What is the role of Blood Urea Nitrogen (BUN) in guiding nutrition in the Neonatal Intensive Care Unit (NICU)?

Role of BUN as a Valuable Tool in NICU Nutrition

Blood urea nitrogen (BUN) serves as a useful marker for monitoring protein adequacy during enteral nutrition in preterm neonates, but it is unreliable during early parenteral nutrition and should not be used to limit amino acid intake in the first 72 hours of life.

BUN During Early Parenteral Nutrition (First 72 Hours)

In the early postnatal period, BUN does not correlate with amino acid intake and should NOT be used to restrict protein delivery. 1

• During the first 3 days of life in very preterm infants (<1250g), there is no correlation between amino acid intake (0-3.7 g/kg/day) and BUN concentration (p=0.2, r²=0.01) 1
• High BUN in early life reflects multiple factors beyond amino acid tolerance, including gestational age, fluid status, and metabolic instability 2
• Gestational age shows a significant negative correlation with BUN during parenteral nutrition, meaning younger gestational age infants have higher BUN independent of protein intake 2
• Limiting amino acid intake based on BUN concentration in the first days of life is not warranted 1

BUN During Enteral Nutrition (After Transition)

Once infants achieve full enteral feeds, BUN becomes a moderately useful marker for protein adequacy. 2

• A moderately positive correlation exists between BUN and protein intake when infants consume 160 mL/kg of milk (r=0.5, p=0.001) and at 36 weeks postmenstrual age (r=0.49, p=0.012) 2
• BUN can represent a useful index for monitoring adequacy of protein intake during established enteral nutrition 2

Adjustable Fortification Using BUN Targets

Adjustable fortification protocols targeting specific BUN levels improve short-term growth in preterm infants, though 63% require protein supplementation beyond standard recommendations. 3

• Infants receiving adjustable fortification guided by BUN targets show significantly higher daily weight gain and weekly head circumference increases compared to standard fortification 3
• At 40 weeks postmenstrual age, weight and head circumference are significantly higher in the adjustable fortification group 3
• Protein supplementation up to 1.6 g/day above standard fortification is safe, feasible, and beneficial when targeting BUN levels 3
• By one year corrected age, growth parameters equalize between groups, suggesting the benefit is primarily during the NICU period 3

Relationship Between BUN, Protein Intake, and Renal Function

Both protein intake and creatinine independently affect BUN levels, requiring consideration of renal function when interpreting BUN values. 4

• As protein intake increases over the first 3 weeks of life, both BUN and creatinine paradoxically decrease significantly 4
• Linear regression models demonstrate that protein intake and creatinine are each significantly associated with mean BUN for each study week 4
• Higher protein intake shows significant amelioration of growth failure during this period 4

Clinical Algorithm for BUN Use in NICU Nutrition

Days 1-3 (Early Parenteral Nutrition Phase):

• Do not use BUN to adjust amino acid delivery 1
• Provide amino acids at 3-4 g/kg/day as recommended by standard protocols 5
• Elevated BUN reflects gestational age and metabolic factors, not protein intolerance 2, 1

Days 4-21 (Transition and Early Enteral Phase):

• Monitor BUN alongside creatinine to assess renal function 4
• Interpret BUN in context of both protein delivery and creatinine levels 4
• Continue advancing protein intake toward 3.5-4 g/kg/day targets 5

Established Enteral Feeding (>160 mL/kg/day):

• Use BUN as a monitoring tool for protein adequacy 2
• Target BUN levels that support optimal growth (specific targets vary by protocol) 3
• Consider adjustable fortification with protein supplementation up to 1.6 g/day above standard if BUN suggests inadequate protein delivery 3
• Reassess if BUN remains low despite adequate protein provision 3

Critical Pitfalls to Avoid

The most common error is restricting amino acids in the first 72 hours based on elevated BUN, which deprives critically ill neonates of essential protein during maximal catabolism. 1

• Early amino acid restriction based on BUN leads to negative nitrogen balance and growth failure 2, 1
• Failing to account for creatinine when interpreting BUN leads to misattribution of elevated BUN to excessive protein rather than renal dysfunction 4
• Not recognizing that BUN utility changes dramatically between parenteral and enteral nutrition phases 2
• Assuming standard fortification is adequate when BUN monitoring during adjustable fortification shows 63% of infants require additional protein 3