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Professor Silvia Iacobelli explores fluid and electrolyte requirements in parenteral nutrition.
A group of water molecules.

In the following audio clip, Professor Silvia Iacobelli introduces this step and the key areas we will cover.

The transcript for this audio can be found here or in the downloads section below.

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Water is the major constituent of the human body.

As such, water is a key component of enteral nutrition as an essential carrier for nutrients and metabolites.

Preterm infants have higher fluid requirements than full term infants due to the following factors:

The factors are as follows: immature renal function with lower maximum urine osmolality, immature skin leads to increased insensible water loss, fluid needs are proportional to growth rates, and higher surface area to body volume ratio.

Fluid requirements show considerable inter- and intra-individual variation, especially in very preterm infants.

Maintenance of water volume and adequate fluid intake is critical for:

The factors listed as bullet points are: body homeostasis, thermoregulation, cardiovascular function, renal function, and sufficient nutrient intake.

The clinical condition of an infant influences the total body water requirements.

The relationship between water and solute quantities in preterm infants is important, as well as whether they are fed fortified breastmilk or formula. This ratio influences:

  • Milk osmolarity
  • Renal solute load
  • Acid load
  • Feed tolerance
  • Acid-base status
  • Water balance

These factors must be well managed in preterm infants, as their renal concentration ability, excretory capacity and regulation of acid-base metabolism are limited.

Recommendations for enteral fluid intake in stable growing preterm infants should be considered in the context of:

A box contains the heading 'context of fluid intake recommendations'. These are listed as follows: total fluid intake, human milk composition, the requirement for fortification, balancing support for normal growth with long term outcome optimisation, avoiding short term metabolic disturbances, and envrionment.

If you require a screen-reader compatible version this is available as a PDF.

Studies to establish enteral fluid intakes for preterm babies are often limited, as they may fail to consider water metabolism and kidney function. Most of these studies are also methodologically challenging as many historical studies were performed when recommended enteral macronutrient and electrolyte intakes were lower and stable preterm infants were typically more mature.

Human milk is the best source of water for preterm infants, but fortification is often necessary. Fortification increases the solute concentration, milk osmolarity and renal solute load.

A bottle containing human milk.

The potential renal solute load (PRSL) are dietary solutes that must be excreted in the urine as they are not diverted towards new tissue growth or lost through extra-renal routes. Few studies have calculated or reported the PRSL of milk-fed preterm infants and no detailed studies have compared differing fluid intakes when nutrient intake is identical.

PRSL can be calculated by the following equation:

The calculation reads as: PRSL = [N]/28 + Na + Cl + K + Pa. Further information is available as a PDF in the downloads section, with screen-reader compatibility.

If you require a screen-reader compatible version this is available as a PDF.

Clinicians are sometimes cautious about increasing fluid intake volumes, especially in infants with patent ductus arteriosus or chronic lung disease. However, in some studies, increased growth has been seen with increased feed volumes up to 200 mL/kg/day. In the short term, these volumes appear well tolerated with improved growth and no adverse outcomes on body composition, but the long-term impact has not been well explored.

Evidence increasingly suggests that prolonged dehydration causes renal hyperfiltration, exacerbated by a high protein intake, which may promote early development of metabolic diseases in adulthood. Therefore, adequate water intake is important.


Conclusions of water. Conclusions cover water requirements, water volume for homeostasis, and assessment. The full conclusion is available as a PDF in the downloads section.

If you require a screen-reader compatible version this is available as a PDF.


Recommendations of water. Recommendations cover fluid intake requirements and fluid intake for homeostasis. The full recommendation is available as a PDF in the downloads section.

If you require a screen-reader compatible version this is available as a PDF.

If you wish, you can download the SDC for this step for future reference.

In the next step, we are going to cover the main electrolytes: sodium, chloride and potassium.

This article is from the free online

Enteral Nutrition in Preterm Infants: ESPGHAN Recommendations

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