BMR (basal metabolic rate)

BMR (Basal Metabolic Rate) is the minimum energy required to maintain basic life functions at rest.

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What it is

Basal metabolism (BMR) is the energy that the human body expends to maintain its basic life functions in a state of complete rest. These functions include breathing, heart activity, regulation of body temperature, brain activity, cellular repair, and protein synthesis. BMR represents the largest component of total daily energy expenditure (TDEE), often reaching 60-75% of it in people with a moderate lifestyle.

BMR is measured under strictly controlled conditions: after 12-14 hours of fasting, 8 hours of sleep, and in a thermoneutral environment. The goal is to eliminate all factors that could affect energy expenditure, such as digestion, physical activity, or stress. The unit of measurement for BMR is calories (kcal) per day.

Although often used interchangeably with the term "Resting Metabolic Rate" (RMR), BMR is slightly lower because RMR does not require such strict conditions (e.g., it is measured only 3-4 hours after the last meal). In practice, however, the differences are minimal, and most calculation formulas provide an estimate of RMR, which is accepted as adequate for BMR.

How it works

The mechanism by which BMR functions is related to the continuous biochemical processes occurring in every cell of the body. Even in a state of absolute rest, the body burns energy to maintain homeostasis. This energy is obtained through the breakdown of macronutrients – carbohydrates, fats, and proteins, which are metabolized into adenosine triphosphate (ATP) – the main energy currency of cells.

The factors influencing BMR are numerous and complex. Here are some of the main ones:

1. Age: BMR decreases with age, mainly due to loss of muscle mass and changes in hormonal balance.
2. Sex: Men generally have a higher BMR than women due to greater muscle mass and lower body fat percentage.
3. Weight and height: Larger bodies require more energy to maintain, thus having a higher BMR.
4. Body composition: Muscle tissue is metabolically more active than fat tissue, so individuals with more muscle mass have a higher BMR.
5. Genetics: Genetic factors can largely determine individual differences in BMR.
6. Hormones: Hormones such as thyroid hormones (thyroxine) and insulin play a key role in regulating metabolism.

Why it's important for athletes

For athletes and active individuals, understanding and correctly calculating BMR is essential for optimizing their nutritional regimen and achieving specific goals – whether it's reducing body fat, increasing muscle mass, or maintaining weight. BMR forms the base upon which energy needs from physical activity and the thermic effect of food are added to determine total daily energy expenditure (TDEE).

For example, if an athlete weighs 80 kg and aims to build muscle mass, their BMR will be a significant part of their total calorie intake. To support muscle growth, the ISSN (International Society of Sports Nutrition) recommendations for protein intake are in the range of 1.6-2.2 g/kg of body weight, but the overall calorie surplus, which is based on BMR + activity, is critical for anabolic processes. If BMR is underestimated, the athlete may not consume enough calories for recovery and growth, despite adequate protein intake, leading to stagnation or even muscle loss. Conversely, when aiming for weight loss, understanding BMR allows for the creation of a sustainable calorie deficit without compromising health or damaging metabolism.

Related concepts

• Energy balance — Represents the ratio between energy intake and expenditure, with BMR being a major component of energy expenditure.
• TDEE (Total Daily Energy Expenditure) — TDEE includes BMR, plus energy expenditure from physical activity and the thermic effect of food, giving the body's total energy requirement.
• Calorie deficit — A state where calorie intake is less than TDEE, used for weight reduction, with BMR being the basis for calculating TDEE.
• Thermic Effect of Food (TEF) — This is the energy expended for digestion, absorption, and metabolism of food, which, along with BMR and physical activity, forms TDEE.