Ever wonder where the calories you burn during exercise come from? Is it the meal you ate for lunch, or is it the fat that’s hanging over your jeans? If you get down to the nitty-gritty, you could confidently say that the fuel your body uses as energy throughout the day and to get you through a vigorous (or not so vigorous) workout comes in the form of ATP (Adenosine Triphosphate). Obviously, ATP is neither food nor fat, but merely the outcome of the metabolism of proteins, carbohydrates and fats— it’s the end result of energy metabolism. But whether the ATP is being derived from your fat stores or glycogen (carbohydrate) stores is debatable.
In the fitness world, there are two occasions where a person would want to burn more fat as fuel: a) to burn more body fat, resulting in faster weight loss; and b) when participating in an endurance-type sport where if you burn fat as fuel, you’re not using your stored carb source (glycogen stores), in an effort to extend your endurance range to get you through to the finish line.
Both scenarios are equally important, depending upon your fitness goals— but for the sake of this article, we’re going to focus on the first. Finding the right balance of carbs in your diet, as well as incorporating the right training and cardio protocol into your routine, will ultimately turn you into a fat-burning machine.
The idea of burning body fat as fuel has a certain ring to it that just makes you feel like the hard work you’re putting in is really helping you rid what most women don’t want on their bodies— fat. If we could find a way to predominantly use our fat stores as fuel, the world would be a much happier place. Unfortunately, there’s no such thing as a quick fix.
The solution: use what we’ve learned through scientific research and “in the trenches” experience, to teach our bodies how to use stored body fat as efficiently as possible.
Energy Pathways to Exercise Freedom
It is important to understand how the body breaks down food, to convert to energy, to fuel your workouts. This is done via energy pathways in the body to produce ATP. During exercise, carbohydrates are the predominant fuel source; fats are used secondarily, and finally, proteins.
If you have a basic understanding of exercise, you may already be familiar with both ways the body converts nutrients to energy: anaerobic metabolism and aerobic metabolism. Anaerobic metabolism takes place in the absence of oxygen, and is associated with short, intense bursts of energy; think of a sprinter. Aerobic metabolism takes place in the presence of oxygen, and is associated with lower-intensity exercise— think of a marathon runner, as well as the daily work of the cell. Your average workout session will use a mixture of both metabolic pathways.
Break it down even further and you’ll find that anaerobic metabolism occurs in two forms: the ATP-CP energy pathway and glycolysis. ATP-CP, oftentimes referred to as the phosphate system, does not require any oxygen and is used during short bursts of exercise like the 100-meter sprint. Initially, this pathway uses ATP to fuel the first 3-4 seconds of the exercise. Once ATP is used up, CP (creatine phosphate) stores are then used to fuel the rest of the exercise (the final 6-8 seconds of work). Once your stored CP is fully tapped out, the body then either switches to the aerobic pathway or continues onto the anaerobic pathway known as glycolysis to produce more ATP to fuel your workout.
Here’s where the fun begins— glycolysis produces ATP exclusively from carbohydrates, with lactic acid being a byproduct. This pathway is used for short bursts of exercise that last for no longer than around 10 seconds or so, without the need for oxygen. Once lactic acid builds up in the muscle and muscle pain and fatigue set in, you’re no longer able to perform the exercise at that intensity— that particular moment is known as the lactate threshold. If you take a short rest, your body will have time to recover and produce more ATP by way of stored carbohydrates.
You can think of performing any given set at the gym; for instance, leg extensions. This exercise is known for generating that brutal burn (lactate threshold) you feel in your quads by the 8th or 9th rep of the set. Once you set down the weight and relax your legs, it’s almost as if you can feel the lactic acid clearing from the muscle. After a short rest period of 30 seconds to one minute, the pain has subsided and you’re able to then perform another set at almost the same intensity level as the first, and so on and so forth. The energy you’ve regained for those 2nd and 3rd sets will ultimately come from the ATP your body produced in the time you took to rest.
I know all of this science mumbo-jumbo is really not that fun— just hold your patience, we’re getting there!
Aerobic metabolism, on the other hand, is used during long-duration exercise like a grueling 45-minute cardio session on the stepmill, or completing a 10k marathon. This mode of energy metabolism uses oxygen to convert nutrients from carbs, proteins and fats into ATP.
During exercise, your body will move through all three metabolic pathways. For instance, during a long-duration jog on the treadmill, your body begins using anaerobic metabolism at the onset of your workout. Once heavy breathing sets in and your heart rate rises, more oxygen becomes available to produce ATP, allowing aerobic metabolism to begin. If the intensity of your workout continues to climb, oxygen will not circulate fast enough to produce more ATP, and lactic acid will then begin to build up, requiring you to lower the intensity of your workout or stop completely. Ever notice that the first 10-15 minutes of your jog are less-than-comfortable on your legs and muscles? But once you push past that first threshold, you seem to be able to complete the duration of your cardio workout with ease. That’s because you’ve naturally slowed the intensity of your jog to what is comfortable for your body to deliver oxygen and clear lactic acid buildup from your legs. You’re then using aerobic metabolism to complete your long jog on the treadmill.
That being said, we still haven’t discussed the use of fat as fuel during your workouts.
Food for Thought
When training, your body will use the carbs stored in your liver and muscle as fuel. Also known as glycogen stores, these stored carbs come from the food you eat. Carbohydrates have been believed to play an important role in exercise performance especially during moderate to very high-intensity exercise. After a strenuous workout, stored muscle and liver glycogen can be completely depleted and to replenish those stores is as simple as consuming some kind of carb source post-workout like Gatorade or a piece of fruit. Any excess carbs in your diet, however, will be converted into fat and stored on your hips and butt. That’s why it’s important to pay close attention to the way our bodies react to the types and amounts of carbs we eat— any excess can wreak havoc on our waistlines.
When we eat carbs, they are first and foremost stored in our cells as glycogen. When we eat fats, they are most definitely stored in adipose tissue (fat), but that is to make them accessible for use in the protection of vital organs, the transport of fat-soluble vitamins, fuel for the cell and contracting muscle tissue, transportation of cholesterol out of the body, fuel for low-intensity exercise, as well as many other important functions in the body. The kicker is that you don’t need a large store of body fat for these important functions to take place. If burning fat and using it as fuel is what we want, then dipping into those fat stores as often as possible is what we need to do.
Enhance Fat Loss: Train On Low Carbs
Because our bodies will use carbs as fuel for moderate- to high-intensity training, stored fat is only used for a short period of time. Stored fat is excellent for use during low-intensity, long-duration workouts like running a marathon, but as soon as exercise intensity is increased, carbs are pulled in as fuel. But what if we didn’t always work out with our carb stores full? I’m not talking about going on a ketogenic (very low-carbohydrate) diet. What I am talking about is working out on an empty stomach and/or keeping carbs on the lower end of your diet totem pole.
As we know, stored glycogen can fuel up to two hours of high-intensity training. However, once your glycogen stores are used up, you’ll hit a wall and in turn be forced to lower the intensity. You can continue training, but unless you replenish your carb stores, you’ll have to train at a lower intensity. This is because your body is now tapping into your fat stores for energy, which in the case of this article is not a bad thing. If you did want to work out more intensely, all you’d have to do is replenish those glycogen stores by eating some carbs.
A study conducted at the University of Birmingham’s School of Sport and Exercise Sciences, compared high and low glycogen training in 14 well-trained cyclists. The purpose of the study was to determine the effects of training with low muscle glycogen on performance and energy metabolism. It’s been said that training with low glycogen stores has negative effects on performance, but this particular study found that’s not really the case.
After testing 14 cyclists over a three-week period, the subjects performed nine aerobic training sessions and nine HIIT sessions. Pre-training and post-training measures were taken via resting muscle biopsy and metabolic measures. Results indicated that those training with low glycogen levels did reduce training intensity, but as far as performance is concerned, there was no marked difference in training with high glycogen levels. However, fat oxidation was increased after training with low muscle glycogen.
In this study, low-glycogen trainees were defined as performing a high-intensity workout one hour after performing a low-intensity workout. It’s assumed that their glycogen stores were diminished via the first workout. Other similar studies in the past have come to similar conclusions.
In this same study, as well as previous studies, endurance training with high glycogen levels did not improve performance or muscle triglyceride levels (fat levels in the muscle). Alternately, TG levels were higher in those who trained with low glycogen levels, allowing more fat to be oxidized during training for use as fuel— this occurred despite the type of training (HIIT or low-intensity). That means that no matter which type of training you engage in— on a low-carb diet or with low glycogen levels— you’re tapping into your fat stores more.
Put It Into Practice
With fat loss as our main goal, the idea is to come up with a plan that gets us from point A (fat ass) to point B (fat loss) the quickest and most efficient way possible. Combining the right mix of training protocols, sports nutrition, nutrient timing and timing of training is where we need to begin. We’ve already found that fat loss is best achieved by exercising on low glycogen levels. That means we either need to train on an empty stomach or train intensely after using our glycogen stores.
Weight training does boost fat loss; however, to really lose your excess baggage, cardio is essential. For best results, incorporate both high-intensity (HIIT) and low-intensity long-duration cardio into your weekly routine and stay consistent. If you use HIIT workouts too often, you risk overtraining and putting yourself out of commission until you fully recover, which would most definitely put a stop to your weight loss and slow down your progress toward your goal(s). That’s why it’s important to space out your HIIT sessions to every other day. Try performing a 20-minute HIIT workout three days per week, while on the other four days you engage in 40-60 minutes of low-intensity long-duration cardio, like climbing the StepMill at a slow pace.
For maximum fat loss, we discussed working out on low glycogen stores. That means you could either a) get up first thing in the morning and get your cardio workout done or b) perform your HIIT or low-intensity cardio session immediately or an hour after your weight training session. The idea behind early morning cardio is that you’ve gone without food throughout the night, so your body is in an unfed state— glycogen stores are most definitely low. Post-workout cardio sessions are great because you’ve used your glycogen stores to power through your weight training workout and can now tap into your fat stores to get you through your cardio workout.
Some people have trouble working out on an empty stomach and some are cool with it. For those who just can’t seem to function, I suggest tossing in your cardio session after you train. And for those early birds, a cup of black coffee is a great fat-loss and energy booster, which may be all you need to get you through your fat-burning sweat session before the sun rises. Once you get started on a consistent cardio program while keeping your carbs low, your body will learn to use fat as its predominant fuel source and you’ll become a fat-burning machine in no time!
Monday: HIIT session post-workout
Tuesday: Early morning long-duration cardio
Wednesday: HIIT session post-workout
Thursday: Early morning long-duration cardio
Friday: HIIT session post-workout
Saturday: Early morning long-duration cardio
Sunday: Early morning long-duration cardio
Post-Workout Glycogen Replenisher Shake
1 scoop chocolate whey protein isolate
8 oz. orange-flavored Gatorade (or other sports beverage of your flavor choice)
Drink separate, or mix together for a chocolate-orange delight!
This post-workout shake provides around 20 grams of fast-absorbing, muscle-repairing protein and 14 grams of fast-acting glycogen-replenishing carbs. On top of that, you’ll get thirst-quenching electrolytes that keep you healthy and strong.
HIIT of Your Choice
Choose your favorite cardio machine. Begin at a moderate pace for one minute. Then ramp up the speed to 90-95 percent of your max and go all-out for 20 seconds. Return to your moderate starting pace for one minute. Repeat until you’ve reached a total of 15-20 minutes. Be sure to cool down for five minutes and finish with a good stretch and glass of water.
C.J. Hulston, et al. Training with Low Muscle Glycogen Enhances Fat Metabolism in Well-Trained Cyclists. Med Sci Sports Exerc, 42:11;2046-2055.
G.D. Foster, et al. Weight and Metabolic Outcomes After 2 Years on a Low-Carbohydrate Versus Low-Fat Diet. Ann Intern Med, 2010;153:147-157.
I. Shai, et al. Weight Loss with a Low-Carbohydrate, Mediterranean, or Low-Fat Diet. N Engl J Med, 359;3:229-241.
Jeukendrup, Asker and Gleeson, Michael. Sport Nutrition: An Introduction to Energy Production and Performance. Champaign, IL: Human Kinetics, 2004.
Kones, Richard. Low-Fat Versus Low-Carbohydrate Diets, Weight Loss, Vascular Health, and Prevention of Coronary Artery Disease: The Evidence, the Reality, the Challenge, and the Hope. Nutr Clin Pract, 2010;25:528-541.