Basal Metabolic Rate (BMR) in Nutrition

Basal Metabolic Rate (BMR)

• BMR is the minimum number of calories required for basic bodily functions at rest, such as breathing and circulation.
• At rest, the body should be at a comfortable temperature and not digesting food.
• Approximately 70% of daily energy is used to maintain organ function, 10% for digestion, and 20% for physical activity.
• Understanding BMR aids in weight management; consuming more calories than BMR leads to weight gain, while consuming less results in weight loss.
• Activity levels significantly influence BMR, with more active individuals having a higher BMR than inactive individuals.

Resting Metabolic Rate (RMR)

• RMR is sometimes used interchangeably with BMR but refers to the number of calories burned at rest including more than life-sustaining functions.
• Activities included in RMR measurements are eating, sweating, shivering, and using the restroom.
• The conditions for measuring BMR are stricter than for RMR, requiring the sympathetic nervous system to remain inactive.

Biological Definition of BMR

• BMR in biology defines the continuous supply rate of some chemical or process, focusing on the allometric relationship between body mass and metabolic rate.
• Allometry, describing the relationship of body size to shape, was first detailed by Otto Snell in 1892.

Calculating Basal Metabolic Rate

• The Mifflin-St. Jeor equation, introduced in 1990, is commonly used to calculate human BMR.
• The formula includes weight (kg), height (cm), age (years), and a constant based on gender (+5 for males, -161 for females) due to muscle mass differences.
• Example BMR calculation for a 30-year-old woman (5'3", 130 lbs) results in 1,263.4 calories for maintenance, while her RMR would be approximately 1,375.6 calories.

BMR Calculation in Animals

• BMR in mammals can be assessed using respirometry to measure oxygen consumption.
• Dr. Max Kleiber’s discovery outlines that BMR closely correlates with mass raised to the 3/4 power, illustrated by the mouse-elephant curve.
• A 2-gram mouse has a BMR of 0.009, whereas a 2-kilogram elephant has a BMR of 1.68, highlighting metabolic differences across species.

Variance in BMR

• Normal BMR range for humans is between -15% to +5%, indicating individual differences even among similar demographics.
• Factors affecting BMR include age, genetics, weather, diet, pregnancy, and supplementation.
• BMR is influenced by temperature; endotherms (mammals) require energy to maintain a constant body temperature, while ectotherms (reptiles, fish, insects) have fluctuating temperatures based on the environment.