Human nutrition - BMR, REE, Energy Balance (2024)

Energy is needed not only when a person is physically active but even when the body is lying motionless. Depending on an individual’s level of physical activity, between 50 and 80 percent of the energy expended each day is devoted to basic metabolic processes (basal metabolism), which enable the body to stay warm, breathe, pump blood, and conduct numerous physiological and biosynthetic activities, including synthesis of new tissue in growing children and in pregnant and lactating women. Digestion and subsequent processing of food by the body also uses energy and produces heat. This phenomenon, known as the thermic effect of food (or diet-induced thermogenesis), accounts for about 10 percent of daily energy expenditure, varying somewhat with the composition of the diet and prior dietary practices. Adaptive thermogenesis, another small but important component of energy expenditure, reflects alterations in metabolism due to changes in ambient temperature, hormone production, emotional stress, or other factors. Finally, the most variable component in energy expenditure is physical activity, which includes exercise and other voluntary activities as well as involuntary activities such as fidgeting, shivering, and maintaining posture. Physical activity accounts for 20 to 40 percent of the total energy expenditure, even less in a very sedentary person and more in someone who is extremely active.

Basal or resting energy expenditure is correlated primarily with lean body mass (fat-free mass and essential fat, excluding storage fat), which is the metabolically active tissue in the body. At rest, organs such as the liver, brain, heart, and kidney have the highest metabolic activity and, therefore, the highest need for energy, while muscle and bone require less energy, and body fat even less. Besides body composition, other factors affecting basal metabolism include age, sex, body temperature, and thyroid hormone levels.

The basal metabolic rate (BMR), a precisely defined measure of the energy expenditure necessary to support life, is determined under controlled and standardized conditions—shortly after awakening in the morning, at least 12 hours after the last meal, and with a comfortable room temperature. Because of practical considerations, the BMR is rarely measured; the resting energy expenditure (REE) is determined under less stringent conditions, with the individual resting comfortably about 2 to 4 hours after a meal. In practice, the BMR and REE differ by no more than 10 percent—the REE is usually slightly higher—and the terms are used interchangeably.

Energy expenditure can be assessed by direct calorimetry, or measurement of heat dissipated from the body, which employs apparatuses such as water-cooled garments or insulated chambers large enough to accommodate a person. However, energy expenditure is usually measured by the less cumbersome techniques of indirect calorimetry, in which heat produced by the body is calculated from measurements of oxygen inhaled, carbon dioxide exhaled, and urinary nitrogen excreted. The BMR (in kilocalories per day) can be roughly estimated using the following formula: BMR = 70 × (body weight in kilograms)3/4.

The energy costs of various activities have been measured (see table). While resting may require as little as 1 kilocalorie per minute, strenuous work may demand 10 times that much. Mental activity, though it may seem taxing, has no appreciable effect on energy requirement. A 70-kg (154-pound) man, whose REE over the course of a day might be 1,750 kilocalories, could expend a total of 2,400 kilocalories on a very sedentary day and up to 4,000 kilocalories on a very active day. A 55-kg (121-pound) woman, whose daily resting energy expenditure might be 1,350 kilocalories, could use from 1,850 to more than 3,000 total kilocalories, depending on level of activity.

Approximate energy expenditure for activity levels
activity category energy as multiple of resting energy expenditure (REE) kilocalories per minute
Source: National Academy of Sciences, Recommended Dietary Allowances, 10th ed. (1989).
resting (sleeping, reclining) REE × 1.0 1–1.2
very light (driving, typing, cooking) REE × 1.5 up to 2.5
light (walking on a level surface at 4 to 5 km/hr [2.5 to 3 mph], golf, table tennis) REE × 2.5 2.5–4.9
moderate (walking 5.5 to 6.5 km/hr [3.5 to 4 mph], carrying a load, cycling, tennis, skiing, dancing) REE × 5.0 5.0–7.4
heavy (walking uphill with a load, basketball, climbing, football, soccer) REE × 7.0 7.5–12.0

The law of conservation of energy applies: If one takes in more energy than is expended, over time one will gain weight; insufficient energy intake results in weight loss, as the body taps its energy stores to provide for immediate needs. Excess food energy is stored in small amounts as glycogen, a short-term storage form of carbohydrate in muscle and liver, and as fat, the body’s main energy reserve found in adipose tissue. Adipose tissue is mostly fat (about 87 percent), but it also contains some protein and water. In order to lose 454 grams (one pound) of adipose tissue, an energy deficit of about 3,500 kilocalories (14.6 megajoules) is required.

Body mass, body fat, and body water

The human body consists of materials similar to those found in foods; however, the relative proportions differ, according to genetic dictates as well as to the unique life experience of the individual. The body of a healthy lean man is composed of roughly 62 percent water, 16 percent fat, 16 percent protein, 6 percent minerals, and less than 1 percent carbohydrate, along with very small amounts of vitamins and other miscellaneous substances. Females usually carry more fat (about 22 percent in a healthy lean woman) and slightly less of the other components than do males of comparable weight.

The body’s different compartments—lean body mass, body fat, and body water—are constantly adjusting to changes in the internal and external environment so that a state of dynamic equilibrium (homeostasis) is maintained. Tissues in the body are continuously being broken down (catabolism) and built up (anabolism) at varying rates. For example, the epithelial cells lining the digestive tract are replaced at a dizzying speed of every three or four days, while the life span of red blood cells is 120 days, and connective tissue is renewed over the course of several years.

Although estimates of the percentage of body fat can be made by direct inspection, this approach is imprecise. Body fat can be measured indirectly using fairly precise but costly methods, such as underwater weighing, total body potassium counting, and dual-energy X-ray absorptiometry (DXA). However, more practical, albeit less accurate, methods are often used, such as anthropometry, in which subcutaneous fat at various sites is measured using skinfold calipers; bioelectrical impedance, in which resistance to a low-intensity electrical current is used to estimate body fat; and near infrared interactance, in which an infrared light aimed at the biceps is used to assess fat and protein interaction. Direct measurement of the body’s various compartments can only be performed on cadavers.

The composition of the body tends to change in somewhat predictable ways over the course of a lifetime—during the growing years, in pregnancy and lactation, and as one ages—with corresponding changes in nutrient needs during different phases of the life cycle (see the section Nutrition throughout the life cycle). Regular physical exercise can help attenuate the age-related loss of lean tissue and increase in body fat.

Human nutrition - BMR, REE, Energy Balance (2024)

FAQs

What is BMR energy balance? ›

Basal metabolic rate (BMR)

The BMR refers to the amount of energy your body needs to maintain homeostasis. Your BMR is largely determined by your total lean mass, especially muscle mass, because lean mass requires a lot of energy to maintain. Anything that reduces lean mass will reduce your BMR.

What is the difference between BMR and REE? ›

Basal metabolic rate (BMR) and REE are often used as interchange terms but, represent different concepts. BMR is defined as the minimal amount of energy to maintain the vital functions such as respiration, heartbeat, normal body temperature while, REE represents the energy to maintain the body functions at rest.

What does ree mean in nutrition? ›

Resting Energy Expenditure (REE): the amount of calories needed to maintain basic body systems and body temperature at rest. 2. Activity Energy Expenditure (AEE): the amount of calories used during activity. Maintenance or change in body weight is summarized in the formulas below: • Weight maintenance.

What is the energy requirement for BMR? ›

The BMR (in kilocalories per day) can be roughly estimated using the following formula: BMR = 70 × (body weight in kilograms)3/4. The energy costs of various activities have been measured (see table). While resting may require as little as 1 kilocalorie per minute, strenuous work may demand 10 times that much.

What is a normal BMR level? ›

Men's BMR tends to be around 1600 - 1800. This means men burn 1600 – 1800 kcal during the day at complete rest. However, remember BMR is based on your own height, weight and age, so it's not advisable to go off average numbers. Most women have a BMR of around 1550 calories a day.

How to improve your ree? ›

To do this you will need to: 1) have an appropriate calorie intake, 2) increase total energy expenditure through physical activity and optimizing lean mass to raise REE, or 3) a combination of both.

Should I eat my BMR or more? ›

You should never eat less than your BMR as this is the basic number of calories your body needs to perform basic functions like breathing and digestion. Use your BMR to calculate your TDEE, and then you can eat less than your TDEE to lose weight.

What is the best BMR to have? ›

There isn't a "good" or "bad" BMR. “Each individual has a different BMR and cannot be compared to one another,” says Carmichael. What is considered to be “healthy” varies depending on the person and their goals. The average BMR for women is around 1400 kcal and about 1700 kcal for men, she says.

What does REE tell you? ›

Indirect calorimetry is a test that measures your Resting Energy Expenditure (REE), or how many calories you burn just to maintain your body's basic functions.

What is a good resting energy for a woman? ›

Typically, BMR is between 1,000 and 2,000, meaning you need 1,000 to 2,000 calories to fuel basic functions. BMR for women average around 1,400, while BMR for men average between 1,600 and 1,800.

Should resting energy be high or low? ›

Resting energy expenditure remains elevated as long as you exercise at least three days a week on a regular basis. Because resting energy expenditure accounts for 60% to 75% of the calories you burn each day, any increase in resting energy expenditure is extremely important to your weight-loss effort.

Is high BMR good or bad? ›

A higher BMR means you need to burn more calories to sustain yourself throughout the day. A lower BMR means your metabolism is slower. Ultimately, leading a healthy lifestyle, exercising, and eating well is what's important,” said Trentacosta.

Is a low BMR good or bad? ›

It's not a good or a bad; it just is. Smaller bodies have lower BMRs because they need less energy. So. For someone dangerously underweight, a low BMR is a bad thing.

How does BMR affect energy balance? ›

The basal metabolic rate accounts for most of our energy needs, about Page 2 ENERGY BALANCE AND METABOLISM MODULE SCRIPT 2 60-75% of total energy expenditure. Because different tissues use varying amounts of energy, the BMR is influenced by the amounts of lean body mass, mostly muscle, in the body.

Should I eat my BMR calories to lose weight? ›

Will You Lose Weight if You Eat Your BMR? Short answer: yes, but it's not sustainable. Remember, your BMR is just the number of calories your body burns at rest and does not account for the calories you need to walk, talk, exercise, etc.

What is the energy balance for weight loss? ›

Simply put, it's a balance between Energy Input and Energy Output. In other words, it focuses on balancing the energy (calories) you consume and the energy (calories) you burn through physical activity. To lose weight, the number of calories we consume must be less than the number of calories we burn.

What does it mean to be in energy balance? ›

Energy balance is defined as the state achieved when the energy intake equals energy expenditure. This concept may be used to demonstrate how bodyweight will change over time in response to changes in energy intake and expenditure.

What does positive energy balance mean? ›

Positive energy balance occurs when energy intake is greater than energy expenditure, usually resulting in weight gain. Negative energy balance is when energy intake is less than energy expenditure, usually resulting in weight loss. Energy intake is made up of the calories we consume from food and beverages.

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