Changes in serum magnesium and phosphate in older hospitalised patients — Correlation with muscle strength and risk factors for refeeding syndrome

Objectives: To evaluate changes in serum magnesium and phosphate over time in hospitalised older patients, examine whether such changes were associated with changes in muscle strength, and assess whether risk factors for refeeding syndrome were associated with falls in serum magnesium and phosphate. Design and Setting: Community dwelling patients aged 70 and over, admitted to a specialist Medicine for the Elderly assessment unit were included in a prospective study. Measurements: Weight, height, triceps skinfold thickness and mid arm circumference were recorded at baseline. Serum magnesium and phosphate was measured on admission, and at days 1, 2, 3, 5, 7, 10, 14, 21, 28 after admission, along with handgrip and quadriceps strength measured in the non-dominant limbs using a portable dynamometer. Results: 43 patients were recruited with a mean age of 83.8 years (SD 7.5). 58% were female. Mean baseline serum magnesium and phosphate levels were 0.89mmol/L and 1.07mmol/L respectively. 10/43 patients had a fall in serum magnesium of at least 0.2mmol/l from baseline and 20/43 had a similar fall in phosphate. No correlation was shown between these changes in electrolytes and muscle strength. Regression analyses did not show that risk factors for refeeding syndrome were associated with falls in electrolyte levels. Conclusion: Changes in serum magnesium and phosphate levels do not correlate with changes in muscle strength in older hospitalised patients. Risk factors for refeeding syndrome did not predict falls in serum phosphate or magnesium.