8.6. A 52-year-old woman presents with fatigue of several months’ duration. Blood studies reveal a macrocytic anemia, reduced levels of hemoglobin, elevated levels of homocysteine, and normal levels of methylmalonic acid. Which of the following is most likely deficient in this woman?
A. Folic acid
B. Folic acid and vitamin B12
C. Iron
D. Vitamin C
Correct answer = A. Macrocytic anemia is seen with deficiencies of folic acid, vitamin B12, or both. Vitamin B12 is utilized in only two reactions in the body: the remethylation of homocysteine (Hcy) to methionine, which also requires folic acid (as tetrahydrofolate [THF]), and the isomerization of methylmalonyl coenzyme A to succinyl coenzyme A, which does not require THF. The elevated Hcy and normal methylmalonic acid levels in the patient’s blood reflect a deficiency of folic acid as the cause of the macrocytic anemia. Iron deficiency causes microcytic anemia, as can vitamin C deficiency.
8.7. A 10-month-old African American girl, whose family recently located from Maine to Virginia, is being evaluated for the bowed appearance of her legs. The parents report that the baby is still being breastfed and takes no supplements. Radiologic studies confirm the suspicion of rickets caused by vitamin D deficiency. Which one of the following statements concerning vitamin D is correct?
A. A deficiency results in an increased secretion of calbindin.
B. Chronic kidney disease results in overproduction of 1,25dihydroxycholecalciferol (calcitriol).
C. 25-Hydroxycholecalciferol (calcidiol) is the active form of the vitamin.
D. It is required in the diet of individuals with limited exposure to sunlight.
E. Its actions are mediated through binding to G protein–coupled receptors.
F. It opposes the effect of parathyroid hormone.
Correct answer = D. Vitamin D is required in the diet of individuals with limited exposure to sunlight, such as those living at northern latitudes like Maine and those with dark skin. Note that breast milk is low in vitamin D, and the lack of supplementation increases the risk of a deficiency. Vitamin D deficiency results in decreased synthesis of calbindin. Chronic kidney disease decreases production of calcitriol (1,25-dihydroxycholecalciferol), the active form of the vitamin. Vitamin D binds to nuclear receptors and alters gene transcription. Its effects are synergistic with parathyroid hormone.
8.8. Why might a deficiency of vitamin B6 result in a fasting hypoglycemia? Deficiency of what other vitamin could also result in hypoglycemia?
Vitamin B6 is required for glycogen degradation by glycogen phosphorylase. A deficiency would result in fasting hypoglycemia. Additionally, a deficiency of biotin (required by pyruvate carboxylase of gluconeogenesis) would also result in fasting hypoglycemia.
Micronutrients: Minerals 29
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I. OVERVIEW