Food and Nutrition Science — Energy Metabolism and Weight Management

Energy Units and Food Energy

Energy units:

kcal (kilocalorie): the heat needed to raise 1kg of water by 1°C
kJ (kilojoule): 1 kcal = 4.184 kJ
Food labeling: kcal is used in Korea and the US, kJ in Europe and Australia

Energy content of macronutrients:

Carbohydrates: 4 kcal/g
Protein: 4 kcal/g
Fat: 9 kcal/g
Alcohol: 7 kcal/g (not a nutrient)
Water: 0 kcal/g

Measuring food energy:

Bomb calorimetry: measures the heat of complete combustion
Atwater factors: adjust for digestibility carbohydrate 97%, protein 92%, fat 95%

Energy density:

Energy per unit weight (kcal/g)
High energy density: fat-rich foods (butter, 7–9 kcal/g)
Low energy density: water-rich vegetables and fruit (0.1–0.5 kcal/g)
Low-energy-density foods → aid satiety and weight management

Energy per serving, by food group:

Grains: about 300 kcal
Meat, fish, eggs, legumes: about 100–150 kcal
Vegetables: about 15–30 kcal
Fruit: about 50 kcal
Oils and sugars: about 45 kcal
Dairy: about 125 kcal

Components of Energy Expenditure

Total Energy Expenditure (TEE):

TEE = BMR + AEE + TEF

Basal Metabolic Rate (BMR):

The minimum energy needed to sustain life (at rest, fasting, in a thermally neutral setting)
60–75% of total energy expenditure
Influencing factors:
- body composition: more muscle mass raises BMR
- sex: men > women (muscle-mass difference)
- age: BMR falls as muscle mass declines with age
- thyroid hormone: regulates metabolic rate
- body surface area: a larger area raises BMR

Resting Metabolic Rate (RMR):

Slightly higher than BMR (includes sleep, sitting, and similar states)

Activity Energy Expenditure (AEE):

Energy spent on physical activity
15–30% of the total (varies widely with exercise habits)
Physical Activity Level (PAL): TEE/BMR sedentary: PAL 1.2–1.4 / active: PAL 1.6–1.9

Thermic Effect of Food (TEF):

Energy used in digestion, absorption, and metabolism
5–10% of the total
Protein has the highest TEF (20–30%) fat has the lowest TEF (0–3%)

BMR prediction formulas:

Harris-Benedict:

Men: 88.4 + 13.4×weight(kg) + 4.8×height(cm) − 5.68×age
Women: 447.6 + 9.25×weight(kg) + 3.10×height(cm) − 4.33×age

A quick estimate:

Adult men: about 1.0 kcal/kg/hour × body weight
Adult women: about 0.9 kcal/kg/hour × body weight

Principles of Weight Management

The energy-balance equation:

Change in weight = energy intake − energy expenditure
Energy surplus → weight gain (fat storage)
Energy deficit → weight loss (fat breakdown)
1kg of body fat ≈ 7,700 kcal

Measuring body fat:

BMI (Body Mass Index): weight(kg) / height(m)² underweight: <18.5 / normal: 18.5–24.9 / overweight: 25–29.9 / obese: ≥30 (South Korea applies lower cutoffs: overweight at 23–24.9, obese at 25 and above)
Waist circumference: an indicator of abdominal fat ≥90cm for men, ≥80cm for women → abdominal obesity (South Korean criteria)
Body-fat percentage: bioelectrical impedance analysis (BIA), DEXA, or underwater weighing

Causes of obesity:

Excess energy intake + insufficient physical activity
Genetic predisposition (the FTO gene, leptin signaling)
Gut microbiota: linked to obesity
Sleep deprivation: disrupts appetite-regulating hormones
Stress: raises cortisol → promotes fat storage

Principles of weight loss:

A 500 kcal/day deficit → targets about 0.5 kg of loss per week
Avoid rapid weight loss: it causes muscle loss and lowers basal metabolic rate
Combine aerobic and resistance exercise: reduces body fat while preserving muscle
Adequate protein intake: protects muscle (1.2–1.6 g/kg/day)
Sustainability: long-term lifestyle change beats extreme dietary restriction

Weight regain (the yo-yo effect):

Regaining weight after rapid loss
Cause: adaptive thermogenesis (a drop in BMR), abandoning dietary restriction
Prevention: gradual weight loss, combined with exercise, and a maintenance-phase strategy

Nutritional-Status Assessment

Nutritional assessment methods (ABCDE):

A — Anthropometric:

Height, weight, BMI, waist circumference
Mid-upper-arm circumference, triceps skinfold thickness
Growth charts (for children)

B — Biochemical testing:

Serum protein: albumin, prealbumin (nutritional status)
Hemoglobin, hematocrit: iron-deficiency anemia
Serum vitamin D, B12, folate
Lipid panel: total cholesterol, LDL, HDL, triglycerides
Blood glucose, HbA1c: assessing diabetes

C — Clinical signs:

Observing deficiency symptoms (angular cheilitis, hair loss, edema, and the like)
Medical history and records

D — Dietary intake assessment:

24-hour recall: recalling the previous day’s intake
Food frequency questionnaire (FFQ): semi-quantitative
Food records: recording intake directly over several days (the most accurate)
National nutrition surveys: South Korea’s Korea National Health and Nutrition Examination Survey (KNHANES)

E — Environmental and other factors:

Socioeconomic conditions
Education, culture, and food access

South Korea’s Dietary Reference Intakes (KDRIs):

Estimated Average Requirement (EAR): meets 50% of a population’s needs
Recommended Nutrient Intake (RNI): meets 97.5% (EAR × 1.2)
Adequate Intake (AI): used when evidence is insufficient
Tolerable Upper Intake Level (UL): the maximum intake without health risk
Acceptable Macronutrient Distribution Range (AMDR): carbohydrates: 55–65% / protein: 7–20% / fat: 15–30%

Meal Planning

Principles of a balanced diet:

Variety: eat across multiple food groups
Adequacy: meet essential nutrient requirements
Balance: match recommended intake levels
Appropriate calories: maintain energy balance
Moderation: limit saturated fat, sodium, and sugar

South Korea’s food-group wheel:

Grains: 6–12 servings (50–65% of energy)
Meat, fish, eggs, legumes: 3–4 servings (protein)
Vegetables: 7–8 servings (vitamins, minerals, fiber)
Fruit: 2–3 servings (vitamins, antioxidants)
Milk and dairy: 2 servings (calcium)
Oils and sugars: 4–5 servings (small amounts)

Reducing sodium (the WHO target is 2,000 mg/day):

South Korea’s average: about 3,200 mg/day (excessive)
Watch out for broths, salted seafood, and processed food
Favor fresh food and choose low-sodium products

Dietary fiber:

Recommended intake: 25–30 g/day
Soluble: oats, barley, beans, fruit → eases blood sugar and cholesterol
Insoluble: whole grains, vegetables, wheat bran → promotes bowel movement

The Mediterranean diet (an evidence-backed healthy diet):

Centers on vegetables, fruit, whole grains, olive oil, and legumes
Fish: at least twice a week
Minimizes red and processed meat
Evidence for preventing cardiovascular disease, diabetes, and cognitive decline

Frequently Asked Questions

Q. How can I raise my basal metabolic rate? A. The most effective way to raise BMR is to build muscle mass. Muscle burns more energy at rest than fat (1kg of muscle burns about 13 kcal/day). Resistance training maintains and increases muscle mass, and adequate protein intake supports its synthesis. It’s also important to avoid drastic dietary restriction — a sudden, large cut in calorie intake pushes the body into energy-conservation mode, lowering BMR.

Q. Can a normal BMI still hide a health problem? A. Yes. BMI can’t distinguish body composition — the ratio of muscle to fat. An athlete with high muscle mass can have a high BMI and still be healthy, while someone with a normal BMI but high body fat and low muscle — sometimes called “normal-weight obesity” — can carry real metabolic risk. Asian populations, including Koreans, tend to have higher body fat and more abdominal fat than Western populations at the same BMI, which makes evaluating waist circumference or body-fat percentage alongside BMI important.