Nutritional Assessment in ICU

Nutritional Assessment in ICU

A common pitfall in nutritional assessment is misinterpreting the effects of acute illness (e.g., albumin shifts in septic shock) as indicators of underlying malnutrition.

History 

• Acute disease effects: Conditions like sepsis, trauma, or inflammation can cause fluid shifts, hypoalbuminemia, and weight fluctuations, which may not reflect true nutritional status.
• Oral intake history: Has the patient been NPO for several days or had restricted access to food?DiarrheaDysphagia,Vomiting,Feeding intolerance
• BMI (Body Mass Index): Low BMI may indicate chronic undernutrition, while high BMI does not necessarily rule out malnutrition (obesity-related sarcopenia).
• Recent weight loss:Clinically significant weight loss:

Time	Significant Loss
1 week	>1–2%
1 month	>5%
3 months	>7.5%
6 months	>10%

• Physical signs :

Assessment	Sites to Examine
Muscle Loss	Temporalis muscle, Clavicles (pectoralis region), Deltoids, Interosseous muscles (hands), Quadriceps, Gastrocnemius (calves)
Subcutaneous Fat Loss

• Orbital fat pads, Buccal fat (cheeks), Triceps, Ribs/Thoracic region (mid-axillary line)

Micronutrient Deficiency Signs

Deficiency	Findings
Zinc	Poor wound healing, dermatitis
Vitamin C	Petechiae, gingival bleeding
Vitamin D	Weakness
Thiamine	Neuropathy, encephalopathy
Iron	Pallor, koilonychia

Nutrition Assessment Tools

1. Subjective Global Assessment (SGA) – Evaluates weight loss, dietary intake, functional status.
2. Nutritional Risk Screening (NRS-2002) – Score ≥3:
   → Nutritional risk,Limitations in ICU: Less accurate in severe critical illness
3. Nutrition Risk in Critically Ill (NUTRIC) Score – ICU-specific scoring system incorporating APACHE II and SOFA scores.Modified NUTRIC (mNUTRIC):Excludes IL-6,Most commonly used

Interpretation

Score	Risk
0–4	Low risk
5–9	High nutritional risk

High scores identify patients likely to benefit from aggressive nutrition therapy.

4. Indirect Calorimetry – Gold standard for measuring energy expenditure.

Measures:

• Oxygen consumption (VO₂)
• Carbon dioxide production (VCO₂)

Calculates resting energy expenditure (REE).Most accurate ICU method.If calorimetry is not available, using VO2 (oxygen consumption) from pulmonary arterial catheter or VCO2 (carbon dioxide production) derived from the ventilator (REE=VCO2 x 8.19)will give a better evaluation on EE than predictive equations.

Weir Equation

REE=(3.941×VO2 )+(1.106×VCO2 )

Advantages:

• Individualized
• Avoids overfeeding/underfeeding

Limitations:Cost,Equipment availability,FiO₂ limitations

5. PREDICTIVE EQUATIONS

Used when indirect calorimetry unavailable.

Examples:

• Harris–Benedict
• Penn State
• Mifflin–St Jeor
• Ireton-Jones

Less accurate in ICU.If predictive equations are used to estimate the energy need, hypocaloric nutrition (below 70% estimated needs) should be preferred over isocaloric nutrition for the first week of ICU stay.

6. BODY COMPOSITION ASSESSMENT

Assessment of lean body mass is increasingly important.

Assessment	Key Points
Ultrasound Muscle Assessment	Increasingly used bedside tool to assess muscle mass. Commonly evaluates the rectus femoris and quadriceps. Detects muscle thickness, cross-sectional area, muscle quality (echogenicity), and progression of muscle wasting. Advantages: Bedside, non-invasive, repeatable, no radiation, useful for serial monitoring in ICU.
Bioelectrical Impedance Analysis (BIA)	Estimates fat mass, lean body mass, skeletal muscle mass, and total body water by measuring electrical impedance. Limitations: Accuracy is significantly reduced in patients with edema, fluid overload, rapid fluid shifts, ascites, or critical illness, making it less reliable in many ICU patients.

7. FUNCTIONAL ASSESSMENT

Muscle function predicts outcomes better than body weight.

Assessment	Key Points
Handgrip Strength	Bedside measure of functional status, muscle strength, and sarcopenia. Low handgrip strength is associated with higher mortality, prolonged hospital/ICU stay, disability, and poorer functional recovery. Limitations: Not reliable in patients with sedation, delirium, mechanical ventilation, neuromuscular weakness, severe pain, or inability to cooperate.
Frailty Assessment	Assesses physiological reserve and vulnerability before critical illness. Frailty independently predicts higher mortality, prolonged ICU/hospital stay, greater complications, increased mechanical ventilation, institutionalization, and poorer long-term functional outcomes. Common tools include the Clinical Frailty Scale (CFS) and Frailty Index (FI).

8 . LABORATORY ASSESSMENT

Parameter	Key Points
Serum Albumin	Poor nutritional marker in ICU. Low levels mainly reflect inflammation, capillary leak, fluid shifts, and liver dysfunction, not nutritional status. Not reliable for acute nutritional assessment.
Prealbumin (Transthyretin)	Shorter half-life than albumin, but still influenced by inflammation, renal failure, and liver disease. Not recommended as a sole marker of nutritional status.
Nitrogen Balance	Estimates protein catabolism/anabolism. Formula: Nitrogen Balance = Protein intake (g)/6.25 − (UUN + 4) (UUN = urinary urea nitrogen; +4 g accounts for insensible nitrogen losses). Interpretation: Negative = catabolic state; Positive = anabolic state. Limitations: Requires accurate 24-hour urine collection; unreliable in renal failure and incomplete urine collections.
Micronutrient Levels	Assess selectively, not routinely. Common tests include Zinc, Selenium, Copper, Vitamin D, Thiamine (B1), and Folate, especially when deficiency is suspected or in prolonged critical illness.

Nutritional Requirements

Nutrient	Recommended Intake
Calories	25-30 kcal/kg/day
Protein	1.3g/kg/day
Carbohydrates	4 kcal/g 3-5 g/kg/day 150 g/d 600 kCal/day	40–60% of total calories Glucose infusion—5 mg/kg/min
Fats	1-1.5 g/kg/day 9 kcal/g	20–35% of total calories
Fluids	25-35 mL/kg/day (adjust For Clinical Status)
Electrolytes	Replace according to losses
Micronutrients	Daily RDA + supplementation if deficient
Fiber	~10-30 grams/day in divided doses	Partially hydrolyzed guar gum (PHGG) is best

Special Conditions

Clinical Condition	Energy Target (kcal/kg/day)*	Protein (g/kg/day)**	Comments
Trauma	25–30	1.5–2.0	Increased catabolism.
Major burns (>20% TBSA)	30–35 (or IC-guided)	1.5–2.0 (up to 2.5 in selected cases)	Hypermetabolic state; IC preferred.
Obesity (BMI 30–50)	11–14 kcal/kg actual body weight/day (hypocaloric)	2.0 g/kg ideal body weight/day	High-protein hypocaloric feeding.
Obesity (BMI >50)	22–25 kcal/kg ideal body weight/day	2.5 g/kg ideal body weight/day	ASPEN recommendation.
AKI (no dialysis)	20–30	1.0–1.3	Do not restrict calories; modest protein if not on RRT.
AKI on CRRT	20–30	1.5–2.5	CRRT causes amino acid losses.
Intermittent Hemodialysis	25–30	1.2–1.5	Increase protein to replace dialysis losses.
CKD (non-ICU)	Individualized	Usually 0.6–0.8 (non-catabolic); ICU: ≥1.2	Critical illness overrides chronic CKD restrictions.

Weight to Use for Protein Prescription in Critically Ill Patients

• Target protein intake: ≥1.2 g/kg/day (higher amounts may be required in selected patients according to disease state and guidelines).
• BMI <30 kg/m²: Calculate protein requirements using the actual body weight (ABW).
• BMI ≥30 kg/m² (Obesity): Use the adjusted body weight (AdjBW) instead of actual body weight to avoid overestimating protein needs.

Formula for Adjusted Body Weight (AdjBW):

Adjusted Body Weight = Ideal Body Weight (IBW) + ⅓ × (Actual Body Weight − IBW)

or

AdjBW = IBW + 0.33 × (ABW − IBW)

 Non-Protein Calories (NPC)

Definition: Calories supplied by carbohydrates and lipids only. Protein calories are excluded because protein is primarily provided to maintain lean body mass rather than as an energy source.

Formula

NPC = Total calories − Protein calories

Protein calories = Protein (g) × 4 kcal/g

Sources

Nutrient	kcal/g
Carbohydrate	4
Fat	9

Example

• Total calories = 2,000 kcal/day
• Protein = 100 g/day (400 kcal)

Non-protein calories = 2,000 − 400 = 1,600 kcal/day

NPC:N Ratio

Nitrogen (g) = Protein (g) ÷ 6.25

Example:

• Protein = 100 g → Nitrogen = 16 g
• NPC = 1,600 kcal

NPC:N ratio = 1,600 : 16 = 100 : 1

Clinical situation	Target NPC:N ratio
Severe stress (burns, trauma, sepsis)	80–100 : 1
Moderate stress	100–120 : 1
Stable/anabolic patients	120–150 : 1

A lower NPC:N ratio provides relatively more protein, whereas a higher ratio provides relatively more energy.

Respiratory Quotient (RQ)

• RQ is the ratio of carbon dioxide produced (VCO₂) to oxygen consumed (VO₂) and reflects which substrate is being oxidized.
• RQ = VCO₂ / VO₂
• Measured by indirect calorimetry.

RQ of Macronutrients

Substrate	RQ
Fat	0.70
Protein	0.80
Mixed diet	0.80–0.85
Carbohydrate	1.00

Interpretation

RQ	Interpretation
<0.70	Underfeeding, ketosis, predominant fat oxidation
0.80–0.90	Appropriate substrate utilization (target range)
>1.00	Overfeeding, excess carbohydrate intake, lipogenesis

Clinical Importance

• Assesses adequacy of nutrition
• Detects overfeeding or underfeeding
• Guides adjustment of carbohydrate-to-fat ratio
• Helps minimize excess CO₂ production in mechanically ventilated patients

Target RQ: 0.80–0.90

Nitrogen Balance

• Nitrogen balance estimates whether the patient is gaining or losing body protein.
• Nitrogen balance (g/day) = Nitrogen intake − Nitrogen losses
• Nitrogen intake = Protein intake (g) ÷ 6.25
• Nitrogen losses are primarily measured using 24-hour urinary urea nitrogen (UUN), with an additional ≈4 g/day added for non-urinary losses (skin, stool, sweat) in most patients.
• Nitrogen balance = [Protein (g) ÷ 6.25] − (UUN + 4)

Interpretation

Nitrogen balance	Interpretation
Positive	Anabolism, recovery, tissue growth
Zero	Maintenance (protein equilibrium)
Negative	Catabolism, ongoing muscle loss

Causes of Negative Nitrogen Balance

• Sepsis
• Major trauma
• Burns
• Surgery
• Inadequate protein intake
• Prolonged fasting
• Uncontrolled hyperglycemia
• Corticosteroid therapy

Clinical Importance

• Estimates adequacy of protein prescription
• Assesses response to nutrition therapy
• Monitors recovery from catabolic illness

Limitations: Nitrogen balance calculations become less reliable in patients with CRRT, large wound exudates, fistulas, burns, massive diarrhea, or incomplete urine collection, and they are largely being replaced by serial clinical assessment and body composition measures where available.

Metabolic Response to Critical Illness

A. Ebb Phase (First 24–48 Hours)

• Reduced metabolic rate
• Reduced oxygen consumption
• Shock physiology and hypoperfusion
• Decreased insulin secretion

Clinical priority:

• Hemodynamic stabilization
• NOT aggressive feeding

B. Flow Phase

Catabolic Phase

• Hypermetabolism
• Insulin resistance
• Increased cortisol/catecholamines
• Muscle breakdown
• Increased nitrogen loss

Anabolic Recovery Phase

• Protein synthesis improves
• Muscle rebuilding begins

• Initiate feeding early at a reduced rate (approximately 25-50% of the caloric goal)-Providing full nutritional support during the early catabolic phase can lead to overfeeding, as the combination of exogenous nutrition and endogenous energy from catabolism may exceed the patient’s actual metabolic needs. To avoid this, full caloric targets should not be met immediately, and nutrition should be gradually increased based on the patient’s metabolic state and tolerance
•  Gradually increase to 100% of target calories over 3-7 days,After day three, energy delivery can be increased up to 80 to 100% of measured EE based on patient tolerance. Priority should be given to meeting full protein requirements(1.2 grams/kg/day ) whenever possible.