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Nutrition Fundamentals & Key Formulas - Quick Reference for Dietitians
1. ANTHROPOMETRIC ASSESSMENT
Body Mass Index (BMI)
The primary screening tool for weight status.
BMI = Weight (kg) / Height² (m)
OR
BMI = 703 × Weight (lb) / Height² (in)
| BMI (kg/m²) | Classification |
|---|---|
| < 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 - 34.9 | Class I Obesity |
| 35.0 - 39.9 | Class II Obesity |
| ≥ 40 | Class III (Severe) Obesity |
Note: BMI may overestimate body fat in trained athletes and underestimate it in the elderly. - Textbook of Family Medicine 9e
Ideal Body Weight (IBW) Formulas
Brocca Index (simple, quick)
IBW (kg) = Height (cm) - 100
Example: Height = 170 cm → IBW = 70 kg
Lorentz's Formula (more precise)
Men: IBW = Height (cm) - 100 - [(Height - 150) / 4]
Women: IBW = Height (cm) - 100 - [(Height - 150) / 2]
Corpulence Index
Corpulence Index = Actual Weight / Desirable Weight
(should not exceed 1.2)
Ponderal Index
Ponderal Index = Height (cm) / ³√Body Weight (kg)
- Park's Textbook of Preventive & Social Medicine
Waist-to-Hip Ratio (WHR)
WHR = Waist circumference (cm) / Hip circumference (cm)
| Risk | Men | Women |
|---|---|---|
| Low | < 0.90 | < 0.80 |
| High | ≥ 0.90 | ≥ 0.80 |
Skinfold Thickness (Body Fat %)
- Measure at 4 sites: triceps, biceps, subscapular, suprailiac
- Sum of measurements: < 40 mm (boys), < 50 mm (girls) - normal
- Use Durnin-Womersley or Siri equation to convert sum to % body fat
2. ENERGY (CALORIE) CALCULATIONS
Step 1 - Calculate Basal Metabolic Rate (BMR) / Resting Metabolic Rate (RMR)
Harris-Benedict Equation (1919) - classic, widely used
Men: RMR = 66.47 + (13.75 × W) + (5.0 × H) - (6.75 × A)
Women: RMR = 665.09 + (9.56 × W) + (1.84 × H) - (4.67 × A)
W = weight in kg | H = height in cm | A = age in years
Mifflin-St. Jeor Equation (1990) - PREFERRED, most accurate especially in obesity
Men: RMR = (9.99 × W) + (6.25 × H) - (4.92 × A) + 5
Women: RMR = (9.99 × W) + (6.25 × H) - (4.92 × A) - 161
Frankenfield et al. (2005) compared multiple equations and found Mifflin-St. Jeor performs best vs. indirect calorimetry. - Textbook of Family Medicine 9e
Simplified Clinical Formula (Bailey & Love's Surgery)
Men: BMR = (10 × W) + (6.25 × H) - (5 × A) + 5
Women: BMR = (10 × W) + (6.25 × H) - (5 × A) - 161
Step 2 - Apply Physical Activity Factor (PAF)
Total Daily Energy Expenditure (TDEE) = BMR × Activity Factor
| Activity Level | Factor |
|---|---|
| Sedentary (little/no exercise; mild stress) | × 1.2 |
| Lightly active (1-3 days/wk; moderate stress) | × 1.4 |
| Moderately active (3-5 days/wk; severe stress) | × 1.6 |
| Very active (6-7 days/wk hard exercise) | × 1.725 |
| Extra active (very hard exercise + physical job) | × 1.9 |
- Textbook of Family Medicine 9e
Step 3 - Quick Weight Goal Estimate (kcal/kg IBW)
| Goal | Low Activity | Moderate | High Activity/Illness |
|---|---|---|---|
| Lose weight | 15 kcal/kg | 20 kcal/kg | 25 kcal/kg |
| Maintain weight | 20 kcal/kg | 25 kcal/kg | 30 kcal/kg |
| Gain weight | 25 kcal/kg | 30 kcal/kg | 35 kcal/kg |
- Textbook of Family Medicine 9e, Table 37-10
Stress Factors (for hospitalized/ill patients)
REE = RMR × 1.2 to 1.3 (baseline resting energy expenditure)
Fever adjustment: REE × 1.1 per °C above normal
Mild stress: × 1.2
Moderate stress: × 1.4
Severe stress: × 1.6
- In general disease/post-op: 20-30 kcal/kg IBW/day
- Severe stress/burns: up to 30 kcal/kg IBW/day
- Always use IBW not actual body weight in obese patients to avoid overfeeding
- Bailey & Love's Short Practice of Surgery 28th Ed.
3. MACRONUTRIENT REQUIREMENTS
Acceptable Macronutrient Distribution Ranges (AMDR)
| Macronutrient | % of Total Calories | Calories per gram |
|---|---|---|
| Carbohydrates | 45 - 65% | 4 kcal/g |
| Protein | 10 - 35% | 4 kcal/g |
| Fat | 20 - 35% | 9 kcal/g |
| Alcohol | - | 7 kcal/g |
Protein Requirements
| Population | g/kg body weight/day |
|---|---|
| Healthy adults (general) | 0.8 g/kg |
| Moderately stressed (hospitalized) | 1.0 - 1.5 g/kg |
| Critically ill / surgical / trauma | 1.5 - 2.0 g/kg |
| Burns, severe catabolism | 2.0 - 2.5 g/kg |
| Athletes (endurance) | 1.2 - 1.4 g/kg |
| Athletes (strength) | 1.6 - 1.8 g/kg |
| Elderly (sarcopenia prevention) | 1.0 - 1.2 g/kg |
| Pregnancy | +25 g/day above baseline |
"Protein should make up 1.5 to 2 g/kg/day... Carbohydrates make up about 70% of the remaining total caloric requirement and lipids about 30%." - Textbook of Family Medicine 9e
Carbohydrate Requirements
- General adults: 45-65% of total kcal; minimum 130 g/day (brain requirement)
- Fiber: 25 g/day (women), 38 g/day (men); or 14 g per 1000 kcal
- Added sugars: < 10% of total calories (WHO/Dietary Guidelines)
Fat Requirements
- Total fat: 20-35% of total kcal
- Saturated fat: < 10% total kcal (< 7% in cardiovascular disease)
- Trans fat: < 1% total kcal (minimize as much as possible)
- Omega-3 (ALA): 1.1 g/day (women), 1.6 g/day (men)
- Omega-6 (LA): 11-17 g/day (age/sex dependent)
4. MICRONUTRIENT KEY REFERENCE VALUES
Major Vitamins (adult RDA/AI)
| Vitamin | Men | Women | Key Function |
|---|---|---|---|
| Vitamin A | 900 mcg RAE | 700 mcg RAE | Vision, immunity |
| Vitamin C | 90 mg | 75 mg | Antioxidant, collagen |
| Vitamin D | 600-800 IU | 600-800 IU | Bone, immune |
| Vitamin E | 15 mg | 15 mg | Antioxidant |
| Vitamin K | 120 mcg | 90 mcg | Coagulation |
| Thiamine (B1) | 1.2 mg | 1.1 mg | Energy metabolism |
| Riboflavin (B2) | 1.3 mg | 1.1 mg | Energy metabolism |
| Niacin (B3) | 16 mg NE | 14 mg NE | Energy metabolism |
| B6 | 1.3 mg | 1.3 mg | Protein metabolism |
| Folate (B9) | 400 mcg DFE | 400 mcg DFE | DNA synthesis (600 mcg in pregnancy) |
| B12 | 2.4 mcg | 2.4 mcg | Nerve function, RBC |
Key Minerals (adult RDA/AI)
| Mineral | Men | Women | Key Function |
|---|---|---|---|
| Calcium | 1000 mg | 1000 mg | Bone, muscle |
| Iron | 8 mg | 18 mg (pre-menop.) | Oxygen transport |
| Zinc | 11 mg | 8 mg | Immunity, wound healing |
| Magnesium | 400-420 mg | 310-320 mg | Enzyme cofactor |
| Potassium | 3400 mg | 2600 mg | Fluid balance, cardiac |
| Sodium | < 2300 mg | < 2300 mg | Fluid balance |
| Iodine | 150 mcg | 150 mcg | Thyroid |
| Selenium | 55 mcg | 55 mcg | Antioxidant, thyroid |
5. NUTRITIONAL ASSESSMENT TOOLS
Subjective Global Assessment (SGA)
Components: weight change history, dietary intake change, GI symptoms, functional capacity, disease-related metabolic demands, physical exam (muscle wasting, fat loss, edema).
- Validated with trained clinicians. Result: A (well-nourished), B (mild-moderate malnutrition), C (severe malnutrition).
Biochemical Markers
| Marker | Normal | Half-life | Use |
|---|---|---|---|
| Albumin | 3.5 - 5.0 g/dL | 20 days | Long-term status |
| Prealbumin (Transthyretin) | 18 - 45 mg/dL | 2 days | Short-term change |
| Transferrin | 200 - 400 mg/dL | 8 days | Medium-term |
| Total Lymphocyte Count | > 1500/mm³ | - | Immune/nutritional |
| C-reactive protein | - | - | Inflammation (confounds albumin) |
5 A's of Obesity Management (clinical framework)
- Ask - for permission; explore readiness
- Assess - BMI, waist circumference, stage, drivers
- Advise - health risks, benefits of modest loss, treatment options
- Agree - realistic targets, behavioral changes, treatment plan
- Arrange/Assist - barriers, resources, follow-up
- Textbook of Family Medicine 9e (Alexander et al., 2011)
6. ENERGY-YIELDING CALCULATION EXAMPLES
Example 1 - Weight Loss Plan
- Patient: Female, 75 kg, 160 cm, 35 years old, sedentary
- Mifflin-St. Jeor RMR: (9.99×75) + (6.25×160) - (4.92×35) - 161 = 749.25 + 1000 - 172.2 - 161 = 1416 kcal/day
- TDEE: 1416 × 1.2 = 1699 kcal/day
- Weight loss target: Deficit of 500 kcal/day → ~1200 kcal/day
- Protein target: 75 × 1.2 = 90 g/day minimum
Example 2 - Post-Surgical Patient
- Patient: Male, 80 kg, moderate stress (e.g., abdominal surgery)
- Energy: 25 kcal × 80 kg = 2000 kcal/day
- Protein: 1.5 g × 80 kg = 120 g/day
- Carbohydrate: 70% of 2000 kcal = 1400 kcal ÷ 4 = 350 g/day
- Fat: 30% of 2000 kcal = 600 kcal ÷ 9 = 67 g/day
7. ENTERAL NUTRITION QUICK GUIDE
| Parameter | Standard Target |
|---|---|
| Standard enteral formula | 1-2 kcal/mL |
| Protein density | up to 0.6 g/mL |
| Starting rate | 10-20 mL/hour |
| Target rate | up to 75 mL/hour (if tolerated) |
| Oral supplement (can) | ~200-250 kcal + 9 g protein per 200 mL |
Enteral route is always preferred over parenteral - preserves gut mucosal barrier, reduces infection rates, better wound healing. - Bailey & Love's Short Practice of Surgery 28th Ed.
Refeeding Syndrome risk: Start at no more than 50% of target energy; advance to full over 24-48 hours. Monitor phosphate, magnesium, potassium daily.
8. MONITORING SCHEDULE (Clinical)
| Frequency | Parameters |
|---|---|
| Daily | Weight, fluid balance, glucose, Na/K/urea/creatinine, Mg+phosphate (if refeeding risk), LFTs, CRP |
| Weekly-fortnightly | CBC, calcium, zinc, copper, albumin, thiamine, triglycerides, B12, folate |
| Every 3-6 months | Ferritin, selenium, manganese, 25-OH vitamin D |
- Bailey & Love's Short Practice of Surgery 28th Ed., Table 25.4
Sources:
- Textbook of Family Medicine, 9th Ed. (Park's chapters on obesity, caloric requirements, macronutrients)
- Park's Textbook of Preventive and Social Medicine (anthropometric assessment)
- Bailey & Love's Short Practice of Surgery, 28th Ed. (nutritional requirements, enteral nutrition)
- Dietary Guidelines for Americans 2020-2025 (AMDR, micronutrient DRIs)
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