The relationship between basal metabolism, basal metabolic rate and calories

**Basal Metabolic Rate, Basal Metabolic Rate, and Calories**

Above, we briefly introduced basal metabolism. Now, let's talk about basal metabolism and its relationship with calorie conversion.

Basal energy expenditure determines the body's most basic energy needs and plays a vital role in life, such as cellular function and protein synthesis. It accounts for approximately 70% of daily energy requirements, or about three-quarters of daily energy expenditure. Compared to energy expenditure from physical activity and the thermic effect of food, basal energy expenditure is more stable, changing little from day to day, making it an important indicator for differentiating energy needs between individuals. Basal energy expenditure is calculated using the basal metabolic rate, which is the body's energy expenditure per minute.

Here, we need to further clarify the concepts of basal metabolism and basal metabolic rate. Basal metabolism refers to the body's energy metabolism in a basal state. The basal metabolism per unit of time is called the basal metabolic rate. The so-called basal state refers to a state where the room temperature is 20-25°C, in the early morning, on an empty stomach, awake, and extremely quiet. In this state, excluding the influence of factors such as muscle activity, ambient temperature, the specific dynamic effect of food, and mental stress, the body's energy consumption is mainly used to maintain basic life activities such as heartbeat, respiration, and body temperature. The value of basal metabolic rate varies physiologically with each individual's gender, age, height, weight, health status, body composition, hormones, etc. Generally speaking, it is higher in men than in women, and higher in childhood than in adulthood. The older the age, the lower the relative basal metabolic rate. The basal metabolic rate is lowest at birth, then surges rapidly, reaching its highest value at age 5, then gradually weakens, remaining roughly constant between 20 and 30 years of age, and then slowly decreasing in old age. With increasing age, muscle loss accelerates, and the basal metabolic rate will also decrease significantly. Adult women have 5% to 7% lower body fat percentages than men because women have a higher body fat percentage than men.

Basal metabolic rate (BMR) is the amount of energy required at rest for respiration, heartbeat, oxygen transport, glandular secretion, kidney filtration and excretion, muscle tension, and cellular function. Simply put, if your BMR is 1200 calories, and you sleep all day without any other activity, you will still burn 1200 calories a day. BMR reflects the metabolic capacity of the body's cells; different cells have different physiological functions, and therefore different metabolic capacities. Generally, adipose tissue and bone tissue metabolize almost no calories. Therefore, BMR is directly proportional to lean muscle tissue, which is the amount of muscle in our body. In other words, the lower the body fat percentage and the more muscle tissue, the higher the BMR.

All energy produced by the body is ultimately converted into heat. It's important to note that the basal metabolic rate (BMR) per unit body surface area varies with age, sex, season, nutritional status, medication use, the subject's physical condition, testing time, and pathological state. Therefore, to compare the energy metabolism levels of different individuals, it can be expressed as the heat dissipated per square meter of body surface area per hour (kJ/h·m²), i.e., the basal metabolic rate (BMR). Clinically, a basal metabolic rate analyzer is commonly used to measure oxygen consumption per unit time (O₂L/h).

Besides these, there are many other methods for measuring basal metabolic rate, such as: (1) Basal metabolic rate % = (pulse rate + pulse pressure) - 111 (Gale); (2) Basal metabolic rate % = 0.75 × (pulse rate + pulse pressure difference × 0.74) - 72 (Read); (3) Basal metabolic rate % = 1.28 × (pulse rate + pulse pressure difference) - 116 (Kosa), etc. Of course, there are other methods as well. Here we introduce a popular and convenient method for calculating basal metabolic rate (Harris-Benedict formula). The formula for calculating basal metabolic rate is:

For men: 66 + (13.7 × weight) + (5.0 × height) - (6.8 × age). For example: Mr. Wang weighs 80 kg, is 180 cm tall, and is 24 years old. His daily basal metabolic rate (BMR) is: 66 + (13.7 × 80) + (5.0 × 180) - (6.8 × 24) = 66 + 1096 + 900 - 163.2 = 1898.8 calories.

For women: 655 + (9.6 × weight) + (1.7 × height) - (4.7 × age); For example: Ms. Li weighs 55 kg, is 165 cm tall, and is 21 years old this year. Her daily basal metabolic rate (BMR) is: 655 + (9.6 × 55) + (1.7 × 165) - (4.7 × 21) = 655 + 528 + 280.5 - 98.7 = 1364.8 calories.

The calculation methods for basal metabolic rate differ between men and women, mainly due to differences in certain body tissues. Furthermore, each person's basal metabolic rate varies depending on their height, weight, and age.

The formula for calculating basal metabolic rate is rather complicated, and it's inconvenient to measure it with a machine at a hospital. Body fat analyzers, which are commonly used by fitness enthusiasts, can measure basal metabolic rate at the same time as body fat percentage, and can be used as reference data. They are also very convenient to use.

The actual value of basal metabolic rate (BMR) differs from the normal average by about 10%, which is within the normal range. If it exceeds the normal value by 20%, further examination is necessary to check for any underlying medical conditions. Generally, in hyperthyroidism, the BMR is more than 25% higher than the normal standard, reaching as high as 80%; in hypothyroidism, the BMR is 20% to 40% lower than the normal standard. Therefore, BMR is commonly used in medicine for thyroid function tests. Other conditions such as adrenal cortex and pituitary hypofunction also lower the BMR. Of the BMR, the mechanical work of the heart, respiratory muscles, digestive tract, and vascular smooth muscle, as well as the functional consumption caused by the secretory activities of the liver and kidneys, and the basal consumption caused by individual cells, account for about 3/4. Under the same environment, the BMR is directly proportional to the body surface area.