Understanding Metabolic Rates and Energy Expenditure, Lecture notes of Physiology

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Metabolic Rates

Page 383

Introduction

• (^) Metabolism : chemical reactions within the body.
• (^) It could be breaking or building molecules.
• (^) Catabolic reaction : breaking down molecules, release energy that maybe used to synthesize ATP (energy). Example, hydrolysis of protein.
• (^) Anabolic reaction : building molecules with addition of ATP (energy). Example, translation in protein.

• (^) The definition of a kilocalorie is the amount of energy required to raise the temperature of 1000g of pure water 1 C, from 14.5 to 15.5 C at STP. So, 1 kilocalorie is equivalent to 1000 calories.
• (^) The process of calorimetry can be used to determine the exact amount of energy present in a particular type of molecule and how much energy is released for every liter of oxygen consumed. For example, 1 mole of glucose contains 673 kcal of energy. It takes 6 moles or 134.4 L of oxygen (6 moles O2 x 22.4 L/mole = 134.4 L oxygen) to completely break down 1 mole of glucose. So the catabolism of glucose produces 5.01 kcal per liter of oxygen consumed (673 kcal/134.4 L O2).

• (^) These conditions minimize factors that raise the oxygen consumption.
• (^) Multiple factors can change BMR. One common factor is altering the amount of lean muscle mass a person has.
• (^) Different types of tissues have different energy demands. At rest, skeletal muscle has an energy demand of 13.0 kcal/kg, and adipose tissue has an energy demand of 4.5 kcal/kg. this is why one of the recommendations for weight loss is to increase muscle mass through strength training.
• (^) Although the density (mass/unit volume) of skeletal muscle tissue is approximately 15% greater than the density of adipose tissue, skeletal muscle tissue has a higher per mass energy demand even at rest, so replacing fat with muscle increases the BMR and can help increase overall weight loss.

• (^) Several hormones can also increase BMR: thyroid hormone, growth hormone, stimulated by sympathetic system. And condition that increase these hormones will increase BMR. And any condition with decrease of these hormones will decrease BMR.
• (^) Age and gender also have an influence on BMR. Males higher BMR than females due to the lower percent body fat and higher percent lean muscle. BMR decreases with age. For every decade after age 20, BMR is expected to drop by 2-3%.

• (^) BMR can be predicted or estimated for an individual based upon the person’s height, weight, age, and gender.
• (^) For Females:
• (^) 655 + (4.35 x weight in lb) + (4.7 x height in inches) – (4.7 x age in yrs) =kcal/24 hrs.
• (^) For Males:
• (^) 66 + (6.23 x weight in lb) + (12.7 x height in inches) – (6.8 x age in yrs) = kcal/24 hrs.
• (^) MR is the total energy expenditure of the body regardless of the conditions.

Respirometry

• (^) Is used to determine BMR and MR. it measures the amount of oxygen consumed by an individual in a given time. The subject inhales from a sealed chamber containing pure oxygen. The subject also exhales into the same chamber. As the subject inhales, the volume of the chamber decreases, and the volume change is recorded on a chart. As the subject exhales, the volume of the chamber increases, and the volume change is recorded on a chart.
• (^) Normally the volume of air inhaled and exhaled is the same, but the chemical composition of the air is different. Inhaled air is 100% oxygen, exhaled air is combination of oxygen and carbon dioxide.

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