This pilot trial is part of a long-term research program leading to a large trial to determine if a strategy of supplementing protein in a subset of critically ill children is superior to standard enteral nutrition care. The investigators hypothesize that protein supplementation to critically ill children with body mass index (BMI) z-score <0 reduces the length of stay in the paediatric intensive care unit (PICU) and hospital, as well as the duration of mechanical ventilation (MV).

Protein malnutrition is prevalent among critically ill children and associated with poor clinical outcomes. Current guidelines for provision of protein in daily management of critically ill children in pediatric intensive care units (PICUs) are based on limited and not high-quality evidence. The current protein supplementation approach, "one-size-fits-all" nutrition prescription, may not be appropriate for every critically ill child. Given the heterogeneity of the patients admitted to the PICU, a targeted approach based on individuals' needs represents a potential advancement in PICU care.

The central hypothesis is that protein supplementation in PICU care leads to improved clinical outcomes in subgroups of patients only. The investigators propose a dual-centre pilot randomized controlled trial in Singapore with two major aims:

1. To obtain key information for planning and conducting a large-scale multicentre study in Asia; and
2. to evaluate the benefit of protein supplementation to critically ill children with body mass index (BMI) z-scores on PICU <0

The main clinical outcome of interest is total number of days of hospital stay (from PICU admission to hospital discharge). Two protein supplementation regimes (≥ 1.5g/kg/day vs. standard care) will be randomly allocated to PICU patients with body mass index (BMI) z-scores <0. The investigators will determine the effect of protein supplementation on total length of hospital stay and other clinical outcomes, and assess the impact of protein supplementation on acquired functional impairment of PICU survivors 6 months after hospital discharge. In addition, the investigators will explore whether muscle ultrasound is a biomarker for protein balance in pediatric critical illness.

Upon completion of this study, the investigators will have strong preliminary data to plan, refine their study approach, and execute a future large multi-centre study across Asia. These data will ultimately guide protein provision and nutritional management of patients in PICU setting.

Inclusion Criteria:

1. Between 28 days and 18 years of age
2. Have a BMI z-score < 0 on PICU admission
3. Requires respiratory support in the form of invasive or non-invasive mechanical ventilation (CPAP or BiPAP) within 24-48 hours of PICU admission, and the attending physician expects the child to require any respiratory support for at least 48 hours of PICU admission
4. Requires enteral nutrition support for feeding (e.g., oro-gastric, nasogastric, gastrostomy, naso-jejunal, oro-jejunal)
5. Have an expected PICU stay of > 48 hours

Exclusion Criteria:

1. Are not expected to survive this PICU admission because of palliative care (e.g., do-not-resuscitate status) or limited life support
2. Progressive neuromuscular disease (e.g., spinal muscular atrophy, Duchenne or other muscular dystrophy, multiple sclerosis, amyotrophic lateral sclerosis)
3. Medical conditions where increased or decreased protein intake is required, including acute kidney injury (stage 3 KDIGO criteria), chronic kidney disease (stage 4 and 5), inborn errors of metabolism, fulminant liver failure, severe burn injury
4. Contraindications to enteral nutrition (e.g., gut hemorrhage, post-gastrointestinal surgery etc.)
5. Diagnosed cow's milk protein allergy
6. Parenteral nutrition
7. Premature infants who are less than corrected gestational age of 44 weeks (gestational age + weeks after birth)
8. Were previously enrolled in this trial
9. Are currently enrolled in a potentially confounding trial
10. Diagnosis of anorexia nervosa and other eating disorders
11. On extra-corporeal membrane oxygenation (ECMO) support
12. Conditions that required significant fluid restriction (≤75% of maintenance fluid) (e.g., post cardiac surgery etc.)

Data from this pilot study may be used for the larger study. If so, these data will not be made available to other researchers till the larger study is completed or deemed not feasible.