3.4 Estimating Energy & Protein Needs

Predictive Equations

Mifflin-St Jeor (preferred)

The Academy of Nutrition and Dietetics's Evidence Analysis Library recommends Mifflin-St Jeor as the most accurate predictive equation for resting metabolic rate (RMR) in healthy and obese non-critically-ill adults:

• Men: RMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
• Women: RMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

The only difference between sexes is the final constant: +5 for men, −161 for women. That sign flip is a favorite exam trap, so commit it to memory.

Harris-Benedict (older)

The Harris-Benedict equation estimates basal energy expenditure (BEE) and is still tested for comparison:

• Men: BEE = 66.5 + (13.75 × wt kg) + (5.003 × ht cm) − (6.75 × age)
• Women: BEE = 655.1 + (9.563 × wt kg) + (1.850 × ht cm) − (4.676 × age)

Harris-Benedict tends to over-predict, especially in obesity, which is why Mifflin-St Jeor is now preferred. A practical scope note: in obese adults, Mifflin-St Jeor is validated using actual body weight, so resist the urge to substitute ideal or adjusted weight unless a specific protocol calls for it.

Adding Activity and Stress

A resting estimate is only the starting point. To find total energy expenditure (TEE), multiply RMR by an activity factor and, in acute illness, an injury/stress factor:

TEE = RMR × activity factor × stress factor

Worked example: a patient with an RMR of 1,400 kcal who is on bedrest (1.2) with a major infection (1.4) needs about 1,400 × 1.2 × 1.4 ≈ 2,350 kcal/day. Note that for critically ill, mechanically ventilated patients, indirect calorimetry is the gold standard; predictive equations are only an estimate.

Indirect calorimetry measures oxygen consumption and carbon dioxide production to derive the respiratory quotient (RQ) and measured energy expenditure; an RQ near 0.85 suggests mixed-fuel use, while an RQ above 1.0 flags overfeeding and excess carbon dioxide production — a tested concept because overfeeding can delay ventilator weaning.

Quick kcal/kg and Protein g/kg Shortcuts

In clinical practice the RD often uses per-kilogram rules of thumb, which the exam tests directly because they are fast and condition-specific:

The most heavily tested contrast is renal: CKD predialysis restricts protein to slow progression, while dialysis raises protein needs because hemodialysis and peritoneal dialysate remove amino acids and (in PD) some protein directly. Mix those two up and you choose the opposite answer. Also note critical illness keeps energy modest (avoid overfeeding) while pushing protein high to limit lean-mass loss.

A second high-yield rule concerns kcal-per-gram fuel values used to back-calculate intake: carbohydrate and protein each provide 4 kcal/g, fat provides 9 kcal/g, and alcohol 7 kcal/g. If a stem gives grams of each macronutrient, multiply and sum to verify total energy. For acceptable macronutrient distribution ranges (AMDR), carbohydrate is 45-65%, fat 20-35%, and protein 10-35% of total calories. Worked check: a 2,000 kcal diet with 250 g carbohydrate supplies 250 × 4 = 1,000 kcal, or 50% of energy — within the AMDR. Being fluent with these conversions lets you confirm or reject a calculated answer quickly.

Fluid Needs

The most common adult estimate of maintenance fluid is 30-35 mL/kg/day, adjusted up for losses (fever, open wounds, surgical drains, high ostomy output, diarrhea) and restricted in heart failure, oliguric renal failure, and the syndrome of inappropriate antidiuretic hormone (SIADH).

Alternative methods include the Holliday-Segar rule (≈1-1.5 mL per kcal) and the weight-based pediatric "4-2-1" / 100-50-20 method:

• First 10 kg: 100 mL/kg
• Next 10 kg (11-20 kg): 50 mL/kg
• Each kg above 20 kg: 20 mL/kg

Worked example: a 25 kg child needs (10 × 100) + (10 × 50) + (5 × 20) = 1,000 + 500 + 100 = 1,600 mL/day. Older adults carry a higher dehydration risk because the thirst response and renal concentrating ability decline with age, so they are monitored closely even at a modestly lower floor (about 25-30 mL/kg). Conversely, fluid is restricted in conditions of volume overload — choose the lower end or an explicit cap when the stem mentions heart failure, end-stage renal disease, or SIADH, and watch for signs of overhydration such as edema and dilutional hyponatremia.