6.2 Computing Desirable Body Weight, Total Energy Requirement, and the Diet Prescription

The table of specifications asks examinees to compute nutrient requirements for protein, carbohydrate, and fat from the patient history and physical assessment, and to compute desirable body weight, BMI, weight control diets, and diabetic diets. These computations appear in almost every clinical case.

Step 1: Desirable Body Weight (DBW)

Tannhauser method (common in Philippine diet manuals):

$$\text{DBW (kg)} = (\text{Height in cm} - 100) - 10\%$$

Example: height 160 cm. 160 - 100 = 60; 10% of 60 = 6; DBW = 54 kg.

Other methods you may be asked about:

• Hamwi: women 100 lb for the first 5 ft plus 5 lb per extra inch; men 106 lb plus 6 lb per inch; plus or minus 10% for frame size.
• BMI-based: DBW = target BMI x height in meters squared. With a target BMI of 22 and a height of 1.60 m: 22 x 2.56 = 56.3 kg.

Then compare actual weight with DBW (percent DBW) and calculate BMI to classify nutritional status, as shown in the clinical assessment section.

Step 2: Total Energy Requirement (TER)

Quick method using DBW and activity level:

$$\text{TER} = \text{DBW} \times \text{kcal/kg for the activity level}$$

DBW is used instead of actual weight so that an overweight person's plan moves toward a healthy weight and an underweight person's plan supports gain.

Formula methods. In clinical settings, resting energy expenditure can be estimated with the Mifflin-St Jeor or Harris-Benedict equations and multiplied by activity and stress factors (see the energy metabolism section). Indirect calorimetry is the reference method in critical care.

Rounding. Many Philippine reviewers round TER to the nearest 50 kcal and macronutrient grams to the nearest 5 g. If a question asks for exact values, do not round.

Step 3: Distribute Energy Among the Macronutrients

The PDRI 2015 acceptable macronutrient distribution ranges for adults are 55-75% carbohydrate, 10-15% protein, and 15-30% fat. A common prescription for healthy adults is about 60% carbohydrate, 15% protein, and 25% fat. Therapeutic diets change these proportions, for example lower carbohydrate in diabetes or lower protein in kidney disease.

$$\text{Grams of carbohydrate or protein} = \frac{\text{TER} \times \%}{4}, \qquad \text{Grams of fat} = \frac{\text{TER} \times \%}{9}$$

Protein can also be set per kilogram: about 0.8-1.0 g/kg DBW for healthy adults, higher in stress, wound healing, or dialysis, and lower in chronic kidney disease not on dialysis.

Worked Example: Adult Weight Management

A 45-year-old man is 168 cm tall, weighs 80 kg, and works in an office.

1. DBW: (168 - 100) - 6.8 = 61.2 kg, about 61 kg.
2. BMI: 80 / (1.68 x 1.68) = 28.3 kg/m2, obese class I by Asia-Pacific cut-offs (overweight by WHO international cut-offs).
3. Percent DBW: 80 / 61 = 131%, obese.
4. TER at DBW: 61 x 30 = 1,830 kcal, rounded to 1,850 kcal.
5. Distribution (60:15:25): carbohydrate 1,850 x 0.60 / 4 = 277.5, about 280 g; protein 1,850 x 0.15 / 4 = 69.4, about 70 g; fat 1,850 x 0.25 / 9 = 51.4, about 50 g.
6. Diet prescription: "1,850 kcal; carbohydrate 280 g, protein 70 g, fat 50 g; low in saturated fat."

Weight-loss adjustment. About 7,700 kcal is stored in 1 kg of body fat, so a deficit of about 500 kcal/day leads to a loss of roughly 0.5 kg/week, and 1,000 kcal/day to about 1 kg/week. Calculating TER from DBW already creates a deficit for most people with obesity. Very-low-calorie diets below about 800 kcal/day need medical supervision.

Worked Example: Child

A quick estimate for children aged 1 to 12 years:

$$\text{TER} = 1{,}000 + (100 \times \text{age in years})$$

A 6-year-old needs about 1,000 + 600 = 1,600 kcal, which matches the PDRI 2015 value of 1,600 kcal for boys aged 6-9 (1,470 kcal for girls). For infants and adolescents, use the PDRI table values, which are based on reference weights.

Worked Example: Pregnancy

A pregnant woman in her second trimester has a TER of 1,900 kcal before pregnancy. The PDRI 2015 adds 300 kcal/day in the second and third trimesters, so her TER becomes 2,200 kcal, and protein increases by 27 g/day. During lactation, add 500 kcal/day and 27 g protein/day.

Writing the Diet Prescription

A complete diet prescription includes:

• The energy level (kcal).
• Grams of carbohydrate, protein, and fat, and sometimes the percentage distribution.
• Modifications in nutrients (sodium, potassium, fiber, purine), consistency (soft, pureed), or meal pattern (small frequent feedings).
• The meal plan in exchanges and its distribution across meals and snacks.

Example: "Diabetic diet, 1,600 kcal; carbohydrate 200 g, protein 70 g, fat 55 g; 3 meals and 2 snacks with consistent carbohydrate; sodium 2 g."