Basal metabolic rate (BMR)

Basal Metabolic Rate (BMR)

Energy

Question 1. Define BMR. Explain factors affecting basal metabolic rate.
Answer.

Definition of BMR

BMR is defined as the energy expenditure of a subject at complete physical and mental rest, awake (and not during sleep) having normal body temperature and in the post-absorption state (12 hours after the last meal) and 8–12 hours after any significant physical activity.

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Factors Affecting BMR

BMR differs among different individuals. It depends on:

• Variable factors
• Invariable factors

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Variable Factors Affecting BMR

• Nutritional state: BMR is low in starvation and undernourishment as compared to well fed state. Starvation leads to an adaptive decrease in BMR, which results from a decrease in lean body mass.
• Body size or surface area: The BMR is directly proportional to the surface area of the subject. Larger the surface area, greater will be the heat loss and equally higher will be the heat production and BMR.

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• Body composition: The BMR is proportionate to lean body mass (LBM). LBM is the body weight minus non-essential (storage triacyl glycerol) weight. Adipose tissue is not as metabolically active as lean body mass. BMR is often expressed as per kilogram of lean body mass or fat free mass. Therefore, higher the percentage of adipose tissue in the body lower the BMR/ kg body weight.
• Endocrinal or hormonal state: In hyperthyroidism, the BMR is increased and in hypothyroidism it may be decreased by upto 40%, leading to weight loss.
• Environmental temperature or climate: In a colder climate, the BMR is higher and in a tropical climate, the BMR is proportionately low. Stress, anxiety and disease states, especially infections, fever, burns and cancer also increase the BMR.
• Drugs: Smoking (nicotine), coffee (coffine) and tea (theophylline) increase the BMR, whereas β-blockers tend to decrease energy expenditure.

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Invariable Factors Affecting BMR

• Gender or sex: The BMR of males is slightly higher than that of females, particularly due to:
• • Women’s lower percentage of muscle mass (lean body mass) and higher percentage of adipose tissue (which hasa  lower rate of metabolism) when compared to men of the same body weight, and
  • The difference in sex hormone profile of the two genders.
• Age: Decrease in BMR with increasing age is probably related to loss of muscle mass (lean body mass) and replacement of muscle with adipose tissue that has a lower rate of metabolism.

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Question 2. Body mass index.
Answer.

Body Mass Index: Quetelet’s Index

The basal mass index is used as a reference standard for assessing the prevalence of obesity in the community.

BMI = Weight in kg /Height in meters

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Ideal body mass index for Indian woman = 19–24

Ideal body mass index for Indian man = 20–26

Once the BMI exceeds the normal limit, the person can be termed as overweight or obese.