iRun because I cannot say no to a second slice of chocolate cake – Emily Shandruk , Vancouver, BC

iRun to stay fit and release those running endorphins – Liliana Plava , Calgary, AB

iRun because I want to qualify for Boston and raise money for charities near and dear to my heart – Christine Gracel , Calgary, AB

iRun because it makes me feel good, allows me to spend time with my friends and gives me a feeling of accomplishment – Helen Kolodziejzyk , Calgary, AB

iRun but not enough – Michael Shaw , New Westminister, BC

iRun because I never thought I would be able to – Gary Morris , Winnipeg, MB

iRun to challenge myself, physically and mentally – Kathleen Keenan , Brampton, ON

iRun because people around me inspire me – Pina Bevilacqua , Caledon, ON

iRun therefore I am – Duncan Walsh , Nottingham, UK

iRun because I live – Georges Schneller , Laval, QC

iRun to be free and enjoy our beautiful country – Cheryl Carter , Clearwater , BC

iRun for overall wellbeing – Trish McCourt , Halifax, NS

iRun because it makes me a better person, a better wife, a better mother and a better friend – Nathalie Joncas-Caissie , St-Antoine, NB

iRun because it makes me feel powerful – Carlene Paquette , Carp, ON

iRun because pecan pie, french fries and beer are chasing me – Teresa Sterling , Ottawa , ON

iRun because it’s in me – Michael Foley , Stittsville, ON

iRun because it reminds me that I am capable of so much more than I have done – James Sauve , Ottawa, ON

iRun for me – Kiza Francis , Ottawa,ON

iRun to prove to myself I can – Lesley McGougan , Brampton, ON

iRun because all the ladies are chasing my sexy runner’s body – Chris Baker , Etobicoke, ON

iRun because I can and I’m grateful – Terry SanCartier , Gatineau, QC

iRun because when I run I feel most alive – Meghan Lynch , Ottawa, ON

iRun to unleash my inner athlete – Adelle Densham , Avonmore, ON

iRun because it cleans up my life, because I drink more water, sleep better and eat healthier foods – Robin McIntyre , Ottawa, ON

iRun because of the peace and strength it brings me – Michelle Jordan , Ottawa, ON

iRun because I need it to soothe the soul, keep me in shape and for overall wellbeing – Beth Neil , Lombardy, ON

iRun because it is my tonic and my salvation – Georgia Ioannou , British Columbia

iRun for relaxation and to motivate my two sons – Keith Bradbury , Newfoundland

iRun because endorphins are free – Cassandra Chouinard , Ontario

iRun because somebody once told me I couldn�t – Heidi Abbey-Der , Saskatchewan

iRun because couch potatoes die young – Cathy Andrew , Ontario

iRun because it�s cheaper than therapy – Leah Boulter , Alberta

iRun because I like buying running clothes – Pamela Blaikie , Ontario

iRun slowly! – Jason Hoffman , Manitoba

iRun because it gives me freedom to relax my brain – Marie-Claude Gregoire , Nova Scotia

iRun because I learn more about who I am with every km – Steph Mansell , Quebec

iRun because iEat – Sherry Maligaspe , British Columbia

iRun and run, and run, and run, and nobody can stop me – Andrei Lucaciu , Ontario

iRun because the wall is meant to be broken – Jonathan Bird , Ontario

iRun because it has saved my life – John Marshall , Alberta

iRun for the challenge to go faster and farther – Steven Matejka , Alberta

iRun to my happy place and some days it�s very – Doreen May , Alberta

iRun because food tastes better afterwards – Patrick Houston , Alberta

iRun because I can�t dance – Mario Javier , Ontario

iRun so I don�t say never ever again – Linda Klaric , Manitoba

iRun because it makes me whole – Denis Ladouceur , Quebec

iRun because it gets my husband out there – Tricia LaLonde , Alberta

iRun away from the negative and towards the positive – Teri Lepard , Alberta

iRun because running is like breathing to – Stephanie McEvoy , Ontario

iRun because I love the solitude – Janene Tailleur , British Columbia

iRun for the moment when both feet are off the ground – Catherine Anderson , British Columbia

iRun to someday win the race – Lindy Dunlop , Yukon

iRun to stay ahead of the weight gain – Myra Abstreiter , Alberta

iRun because otherwise I�m grumpy – Alexandre Charest , Quebec

iRun because I get foot rubs afterward – Kate Howerton , British Columbia

iRun because iLoves my man – Beverly Huang , Alberta

iRun because not everyone can – Olivia Harvey , New Brunswick

iRun to get to know myself, my strength and my spirit – Lisa Groulx , Ontario

iRun whenever I feel the need to escape – Iona Hillis , Ontario

iRun because it�s like flying, only lower – Glenn Johnson , Ontario

iRun because it makes me feel powerful – Sarah Kallaghan , Alberta

iRun because I�ve lost 80 lbs and running has become fun – Cheryl Kelly , Ontario

iRun because there is no finish line – Claire Kilgour , Ontario

iRun so my daughters know that they can, too – Shelley Kirkpatrick , New Brunswick

iRun because it reminds me of how strong I can be – Monique Lavoie , Ontario

iRun because it�s a great way to see the world – Sherry Mahoney , British Columbia

iRun because my heart tells me to – William Martin , Manitoba

iRun to prove to them that iCan – Catherine Smith , Manitoba

iRun because it�s fun when it�s done – Sue Matte , Ontario

iRun because I am not as clumsy I thought I was – Hanna Baer , Quebec

iRun see where my feet will take me today – Megan Dolinskas , New York

iRun for the cool t-shirts! – Pina Bevilacqua , Ontario

iRun because I want to be a role model for our six kids – Catherine Empey , British Columbia

iRun to inspire my kids to try – Glen Johnston , Nunavut

iRun so I can eat ice cream – Sandy Bolan , Ontario

iRun because I want to live to be 100! – Colette DeJean , Ontario

iRun for health, iRun for life – Pat Cheung , British Columbia

iRun because it gives my day a boost of energy – Sara Campbell , Nova Scotia

iRun because it�s better than almost everything else – Nathan Carey , Ontario

iRun at 50 years old because at 43 I couldn�t – Peter Cicalo , Ontario

iRun iRun because it is my tonic and my salvation – Georgia Ioannou , British Columbia

iRun iRun for relaxation and to motivate my two sons – Keith Bradbury , Newfoundland

iRun iRun because endorphins are free – Cassandra Chouinard , Ontario

iRun iRun because somebody once told me I couldn't – Heidi Abbey-Der , Saskatchewan

iRun iRun because couch potatoes die young – Cathy Andrew , Ontario

iRun iRun because it's cheaper than therapy – Leah Boulter , Alberta

iRun iRun because I like buying running clothes – Pamela Blaikie , Ontario

iRun iRun slowly! – Jason Hoffman , Manitoba

iRun iRun because iEat – Sherry Maligaspe , British Columbia

iRun iRun because I learn more about who I am with every km – Steph Mansell , Quebec

iRun iRun because it gives me freedom to relax my brain – Marie-Claude Gregoire , Nova Scotia

iRun iRun at 50 years old because at 43 I couldn't – Peter Cicalo , Ontario

iRun iRun because it's better than almost everything else – Nathan Carey , Ontario

iRun iRun for my heart, so it runs for me! – Cathy Brzoza , British Columbia

iRun iRun to inspire my children! – Wendy Bowen , Manitoba

iRun iRun because it sure beats the bus – Robin Robbins , Alberta

iRun iRun for the challenge and to remember to fully live – Pascale Synnott , Qu�bec

iRun iRun to kickstart my day – Sharon Strueby , Saskatchewan

iRun iRun for me! – Judi Wearing , Saskatchewan

iRun iRun because it's a great stress release – Brooke McKenzie , Yukon

iRun iRun because i love to – Mirella Petriello , Ontario

iRun iRun because it helps me see things more clearly – Jennifer Pitts , Ontario

iRun iRun to eat – Maureen Tritscher , Alberta

iRun iRun to correct years of sedentary living! – Mike Scott , Ontario

iRun iRun away from the abyss – Charlene Thomas , Ontario

iRun iRun all the livelong day – Pierre Saint-Laurent , Qu�bec

iRun iRun to challenge my perceived limitations – Cassandra Williams , Ontario

iRun iRun to maintain a strong physical and mental state – Tammy Rainville , Ontario

iRun iRun so that I can live longer and stronger – Derek MacPhail , Ontario

iRun iRun to feel great – Kathryn Rachar , Saskatchewan

iRun iRun because I like to be healthy – Melanie Oickle , New Brunswick

iRun iRun to eat more, especially sweet potatoe fries – Joanna Skomra , Ontario

iRun iRun for the fresh air and adrenalin – Charlyn McGregor , Saskatchewan

iRun iRun for the individual pursuit – Robert Pelletier , New Brunswick

iRun iRun to satisfy the irresistible urge – Tim Nixon , British Columbia

iRun iRun because I love the sense of accomplishment – Amber Moase , Nova Scotia

iRun iRun to challenge my mind, body and soul – Sonia Mendes , Ontario

iRun iRun because walking is too slow – Barry Knapp , Ontario

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January 2010

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Carbs vs. Fats

What powers your running engine?

By Beth Mansfield

When it comes to nutrition, runners tend to prioritize carbohydrates as an energy source. Indeed, one of the key aspects of training involves conditioning the runner’s body to maximize its ability to store carbohydrate in the skeletal muscle cells. Yet this finite form of stored energy pales in comparison to the potentially unlimited amount of endogenous fat (adipose tissue and muscle triglyceride) available as fuel for working muscle cells. Unbeknownst to many runners, fat provides the largest source of energy for any athlete and has some serious advantages over carbohydrate.

Did you know? No matter how well you’ve carbo-loaded, the carbohydrate reserves in the human body are limited, and the depletion of muscle and liver glycogen (the stored form of carbohydrate) tends to coincide with fatigue and decreased speed in both endurance running and shorter, higher-intensity events.

It has been estimated that most fit, lean runners have sufficient stored fat to sustain skeletal muscular contraction for roughly 120 hours at marathon running pace compared to about 90 minutes worth of running if relying solely on carbohydrate-based muscle glycogen.

So, why has there been so much focus on carbo-loading, rather than on how to best access the energy laden in our fat stores?

Can we train our bodies to run on fat?
In the last few decades there has been a growing interest in nutritional strategies that have the potential to promote the use of fat as a fuel, sparing muscle glycogen, and maintaining and/or improving exercise capacity. Some of these strategies include:

> Ingestion of fat and caffeine before training.
> Medium chain fatty acid feedings and/or high fat sport bars during exercise.
> Chronic adaptation to high fat, carbohydrate-modified diets

However, while in theory using fat as fuel should have multiple advantages, in both research and practice, no matter how much energy we have banked up in our fat stores our performance during any prolonged, higher-intensity training or racing session – which is to say, 90 minutes or longer – is always limited by/dependent upon the availability of carbohydrate for working muscles.

Muscle biopsy techniques started in the 1960’s have allowed sports physiologists and nutritionists to directly measure and determine the factors that impair or enhance muscles’ ability to store and access carbohydrate and fat. While the energy density of fat is more than double that of carbohydrate (9 kcal/gram vs. 4 kcal/gram) providing double the energy for muscular work it does so at a cost of greater oxygen use. This effectively limits the use of fat during the higher power outputs and intensities sustained throughout anaerobic and high-level aerobic interval training sessions, stair and hill running repeats. The harder you push, the more you will be drawing on carbohydrates as fuel.

Let’s Do the Math:
A trained runner at rest can expend approximately 100-120 kcal/hour, 50 per cent of which will be drawn from fat, and 50 per cent of which will come from carbs.

Similarly, this trained runner at a low-intensity, easy run pace would likely expend approximately 300-400 kcal//hr, 50 per cent of which will come from fat, and 50 per cent will come from carbs.

However, the balance begins to shift to draw more on carbohydrates as the run intensity increases. At race pace this trained runner could expend up to 600-800 kcal/hour, but only 20 per cent of these calories will come from fat, while a whopping 80 per cent of them will be drawn from carbohydrate stores.

In a high-intensity interval training session this runner could expend upwards of 800-1000 kcal/hour with only five per cent of those calories coming from fat, and 95 per cent of them coming from carbs.

In other words, the harder you push in training/racing the more you’ll be drawing your energy from your carbohydrate stores, which will need to be topped up accordingly.

Beth Mansfield, MSc, RD, is a Registered Dietitian, Sport Nutritionist, and Certified Exercise Physiologist. Through her company, Peak Performance (peakperformance.ca), Beth educates Canadian athletes of all levels — from Olympians to recreational runners — on sport nutrition for health and performance. Beth is currently working on her PhD at McGill University, focusing on the specific nutrient needs of female runners.