Clinical Assessment: Metabolism

Today’s post is another excerpt from “Best Weight: A Practical Guide to Office-Based Weight Management“, recently published by the Canadian Obesity Network.

This guide is meant for health professionals dealing with obese clients and is NOT a self-management tool or weight-loss program. However, I assume that even general readers may find some of this material of interest.


People often talk about their extra weight as a consequence of having a “slow metabolism.” In this context, metabolism refers to total daily energy expenditure and is made up of three components: basal metabolic rate (BMR), which is the energy spent on basal metabolism, energy spent on physical activity, and increases in resting energy expenditure in response to different stimuli such as thermogenesis or the thermic effect of food.

BMR is, for most people, the largest component of energy expenditure. It typically accounts for between 60% and 75% of total daily energy expenditure. BMR can be measured through indirect analysis of the amount of heat produced by an individual, using the amount of oxygen consumed and the Weir equations. This means of analysis is called indirect calorimetry and is the current gold standard in office-based BMR measurement. Ideally, it should be measured under standardized conditions, with the patient awake, lying in the supine position, in a resting state in a comfortable warm environment, in the morning, and 10-12 hours after the last meal. In many weight management practices, strict adherence to these conditions is often overlooked. Testing involves connecting the patient to an indirect calorimeter, which measures how much oxygen is inspired and expired during the test. Most tests take 20 minutes to complete.

By far the most important determinant of BMR is body size, in particular fat-free (lean) body mass. Even then, BMR can vary by up to 10% in individuals of the same age, gender, body size, and fat-free mass, suggesting that genetic or other factors are also involved.

Patients often overlook the fact that as they lose weight, they lose not only fat but also fat-free mass, as their muscles adapt to the lighter workload after carrying around extra weight. With enough weight loss, BMR tends to decrease. In addition, hormonal and metabolic responses to weight loss will further reduce energy requirements. Progressive weight loss means people will need fewer and fewer calories because their total daily energy expenditure has decreased.

© Copyright 2010 by Dr. Arya M. Sharma and Dr. Yoni Freedhoff. All rights reserved.

The opinions in this book are those of the authors and do not represent those of the Canadian Obesity Network.

Members of the Canadian Obesity Network can download Best Weight for free.

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