What electrolytes are typically deranged in cases of vomiting and anorexia?

Electrolyte Derangements in Vomiting and Anorexia

In patients with vomiting and anorexia, the primary electrolyte derangements are hypokalemia (low potassium), hypochloremic metabolic alkalosis (low chloride with elevated bicarbonate), and hyponatremia (low sodium), with the severity and pattern depending critically on whether purging behaviors are present.

Key Electrolyte Abnormalities

Hypokalemia (Most Common with Purging)

• Hypokalemia occurs in approximately 20-28% of patients with anorexia nervosa who engage in vomiting, making it the most clinically significant electrolyte disturbance 1.
• Potassium depletion develops rapidly with severe diarrhea, especially when associated with vomiting, and is virtually certain evidence that a patient is purging at least daily 2, 3.
• Patients with purely restricting anorexia nervosa (without purging) are generally normokalemic even at very low body weights, indicating that vomiting or laxative abuse—not starvation alone—drives potassium loss 3.
• Hypokalemia manifests as weakness, fatigue, cardiac rhythm disturbances (primarily ectopic beats), prominent U-waves on ECG, and in severe cases, flaccid paralysis 2.

Hypochloremic Metabolic Alkalosis

• Potassium depletion from vomiting is usually accompanied by concomitant chloride loss, manifesting as hypokalemia and metabolic alkalosis 2.
• The urinary chloride/sodium ratio is characteristically low in patients who vomit and serves as a reliable indicator of purging behavior, with values that do not overlap those of non-vomiting patients 1.
• This sodium chloride-responsive metabolic alkalosis is one of the most common serum changes resulting from purging behaviors 4.

Hyponatremia

• Hyponatremia can occur through water manipulation as part of weight control behaviors or from excessive fluid intake 4.
• The American Gastroenterological Association defines severe eating disorder complications as including electrolyte imbalances alongside dehydration and significant weight loss 5.

Additional Electrolyte Concerns

• Hypophosphatemia, hypomagnesemia, and hypocalcemia may become apparent only after initial metabolic derangements are corrected, particularly during refeeding 6.
• These "refeeding syndrome" electrolytes require serial monitoring as nutritional rehabilitation begins 6.

Clinical Implications and Monitoring

Risk Stratification

• Test results are normal in most patients with eating disorders; normal results do not exclude serious illness or medical instability 6.
• Hypokalemia in an eating disorder patient is virtually certain evidence of daily purging behavior, even if denied 3.
• Lower body weight and body mass index correlate significantly with hypokalemia risk 3.

Monitoring Recommendations

• Serial monitoring of plasma electrolytes is essential, as electrolyte derangements (hypokalemia, hypophosphatemia, hypomagnesemia) often become apparent only after admission and initial correction of metabolic abnormalities 6.
• Blood tests should assess for hypokalemia and changes in other electrolytes, particularly in patients with persistent vomiting or diarrhea 6.

Treatment Priorities

Potassium Replacement

• Potassium supplementation should be aggressive in patients with documented hypokalemia, as stopping vomiting alone is most important for correcting the deficiency 1.
• Potassium chloride is the appropriate replacement form when hypokalemia is associated with metabolic alkalosis and hypochloremia 2.
• In severe cases with potassium <3.3 mEq/L, aggressive potassium replacement must precede other interventions to prevent fatal cardiac arrhythmias 5.

Addressing the Root Cause

• The definitive treatment requires cessation of purging behaviors; electrolyte replacement alone without behavioral intervention will not achieve lasting correction 1, 4.
• Continuous monitoring is recommended when electrolyte losses are ongoing 6.

Common Pitfalls

• Do not assume normal electrolytes exclude serious eating disorder pathology—most patients have normal labs despite significant illness 6.
• Do not overlook hypokalemia as a screening tool for denied purging—while routine screening has low yield (only 4.6% hypokalemic in outpatients), its presence is highly specific for active purging 3.
• Do not neglect serial monitoring during refeeding—phosphate, magnesium, and calcium abnormalities emerge as metabolic correction proceeds 6.