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Chapter 26. Digestion, Absorption, & Nutritional Principles

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A premenopausal woman who is physically active seeks advice from her primary care clinician regarding measures she can take to ensure adequate availability of dietary calcium to ensure bone health later in life. Which of the following dietary components should enhance calcium uptake?

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A. Protein

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B. Oxalates

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C. Iron

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D. Vitamin D

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E. Sodium

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The correct answer is D. Calcium is absorbed across the brush border of intestinal epithelial cells via TRPV6 channels and sequestered in the cytosol by calcium-binding proteins (see Chapter 21). Vitamin D increases expression of these proteins as well as those mediating calcium export across the basolateral membrane. Dietary iron and oxalate can chelate calcium, potentially reducing its absorption (note that kidney stones are most commonly made up of calcium oxalate) (rules out options B and C). Adequate protein intake is important for bone health, but this effect is related to the synthesis of bone matrix proteins rather than an influence on calcium absorption (rules out option A). Finally, the mechanism for apical calcium uptake from the diet is via a channel rather than a Na+/Ca++ exchanger, so dietary sodium should not influence calcium uptake. Na+/Ca++ exchange may contribute to basolateral calcium export, but this process would be inhibited rather than stimulated by an increase in cytosolic sodium ions (rules out option E).

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A child is brought to his pediatrician for an investigation of his failure to thrive. Genetic tests reveal a congenital absence of enterokinase. A decrease in which of the following is expected in this patient?

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A. Incidence of pancreatitis

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B. Carbohydrate assimilation

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C. Bile acid reabsorption

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D. Gastric pH

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E. Protein assimilation

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The correct answer is E. Enterokinase, localized to the intestinal brush border, is responsible for activating pancreatic trypsin in the gut lumen, which in turn activates the remaining pancreatic proteases. In the absence of protease activation, dietary proteins will not be digested to forms that can be assimilated. Absorption of carbohydrates should be unaffected because amylase is released by the pancreas in active form (rules out option B). The incidence of pancreatitis should be unaltered since the defect is in the intestine (rules out option A). Active bile acid reabsorption takes place in the terminal ileum via a sodium-coupled pathway and should not be affected by a lack of enterokinase (rules out option C). ...

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