To the Editor: In Dr. Mehler's discussion of the physiologicalcomplications of bulimia nervosa (Aug. 28 issue),1 he emphasizesthe diagnostic usefulness of urinary electrolyte measurementsand concludes that low urinary potassium and sodium concentrationsare compatible with vomiting. The amount of potassium in gastricsecretions is actually trivial, and the mechanism of hypokalemiain such persons is urinary potassium loss, which occurs as highdistal delivery of bicarbonate enhances potassium secretionin the cortical collecting duct.2,3,4 In addition, there isan obligatory loss of sodium with bicarbonate after each boutof vomiting. Therefore, the urinary hallmarks of active vomitingare an elevated potassium concentration, a low urinary chlorideconcentration, and depending on the degree of urinary bicarbonate,sodium loss. Only urinary electrolyte measurements performedat a time that is remote from the episode of vomiting and inthe absence of distal bicarbonate delivery might show low urinarypotassium and sodium concentrations.
Jordan J. Weinstein, M.D. University of Toronto Toronto, ON M5B 1W8, Canada j.weinstein{at}utoronto.ca
References
Mehler PS. Bulimia nervosa. N Engl J Med 2003;349:875-881. [Free Full Text]
Kassirer JP, Schwartz WB. Correction of metabolic alkalosis in man without repair of potassium deficiency. Am J Med 1966;40:19-26. [CrossRef][Web of Science][Medline]
Carlisle EJ, Donnelly SM, Ethier JH, et al. Modulation of the secretion of potassium by accompanying anions in humans. Kidney Int 1991;39:1206-1212. [Web of Science][Medline]
Kamel KS, Ethier JH, Richardson RM, Bear RA, Halperin ML. Urine electrolytes and osmolality: when and how to use them. Am J Nephrol 1990;10:89-102. [Web of Science][Medline]
The author replies: Dr. Weinstein is concerned about the discussionof the role of urinary electrolytes in the diagnosis of bulimia.I agree with him that with vomiting, most of the potassium lossis in the urine as a result of metabolic alkalosis and the bicarbonatediuresis that ensues, resulting in loss of potassium as theaccompanying cation. Rates of renal potassium secretion arefurther increased in the presence of an elevated aldosteronelevel secondary to volume contraction, causing worsening hypokalemia.My comment about the usefulness of finding a low urinary potassiumlevel in a patient with bulimia was in reference to diarrhealstates due to laxative abuse and to the differentiation of thismode of purging from vomiting, wherein the urinary potassiumlevel is elevated. However, there is an error in Table 2 ofthe article: urinary levels of potassium are indeed increasedwith excessive vomiting, not decreased, as printed.
The key points of the paragraph to which Dr. Weinstein refersremain the same — namely, that hypokalemia is highly specificfor the diagnosis of bulimia in otherwise healthy young womenand that patients with strict nonpurging anorexia nervosa shouldnot have metabolic disturbances.
Philip S. Mehler, M.D. Denver Health and Hospitals Denver, CO 80204 pmehler{at}dhha.org
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