Effects of magnesium infusions on magnesium and nitrogen balance during parenteral nutrition.

The effects of magnesium infusions on urinary and fecal magnesium excretion, serum magnesium and nitrogen balance were examined in seven well-nourished and three nutritionally depleted adult surgical patients receiving total parenteral nutrition. They were maintained on constant nitrogen and caloric intake for 14 +/- 2 days. Magnesium doses ranged from 0 to 664 mg/d and were given in varying crossover patterns. In both groups, urinary magnesium excretion increased as the amount of magnesium infused increased but, at comparable magnesium infusions, depleted patients excreted significantly less magnesium. Renal conservation was most pronounced in well-nourished patients on magnesium-free intake and in depleted patients given 70 mg magnesium daily. Urinary magnesium losses were 40 +/- 5 mg/d and 33 +/- 8 mg/d, respectively, in these two groups. Endogenous fecal magnesium excretion was minimal and ranged from 2 to 38 mg/d. At each level of magnesium intake, serum levels of well-nourished patients were normal. With infusions of less than 200 mg/d, serum magnesium concentrations in depleted subjects averaged 1.6 mg/dl. Reduced urinary magnesium excretion as well as borderline serum levels measured in depleted adults suggest that the magnesium dosage should be higher than that usually recommended during total parenteral nutrition. In both groups a positive correlation between magnesium and nitrogen balance was noted.

The current status of protein sparing.

Solutions of crystalline amino acids infused without dextrose produce a marked improvement in nitrogen balance. Increasing the infusion level of amino acid from 1.0 to 1.7 grams per kilogram further improves nitrogen balance. The addition of dextrose to the amino acid solutions did not affect nitrogen balance and proved that the role of insulin during protein sparing has been overemphasized. Nitrogen balance is slightly, but not significantly, superior when nonprotein dextrose calories are administered. However, amino acid solutions are isotonic and can be infused peripherally, whereas adding dextrose doubles the concentration and renders peripheral infusion more difficult. Protein sparing may be useful for short term nutritional support when the potential risks of total parenteral hyperalimentation are not justified. Endogenous body fat is mobilized. Hence, protein sparing also prevents the development of fatty acid deficiency and may be useful in treating fatty infiltration of the liver. Protein sparing provides suboptimal caloric replacement and should only be used for temporary nutritional support until oral alimentation is resumed or until there is an absolute indication for intravenous hyperalimentation. Expense and the fact that most patients do well after elective abdominal operations militate against the proposition that amino acids should become a routine substitute for 5 per cent dextrose therapy post-operatively.

Calcium requirements during total parenteral nutrition in well-nourished individuals.

Eleven patients, receiving all nutrition intravenously, were given varying doses of calcium (0-20 mg/kg/day) to determine an optimal level for calcium administration during postoperative parenteral nutrition. During each study period, nitrogen, phosphorus, vitamin, and caloric intakes were constant. Negative calcium balance resulted when less than 2 mg Ca++/kg body weight was given daily. During excessive urinary calcium losses, serum calcium concentration remained in the normal range. Increasing calcium intake to approximately 5 mg/kg/day (500 mg/day) yielded an apparent retention of calcium, as did higher doses. Serum calcium did not rise at this time. Urinary calcium excretion was directly proportional to calcium intake. The preliminary data suggest that a minimum dose of 5 mgCa++/kg/day is necessary to attain equilibrium between intake and urinary output. This value is higher than recent suggestions for calcium replacement during intravenous feeding.