Primary aim: to assess the impact of late (≥7 days postnatal) sodium supplementation of premature neonates with birth weight less than 2500 gm on their postnatal short-term catch-up growth.

Secondary aim: to find out the effect of this sodium supplementation on fractional excretion of sodium, hemodynamics and prematurity-related short-term neonatal outcomes including morbidities and morality.

Preterm birth is a global health problem and the primary contributor to neonatal mortality and morbidity [2].

Poor growth after preterm birth is challenging and arises from a combination of various factors, including nutritional needs, hormonal abnormalities, central nervous system damage, feeding difficulties, and administration of drugs that affect nutrient metabolism [3].

Typically, weight gain in the neonatal period begins after the first week of life, considered a period of physiological weight loss. With a mean period of 10.6 days, preterm neonates experience an average weight gain of 16.7 g/kg per day after reaching their birth weight [4].

The shift from the intra- to the extrauterine environment is associated with significant alternations in water and electrolytes, especially sodium homeostasis. In the early phase, this is primarily marked by decreased extracellular fluid volume and sodium loss. This adaptation becomes considerably more complex in premature infants due to immature kidneys, which lack full regulatory functionality and increased transdermal water loss [5]. Consequently, premature infants are at risk to hypernatremia early in life. In contrast, after the initial postnatal period with skin maturation, these infants become susceptible to hyponatremia because of the inability of the premature kidney to retain salt [6]. This often necessitates high sodium substitution to ensure adequate growth [5].

Sodium plays a crucial role for protein synthesis, bone mineralization, maintenance of extracellular space, and enabling the transport of glucose across the cell membranes [7]. Sodium can be considered a growth factor that stimulates protein synthesis and increase cell mass, and thus inadequate sodium intake can lead to chronic sodium depletion and thus growth failure [1].

The European Society of Pediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) Committee of Nutrition recently issued updated recommendations for sodium intake of 3-8 mEq/kg/day for preterm infants during the first few months of birth[1,8].

Inclusion Criteria:

• Preterm from 28 to 34 weeks gestation, admitted to NICU.
• Birth weight (<2500 gm). Postnatal age ≥7 days

Exclusion Criteria:

• - Major congenital malformations.
• Congenital heart disease
• Renal insufficiency (defined as an increase in serum creatinine by ≥0.5 mg/dL/d, urine output <0.5 mL/kg/h) [10].
• Disease states characterized by fluid retention: as cardiac, hepatic or renal.
• Receiving diuretic therapy.
• Full term less than 2500 gm.