventions.

Question 2

A person already consuming adequate calcium doubles their daily intake. What happens to the *fraction* of calcium absorbed?

• A. It doubles, since more calcium is available in the lumen
• B. It stays constant at roughly 30–40%, as absorption is fixed
• C. It decreases, as the intestine down-regulates transport machinery when stores are replete
• D. It increases slightly due to passive paracellular diffusion at higher luminal concentrations

Answer: (C)

Calcium absorption is subject to adaptive regulation: when body status is adequate, the intestinal lining down-regulates active transporter expression, so a smaller fraction of a larger dose is absorbed. This inverse relationship between intake and fractional absorption is a key homeostatic mechanism — it prevents toxicity during high intake and protects against deficiency during low intake. The common misconception is that more intake linearly produces more absorbed mineral.

Question 3

Calcium carbonate and calcium citrate supplements are equally effective for patients taking proton pump inhibitors.

• T. True
• F. False

Answer: False

Calcium carbonate requires gastric acid for dissolution and ionization; in patients on PPIs (which suppress acid secretion), dissolution is impaired and absorption is reduced. Calcium citrate is pre-ionized and dissolves independently of stomach acid, making it the preferred form for patients with achlorhydria, atrophic gastritis, or those on acid-suppressing medications.

Question 4

Heme iron achieves higher fractional absorption than non-heme iron partly because it is taken up as an intact porphyrin ring via a dedicated transporter, bypassing many dietary inhibitors.

• T. True
• F. False

Answer: True

Heme iron (from meat, poultry, fish) is absorbed as the intact heme molecule by a dedicated apical transporter on enterocytes, then the iron is released intracellularly. Non-heme iron must be reduced to Fe²⁺ by a brush-border enzyme before uptake via DMT1, and it is readily inhibited by phytates, oxalates, calcium, and polyphenols. This mechanistic difference explains why heme iron is absorbed at 15–35% versus 2–20% for non-heme iron.

Question 5

Why does a person with very low iron stores absorb a higher fraction of a given iron dose than someone with adequate iron stores?

Model answer: Iron absorption is adaptively regulated through hepcidin. When iron stores are low, the liver secretes less hepcidin, so more ferroportin is expressed on enterocyte surfaces, allowing absorbed iron to flow into the bloodstream. The duodenal transporter DMT1 is also upregulated in deficiency. When stores are adequate, hepcidin rises, ferroportin is degraded, and more iron is retained inside enterocytes and eventually lost as cells shed. The result is that fractional absorption is inversely related to iron status.

This adaptive regulation prevents both iron overload (dangerous because the body has no excretory mechanism for excess iron) and protracted deficiency. The clinical implication is that a patient with severe deficiency will absorb a much higher fraction of a supplement dose than a replete individual — but even so, hepcidin elevation from inflammation can override this, which is why anemia of chronic disease is resistant to oral supplementation.