13.2 Protein & Fluid Requirements

Protein Requirements Across Childhood

Protein needs per kilogram mirror growth velocity: highest in premature and young infants, declining steadily to adult values. Exam-worthy reference points:

Elevated needs in illness. Catabolic states, wound healing, and malabsorption raise requirements well above the RDA: critically ill children generally need at least 1.5 g/kg/d to achieve positive nitrogen balance; major burns and large wounds may need 2–3 g/kg/d; cystic fibrosis (CF) guidelines recommend roughly 1.5–2 times the age-specific DRI to overcome maldigestion and chronic inflammation; nephrotic syndrome and peritoneal dialysis add urinary/dialysate losses of roughly 0.1–0.3 g/kg/d on top of baseline. Adequate energy must accompany protein, or amino acids are oxidized for fuel — the classic target protein:energy ratio for catch-up growth is roughly 8–12% of calories from protein (about 2–3 g protein per 100 kcal).

Worked example. A 10-kg burned toddler prescribed 2.5 g/kg/d protein needs 25 g protein/d; at an energy goal of 1,200 kcal/d that is 100 kcal from protein ≈ 8.3% of energy — consistent with the catch-up ratio.

Fluid Requirements

Holliday-Segar Method

The Holliday-Segar method estimates maintenance water from caloric expenditure, weight-banded:

• 100 mL/kg/d for the first 10 kg
• 50 mL/kg/d for each kg from 11–20 kg
• 20 mL/kg/d for each kg above 20 kg

The hourly equivalent is the "4-2-1" rule: 4 mL/kg/h for the first 10 kg, 2 mL/kg/h for the next 10, 1 mL/kg/h thereafter. Worked example. A 24-kg child: (10 × 100) + (10 × 50) + (4 × 20) = 1,000 + 500 + 80 = 1,580 mL/d, or 40 + 20 + 4 = 64 mL/h.

Body-Surface-Area Method and Insensible Losses

The body surface area (BSA) method estimates maintenance at roughly 1,500–2,000 mL/m²/d and is preferred for larger children and in renal calculations. Insensible water losses (skin + respiratory, ~300–400 mL/m²/d, or about one-third of maintenance) rise with fever, tachypnea, low humidity, radiant warmers, and phototherapy in preterm infants — a key reason ELBW fluid orders are individualized day by day. Losses fall with humidified ventilator circuits and cool ambient temperatures.

Maintenance Electrolytes and Special Situations

Maintenance fluid prescriptions pair water with electrolytes — classically sodium 2–3 mEq/kg/d and potassium 1–2 mEq/kg/d (the origin of hypotonic maintenance fluids such as D5 0.45% saline). Because hospital-acquired hyponatremia from excess antidiuretic hormone is a real danger in acutely ill children, current guidance favors isotonic maintenance fluids for most hospitalized patients. Preterm infants are a distinct case: fluids start around 60–80 mL/kg/d on day 1 and advance by ~20–30 mL/kg/d to 140–160 mL/kg/d, with sodium deliberately withheld until the physiologic postnatal diuresis and weight contraction occur (adding sodium too early promotes edema, bronchopulmonary dysplasia, and patent ductus arteriosus). Children on ketogenic diet therapy have fluids and carbohydrates restricted together — free water is not limited, but all carbohydrate-containing IV fluids and medications must be avoided to preserve ketosis. Finally, remember that enteral formula is mostly water: standard 1.0 kcal/mL formulas are ~85% free water, so high-volume enteral feeding can deliver well over maintenance fluid, while concentrated or high-calorie-density formulas deliver proportionally less free water and may need supplemental flushes.

Fluid Restriction, Dehydration, and Rehydration

Restriction in Cardiac and Renal Disease

In congestive heart failure, fluid is typically restricted to ~80–90% of maintenance (more tightly in decompensation), often combined with diuretics and calorie-dense feeds so nutrition is not compromised by the volume cap. In oliguric or anuric renal failure, the standard formula is insensible losses (~300–400 mL/m²/d, or ~1/3 maintenance) plus measured output (urine, stool, drains) replaced milliliter-for-milliliter — the "insensible + output" approach. Hyponatremia, hyperkalemia, and rising blood urea nitrogen signal that free water and electrolytes need further tightening.

Dehydration Assessment

Percent dehydration is estimated from acute weight loss and clinical signs; thresholds differ by age because infants carry a larger extracellular fluid compartment:

Oral Rehydration Therapy

For mild-to-moderate dehydration, oral rehydration therapy (ORT) with a glucose-electrolyte solution (Na ~75 mEq/L, osmolality ~245 mOsm/kg, e.g. WHO reduced-osmolarity ORS or Pedialyte) is first-line: replace 50–100 mL/kg over 2–4 hours in small frequent aliquots (5–10 mL every 1–5 minutes), then replace ongoing losses (~10 mL/kg per stool, ~2 mL/kg per emesis). ORT works via sodium–glucose cotransport even in secretory diarrhea and avoids unnecessary IVs. Contraindications/cautions: severe dehydration or shock (IV isotonic boluses of 20 mL/kg instead), altered mental status, intractable vomiting, ileus, or suspected surgical abdomen. Juice and sports drinks are too hyperosmolar and too low in sodium to serve as ORS.