Fat Burning Essentials

Alan Couzens, MS (Sports Science), CSCS, PES

One of the key physiological adaptations of endurance training for the Ironman athlete (and for the fitness athlete who wants to ‘lose a few pounds’ is improving their ability to generate energy from fat. I wanted to devote a short article to the ‘whys’ of the importance of improving this ability in athletes along with the practical ‘hows’ of doing so from both a training and a nutritional perspective.

Why do I want to improve my ability to burn fat?

For all intents and purposes, the body generates energy (ATP) from either carbohydrate or fat. More specifically, the body gets this fuel from:

a) Ingested Carbohydrate (gels, sports drinks etc)

b) Intramuscular and liver (hepatic) carbohydrate stores (stored carbohydrate is called glycogen)

c) Intramuscular and adipose fat stores, i.e. the ‘love handles’ :-)

Each of these fuel sources has pros and cons. The big con for the ingested and liver carbohydrates and the fat stores is the rate of energy production. Assuming a fair level of fatburning, the combined energy generation from ingested carbohydrate and fat stores equates to ~480kcal/hour, or about 90-120W on the bike or 4-5mph on the run. Clearly, most of us will need to bring in the third energy store to generate energy at the rate that we need for a competitive Ironman.

As mentioned, the big pro for the intramuscular carbohydrate stores is the rate at which energy can be produced. Energy can be produced from carbohydrate up to very high levels of intensity. In fact, even a flat out 400m run around the track uses carbohydrate as it’s primary fuel source.

The downside to carbohydrate burning is that the carbohydrate stores in the body are quite limited – somewhere in the range of 1500-3000kcal depending primarily on how well trained the athlete is. The fat stores on the other hand can represent up to 50 times that amount!!!

At typical marathon intensities for the average runner (~17-18kcal/min) the average athlete will be out of juice before the 2hr mark. Unless your name is Haile Gebrsellassie, this obviously presents a problem :-)

In fact, the longer the race becomes, the more the limiter becomes metabolic rather than aerobic, as many formidable 10K runners discover when they try their first marathon. Quick example -
Two athletes (both 160lb):

From the table above, obviously athlete A is traditionally ‘fitter’ with that big VO2max that we would all like to have. However, assuming similar glycogen stores and running economy and all of those contributing variables, Athlete B will take longer to fatigue because at that pace of running, costing 18kcal/min for both runners, Athlete B is able to kick in 10% or 1.8kcal of the energy from fat. Therefore in carbohydrate terms, it is only costing him 16.2kcal/min. Or, put another way, like the Energizer Bunny, he just lasts longer!!

Clearly, it is not always the athlete with the highest VO2max that wins the day. Providing certain aerobic fitness pre-requisites are met, if the race is longer than 2hrs it is generally the athlete who spares the most carbohydrate (by burning the most fat) who wins out.

Additionally, for those athletes with fat loss goals (i.e. all of us!!) the amount of fat that we burn in any one session or one day of training is greatly affected by how good we are at burning fat. In the course of our lab testing we have seen a wide range of fat burning abilities at ‘normal’ easy-steady training intensities, anything from 2kcal/min to 10kcal/min. Or, put in other terms to lose 1lb of fat for our carb burner would require 29hrs of aerobic training, whereas for our lean, mean fat-burning machine, it would take less than 6. ‘Nuff said!!!

How do I improve my ability to burn fat?

a) Nutrition
There has been a substantial amount of research on improving resting fat burning capacity. While the mechanism for doing so is easy, it’s application is anything but! Studies have shown that providing caloric requirements are met that an individual’s ‘burn rate’ will match their ‘intake rate’ with respect to Carbohydrate and Fat after a (quite unpleasant) 3-4 day adaptation period.

Very good ultra-endurance athletes typically metabolize less than 40% of their energy from Carbohydrate. Accordingly, approximately 40% of their nutritional intake comes from Carbohydrate.

Additionally, studies have shown that those athletes who burn more fat at rest also burn more fat at all aerobic exercise intensities. In this case, a resting 60/40 split typically results in fat burning of 30-40% at Half Ironman-Ironman intensities. More commonly fat burning is less than 20% for average Ironman athletes.

So, if some is good, is more better? Why not shoot for 100% fat burning? While close to 100% fat burning is physiologically possible, it’s not conducive to the requirements of Carbohydrate replenishment that comes with high volume aerobic training.

Additionally, while possible to alter energy substrates to a large degree via diet, there are a number of hormonal and genetic factors which come into play to affect the level of satiety that diets of different compositions of macronutrients provides. For this reason, a vigilant but steady move towards a more moderate carbohydrate diet is suggested.

The efficacy of the 40/30/30 diet was touted long ago by Phil Maffetone, advisor to the greats (Mark Allen, Mike Pigg, The Debooms etc.) and now a growing body of scientific literature is backing him up.

b) Training
While improving FFA availability by altering your habitual diet is necessary, it is not, in and of itself, sufficient to ensure improved fat oxidation during exercise. After you have liberated the fatty acids so that they are ready to be burned, you still need sufficient power plants to burn them.

At low exercise intensities, fat oxidation is largely influenced by the mitochondrial density within each (low intensity) muscle fiber which, in turn, is mediated simply by the number of contractions each fiber performs.

In summary, a moderate carbohydrate, moderate fat, moderate protein diet coupled with a lot of easy-steady training represents the best method for turning yourself from a sugar-burner into a lean mean fat burning machine.