Q.An 18-year-old boy weighs 65 kg and is 175 cm tall. Using the revised Harris–Benedict equation for males, BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age in years), compute his BMR in kcal/day, showing each step.

Basal Metabolic Rate Factors

Think of your body as a house that is always running — lights on, refrigerator humming, AC working, even when everyone is asleep. That constant energy your body burns just to stay alive, without any activity at all, is your Basal Metabolic Rate (BMR). It is the energy needed for breathing, blood circulation, cell production, and maintaining body temperature.

Now, what decides whether your "house" burns energy fast or slow? That is where the factors come in.

The Core Idea

BMR is not the same for everyone. Two people of the same age and weight can have very different BMRs. The factors that influence it are largely biological and cannot be changed by willpower. Understanding them helps explain why some people seem to eat a lot without gaining weight, while others gain weight easily.

Key Factors That Influence BMR

Age — BMR is highest during growth periods like childhood and adolescence. As you age, especially after 20, your BMR gradually declines. This is why older adults often need fewer calories than younger people. The body's metabolic engine naturally slows down.

Gender — Men generally have a higher BMR than women. This is because men typically have more muscle mass and less body fat than women, and muscle tissue burns more energy than fat tissue even at rest. It is not about superiority — it is about body composition.

Body Composition — This is the most important factor. Muscle is metabolically active; it burns calories even when you are doing nothing. Fat tissue is much less active. So a person with more muscle will have a higher BMR than someone with the same weight but more fat. This is why strength training can help increase BMR over time.

Body Size and Surface Area — Larger bodies have higher BMRs because they have more cells to maintain. A taller, heavier person burns more energy at rest than a smaller person. Surface area matters too — more skin means more heat loss, so the body works harder to maintain temperature.

Genetics — Some people are simply born with a faster or slower metabolism. This is inherited and can explain why members of the same family often have similar body types and weight patterns.

Hormones — The thyroid gland is the master controller of metabolism. An overactive thyroid (hyperthyroidism) raises BMR, while an underactive thyroid (hypothyroidism) lowers it. Other hormones like insulin and cortisol also play roles. …