Q3 LONG ANSWER 10M ★ High-yield

Define a balanced diet. Describe BMR, specific dynamic action and respiratory quotient with their significance.

Biochemistry · 1st Professional — Pre-Clinical

Balanced Diet

A balanced diet contains all nutrients in the correct proportions and adequate amounts to meet the requirements of the body, with a small reserve for periods of stress.

Nutrient	Share of total calories	Energy yield
Carbohydrate	55–70%	4 kcal/g
Fat	20–30%	9 kcal/g
Protein	10–15%	4 kcal/g
Alcohol	—	7 kcal/g — "empty calories", no nutrients

Must also supply vitamins, minerals, essential amino acids, essential fatty acids, fibre and water
The Indian diet is typically cereal-based, so it needs pulses for protein complementation, and is often low in iron, calcium and vitamin A

Basal Metabolic Rate

BMR = the energy expended by the body at complete physical and mental rest, in a thermoneutral environment, 12–18 hours after the last meal, while awake.

About 24 kcal/kg body weight/day, or 1 kcal/kg/hour
Adult male 1600–1800 kcal/day; female 1300–1500
It accounts for 60–70% of total daily energy expenditure
Measured by indirect calorimetry (oxygen consumption) or the Benedict–Roth apparatus

BMR increased by	BMR decreased by
Hyperthyroidism (up to +100%)	Hypothyroidism (−30 to −40%)
Fever — about +13% per °C	Starvation and undernutrition (adaptive fall)
Male sex (greater muscle mass)	Female sex; ageing
Larger body surface area	Sleep
Cold exposure; pregnancy and lactation	Hot climate
Adrenaline, growth hormone; infection, burns, trauma	—

BMR correlates best with body surface area, and with lean body mass — adipose tissue is metabolically inactive

Specific Dynamic Action

Specific dynamic action (thermic effect of food) = the increase in metabolic rate that follows ingestion of food, above the basal level.

Nutrient	SDA
Protein	20–30% — the highest
Carbohydrate	5–6%
Fat	2–3% — the lowest
Mixed diet	About 10%

Due to the energy cost of digestion, absorption, transport and metabolic processing — deamination and urea synthesis make protein the most expensive
Extra calories must be allowed for SDA when calculating requirements; the high SDA of protein is one reason high-protein diets aid weight control

Respiratory Quotient

RQ = volume of CO2 produced ÷ volume of O2 consumed

Substrate	RQ	Reason
Carbohydrate	1.0	Fully oxygenated — CO2 equals O2
Protein	0.8	—
Fat	0.7	Highly reduced — needs more oxygen
Mixed diet	0.85	Usual value
Lipogenesis	> 1.0	Carbohydrate converted to fat releases CO2
Ketosis, starvation, diabetes	< 0.7	Fat is the sole fuel

RQ indicates which fuel is being oxidised — a practical use in metabolic studies and in ventilated patients
Overfeeding carbohydrate raises CO2 production and can make weaning from a ventilator difficult

Energy Requirement

Activity level	Male (kcal/day)	Female (kcal/day)
Sedentary	2320	1900
Moderate	2730	2230
Heavy	3490	2850
Pregnancy	—	+350
Lactation (0–6 months)	—	+600

Total expenditure = BMR + physical activity + SDA + growth
Body mass index = weight (kg) ÷ height (m)2; normal 18.5–24.9. Asian cut-offs are lower — overweight above 23, obese above 25

Applied Aspects

BMR was historically used to assess thyroid function; it has been replaced by serum TSH and free T4, which are far more specific
Fever raises requirements by about 13% per degree — important in feeding the septic or burned patient
Obesity results from sustained positive energy balance; a surplus of only 100 kcal a day adds about 5 kg in a year
India carries a double burden — undernutrition and stunting alongside rising obesity and type 2 diabetes, often within the same community
The "thin fat Indian" phenotype — a normal BMI with high body fat and central obesity, which is why lower Asian cut-offs are used

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