Urinary and faecal excretion of marker calcium (46Ca) by low birthweight infants

Area of Science:

• Neonatal physiology
• Nutritional science
• Tracer methodology

Background:

• Accurate assessment of calcium absorption is crucial for low birthweight infants.
• Understanding calcium kinetics aids in optimizing nutritional support and growth.
• Stable isotopes offer a safe method for studying mineral metabolism in neonates.

Purpose of the Study:

• To quantify the absorption rate of calcium in low birthweight infants.
• To estimate the size of the exchangeable calcium pool using a 46Ca tracer.
• To evaluate the completeness of fecal collection for accurate calcium balance studies.

Main Methods:

• Administration of a single feed containing calcium-46 (46Ca) to low birthweight infants.
• Monitoring of urinary specific activity of 46Ca over time to determine absorption.
• Extrapolation of urinary specific activity data to estimate the exchangeable calcium pool.
• Analysis of fecal samples for 46Ca to assess completeness of excretion and absorption.

Main Results:

• Urinary 46Ca specific activity indicated largely complete absorption within approximately 4 hours.
• The rate of decrease in urinary specific activity after 3.5 hours was exponential and consistent across infants.
• Extrapolation suggested an exchangeable calcium pool size of about 200 mg/kg body weight.
• No detectable 46Ca was found in feces excreted more than 48 hours after the initial marker-containing stool.

Conclusions:

• Calcium absorption is rapid in low birthweight infants, largely completed within 4 hours.
• The exchangeable calcium pool can be estimated using urinary 46Ca excretion patterns.
• While 48-hour fecal collection is often sufficient, potential errors exist, necessitating careful consideration.
• Accurate fecal collection is paramount for precise determination of calcium absorption and endogenous excretion.