BMI – BMR
Metabolism
Metabolism refers to all the chemical processes going on continuously inside your body that allow life and normal functioning which is maintained in the body by what is called homeostasis. These processes include breaking down nutrients from our food, building and repairing our body. Building and repairing the body requires energy that ultimately comes from our food. The amount of energy, measured in kilojoules (kJ), that our body burns at any given time is affected by our metabolism.
Our metabolism has two processes, which are carefully regulated by the body to make sure they remain in balance, which include:
- Catabolism, the breakdown of food components (such as carbohydrates, proteins, and dietary fats) into their simpler forms, which can then be used to provide energy and the basic building blocks needed for growth and repair.
- Anabolism, the process in which our body is built or repaired. Anabolism requires energy that ultimately comes from our food. When we eat more than we need for daily anabolism, the excess nutrients are typically stored in our body as fat.
Body Mass Index (BMI)
Body mass index is a value derived from the mass and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is expressed in units of kg/m², resulting from mass in kilograms and height in meters.
The accuracy of BMI
BMI is not used for muscle builders, long distance athletes, pregnant women, the elderly or young children. This is because BMI does not consider whether the weight is carried as muscle or fat, just the number. Those with a higher muscle mass, such as athletes, may have a high BMI but not be at greater health risk. Those with a lower muscle mass, such as children who have not completed their growth or the elderly who may be losing some muscle mass may have a lower BMI. During pregnancy and lactation, a woman’s body composition changes, so using BMI is not appropriate.
The Canadian Guidelines for Body Weight Classification in Adults is an update of the weight classification system that has been in use in Canada since 1988. The Guidelines describe a body weight classification system used to identify health risks associated with body weight in individuals and in populations. They are aligned with the World Health Organization’s recommendations widely adopted internationally.
The system uses two measures to indicate health risks: the body mass index (BMI) and the waist circumference (WC). BMI is an indicator of health risk associated with underweight and overweight, and WC is used as an indicator of health risk associated with abdominal obesity. The National Institute of Diabetes and Digestive and Kidney Diseases notes that men should aim for a waist circumference under 40 inches and women should aim for a waist circumference under 35 inches in order to reduce the risk of diseases associated with obesity.
Basal Metabolic Rate (BMR)
The Basal Metabolic Rate (BMR) refers to the amount of energy our body needs to accomplish its most basic (basal) life-sustaining functions, such as breathing, circulation, nutrient processing, and cell production. BMR, also called resting energy expenditure (REE), is often used interchangeably with resting metabolic rate (RMR).
BMR is largely determined by the total lean mass, especially muscle mass, because lean mass requires a lot of energy to maintain. Basically, anything that reduces lean mass will reduce the BMR. An average man has a BMR of around 7100 kJ per day, while an average woman has a BMR of around 5900 kJ per day. Energy expenditure is continuous, but the rate varies throughout the day. The rate of energy expenditure is usually lowest in the early morning.
Age is one of the most important factors of changes in energy metabolism. The basal metabolic rate (BMR) decreases almost linearly with age. Skeletal musculature is a fundamental organ that consumes the largest part of energy in the normal human body. The total volume of skeletal muscle can be estimated by 24-hours creatinine excretion.
The volume of skeletal musculature decreases, and the percentage of fat tissue increases with age. It is shown that the decrease in muscle mass relative to total body may be wholly responsible for the age-related decreases in BMR. Energy consumption by physical activity also decreases with atrophic changes of skeletal muscle. Thus, energy requirement in the elderly decreases.
BMR is also influenced by multiple other factors working in combination, including:
- Body size, larger adult bodies have more metabolizing tissue and a larger BMR.
- Amount of lean muscle tissue, muscle burns calories more rapidly.
- Amount of body fat, fat cells burn fewer calories than most other tissues and organs.
- Growth, infants and children have higher energy demands per unit of body weight due to the energy demands of growth and the extra energy needed to maintain their body temperature.
- Gender, generally, men have faster metabolisms.
- Genetic predisposition, our metabolic rate may be partly decided by your genes.
- Hormonal and nervous controls, BMR is controlled by the nervous and hormonal systems. Hormonal imbalances can influence how quickly or slowly the body burns calories.
- Environmental changes, such as increased heat or cold forces the body to work harder to maintain its normal temperature and increases BMR.
- Infection or illness, BMR increases because the body must work harder to build new tissues and to create an immune response.
- Amount of physical activity, hard-working muscles need plenty of energy to burn.
- Drugs, medications such as antidepressants and steroids increase weight gain regardless of what we eat. Caffeine and nicotine can increase our BMR as well.
- Diet, alter our metabolism. What and how we eat has a big influence on our BMR. for example, a diet low in iodine reduces thyroid function and slows the metabolism.
