Metabolism: The fat and thin of it
Thursday, September 1st 2005
How often do you hear someone (usually someone who is overweight) lament: "I've only got to look at that piece of cake and I get fat whereas you seem to be able to eat anything you want"? What they are trying to say is that somehow eating that piece of cake provides them with more energy, and thus a greater propensity to store that energy as fat, than their skinny counterpart.
In actual fact, healthy gastrointestinal tracts are all pretty efficient at digestion, so the same size piece of cake results in the same energy (as fat, carbohydrate, and protein) being absorbed into the bodies of both Laurel and Hardy. Once inside the body, the First Law of Thermodynamics applies:
"Energy can be changed from one form to another, but it cannot be created or destroyed"
What this means is that the chemical energy from the cake just doesn't disappear once inside our thin person, or multiply inside our fat person. It means that if two people eat the same food, the reason why one person becomes fat and one stays thin, is that the rate at which they burn off that energy - their metabolic rate or metabolism - is different.
So, why do different people have different metabolic rates? Metabolism (energy expenditure) is best divided into energy expended at rest (resting metabolic rate) and the energy expended on top of that during physical activity. We know that, although the rate of energy expenditure at rest is low, because it occurs 24 hours per day, it generally provides for the bulk of our daily energy expenditure. During exercise, our metabolic rate can increased many-fold, because most of us don't exercise for more than a few hours per day, and usually not at a high intensity, physical activity is a lesser contributor to daily energy expenditure.
This begs the question as to whether our fat person somehow has a lower resting metabolic rate, and if so, why?
The main factors that affect resting metabolic rate are overall body size, lean body mass, and body surface area, with larger more muscular people expending more energy at rest. Resting metabolic rate is also modulated by the activity of particular thyroid hormones, medications, and environmental conditions. However, assuming the latter are the same for both people, a fatter person will have a greater resting metabolic rate than a thinner person of the same stature simply because they are bigger. This, indeed, is the opposite of what we would have expected for our cake-watcher! So, if the answer is not in the resting part of our energy expenditure, the answer must lie with the energy expended in physical activity.
Studies show that even small amounts of muscular contraction such as fidgeting or standing rather than sitting add significantly to daily energy expenditure. Even more important than this 'hidden' physical activity are factors such as taking the stairs rather than riding the escalator at the shopping centre.
If our fat person and our thin person truly have the same diet, it is highly likely that our thin person actually does more physical activity each day, and thus their metabolic rate is a little higher. Of course our fat person may also be fatter because they have an extra piece of cake when no one is looking.
About the author
Dr Stephen Stannard is a senior lecturer at Massey University with expertise in metabolism and exercise; he also has professional interests in prolonged exercise, the effects of fasting and type II diabetes.
Article reprinted with the kind permission of Body Concepts
Footnote from Ideal Health:
The following products are all useful for Metabolism:
Cell Food Weight Management
IsoWhey Weight Management - Ivory Coast Chocolate®
Super African Mango
Related health information can be found here:
Bitter Orange as an appetite suppresssant and to aid fat loss
Good nutrition for positive weight control
How to read a food label
Water, the elixir of life
What is cholesterol?
Weight loss naturally
Related articles can be found here:
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Have your cake and eat it
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Take the HoodiaSlim Option
If you need help or advice, you are welcome to email our naturopathic team with your health question.
Disclaimer: The health information presented here has been written for the New Zealand health consumer. It is of a general nature and is only intended to provide a summary of the subjects covered. The information is not intended to be comprehensive or to provide medical advice to you. While all care has been taken to ensure the accuracy of the information, no responsibility or liability is accepted, and no person should act in reliance on any statement contained in the information provided. All health ailments should be treated by a qualified health professional.
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