Fast Five Quiz: Common Nutritional Deficiencies

Romesh Khardori, MD, PhD


February 26, 2019

In patients with borderline vitamin B12 levels and clinical features of vitamin B12 deficiency, measure MMA and HC. In vitamin B12 deficiency, MMA and HC are elevated, although HC elevation occurs by itself. MMA is more sensitive than HC. In folate deficiency, MMA is within the reference range and HC is elevated. MMA and HC are considered abnormal when > 3 standard deviations above the mean. Reference range values are not age-dependent for MMA and are 70-350 nM/L. For patients younger than 60 years, reference range values are 5-15 µM/L for HC. In people older than 60 years, the cutoff for HC is 20 µM/L. If both metabolites are within the reference range, vitamin B12 deficiency is effectively ruled out.

Total body stores of vitamin B12 are 2-5 mg, of which half is stored in the liver. The recommended daily intake is 2 µg/day in adults; pregnant and lactating women require 2.6 µg/day. Children require 0.7 µg/day, and adolescents 2 µg/day. Because vitamin B12 is highly conserved through the enterohepatic circulation, cobalamin deficiency from malabsorption develops after 2-5 years and deficiency from dietary inadequacy in vegetarians develops after 10-20 years. Its causes are mainly nutritional and malabsorptive, pernicious anemia being most common.

Clinical manifestations due to vitamin B12 deficiency are unrelated to etiology. Although the clinical features of vitamin B12 deficiency may consist of a classic triad of weakness, sore tongue, and paresthesias, these are not usually the chief symptoms. Onset is subacute or gradual, although more acute courses have been described, in particular after N2O exposure.

In most patients with vitamin B12-associated neuropathy/myelopathy, no restriction on physical activity is necessary unless weakness or gait ataxia is severe. Also, severe encephalopathy may lead to 24-hour supervision. In severe anemia or congestive heart failure, the patient should limit strenuous exercise.

Read more about vitamin B12-associated conditions.


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