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What Is Basal Metabolic Rate (BMR)?

Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest—just to keep you alive. These calories support essential functions like breathing, circulation, body temperature regulation, and cell repair. Knowing your BMR helps you better understand your metabolism, calorie needs, and overall health.

Why BMR Matters

BMR is the largest part of your daily calorie burn, making up about 60–75% of your total daily energy expenditure (TDEE). It reflects the minimum amount of energy your body needs to function even if you did nothing all day.

Because BMR is measured under strict laboratory conditions (after a 12-hour fast, in a thermoneutral room, lying still without physical or mental stress), all practical BMR values—including those in the SiPhox app—are estimates based on validated equations.

How We Calculate Your BMR

SiPhox uses the Harris–Benedict equation, a widely accepted method for estimating BMR based on your age, weight, and height.

Harris–Benedict Equation

Men:
BMR = 66 + (6.23 × weight in lbs) + (12.7 × height in inches) − (6.8 × age)

Women:
BMR = 655 + (4.35 × weight in lbs) + (4.7 × height in inches) − (4.7 × age)

This formula provides a reasonable approximation, but actual BMR can differ by up to ~20% depending on muscle mass, body composition, genetics, and hormonal factors.

(Note: Many modern apps use the Mifflin–St Jeor equation. We use Harris–Benedict because it aligns cleanly with the units most users enter—pounds, inches, and years.)

BMR vs. Other Metabolic Terms

  • Resting Metabolic Rate (RMR): Similar to BMR but measured under less strict conditions; usually 10–20% higher.

  • Total Daily Energy Expenditure (TDEE): Your total calories burned per day = BMR + physical activity + calories used for digestion.

  • Thermic Effect of Food (TEF): The energy cost of digestion; ~10% of daily calorie burn.

What Affects Your BMR

  • Age: Gradually declines as muscle mass decreases.

  • Body composition: More muscle = higher BMR; more body fat = lower BMR.

  • Gender: Men generally have higher BMR due to greater lean mass.

  • Genetics: Plays a large role—up to ~80% of metabolic variability.

  • Environment: Cold exposure can increase BMR to keep you warm.

  • Lifestyle: Poor sleep, stress, and crash dieting can significantly reduce BMR.

Hormonal Influences on BMR

Thyroid hormones are the most powerful regulators of metabolic rate:

  • Hypothyroidism can reduce BMR by 20–40%.

  • Hyperthyroidism can increase BMR by a similar amount.

Other hormones—cortisol, testosterone, insulin, and growth hormone—also influence how quickly your body burns energy. Monitoring biomarkers such as TSH, Free T3, and Free T4 helps identify thyroid-related changes in metabolism.

How to Support a Healthy BMR

  • Build or maintain muscle: Strength train 2–4×/week; muscle burns more calories at rest.

  • Eat enough protein: ~0.8–1.2 g/lb of body weight; high thermic effect helps raise calorie burn.

  • Stay hydrated: Even mild dehydration can lower BMR by several percent.

  • Prioritize sleep: 7–9 hours helps stabilize hormones that regulate metabolism.

  • Manage stress: Lower cortisol = better metabolic stability.

  • Avoid extreme diets: Harsh calorie cuts can drop BMR by 20–30%.

  • Consider thermogenic foods: Green tea, caffeine, and capsaicin provide small, temporary boosts.

Where Can I Learn More?

DISCLAIMER: IF YOU ARE CONCERNED ABOUT YOUR METABOLIC RATE OR RELATED HEALTH FACTORS, PLEASE CONSULT WITH YOUR PHYSICIAN.