Fats and carbohydrates are the principal fuels for muscle tissue. While carbs, in the form of glycogen stores, are widely considered an efficient source of energy, the potential role of stored fat as a source of energy for skeletal muscle during exercise remains controversial.
Most bodyfat is stored as triacylglycerol (TG) in adipose tissues; however, smaller quantities of TGs are deposited as lipid droplets inside muscle fibers. Intramuscular TG stores have recently piqued scientists’ interest.
It’s been generally assumed that accumulation of intramuscular fat is associated with the development of skeletal muscle insulin resistance, adversely affecting muscles’ capacity to use carb or fat fuels. Improved detection technology used in recent studies at the Nutrition Research Institute, Maastricht University in the Netherlands, however, helped demonstrate that intramuscular fat stores function as an important and effective substrate energy source, in particular during intense prolonged exercise.
Muscle fat use during exercise also seems to be largely determined by variables such as diet composition (in particular the ratio of fat to carb), level of fitness, gender and age. (Note that indirect evidence suggests that the capacity to mobilize muscle fat stores and convert them to energy is substantially impaired in obese or diabetic cases.)
Studies at the Institution of Exercise and Sport Science at the University of Copenhagen have demonstrated that women have a higher capacity to mobilize and metabolize fat in muscle tissues than men. Even so, trained men showed significant improvement in their capacity to use intramuscular fat when the intensity and duration of their exercise were increased.
Another variable that affects muscle fat is the ratio of carbs to fat in the diet. In England, researchers at the School of Sport and Exercise Sciences at the University of Birmingham found that minimizing carb intake before exercise forces the body to shift from carb fuel to fat fuel, with significant increase in muscle fat burning during exercise and reduced levels of plasma TGs. Minimizing carb intake before exercise was also associated with decreased plasma cortisol and adrenaline levels. Conversely, there was a significant increase of those stress hormones, in particular cortisol, in individuals whose carb intake was high before or during exercise.
All the above information leads to some interesting conclusions:
‘Minimize your carbs before exercise. Instead, have carbs in your recovery meal after the workout. That way you can minimize cortisol output during exercise while taking advantage of the carb-and-insulin anabolic effect after exercise
‘Workout programs that incorporate high intensity and high volume, such as multiple sets of low reps, as well as forced reps and/or X Reps, help accelerate muscle fat mobilization and use and increase the capacity to perform under intense stress. [Note: X Reps are end-of-set partials done at the point of max-force generation; for more info and how they can make you bigger and leaner, visit www.X-Rep.com.]
‘Women and intensely trained men should take advantage of their naturally higher capacity for using fat as fuel by incorporating high-fat, low-carb days during the week and see how it affects their performance and body composition.
It’s advantageous to train the body to shift from carb to fat fuel and vice versa. Rotating between high-carb and high-fat days forces the body to improve overall fuel utilization. While carb fuel serves as an immediate source of energy, fat fuel, with its higher octane, can be used for a prolonged period, providing muscles the energy they need for sustaining performance under intense stress. For an active athlete or a bodybuilder who wishes to grind to his or her limits, improving fat use in muscle tissues can make the big difference.
Editor’s note: Ori Hofmekler is the author of the books The Warrior Diet and Maximum Muscle & Minimum Fat, published by Dragon Door Publications (www.dragondoor.com). For more information or for a consultation, contact him at [email protected], www.warriordiet.com or by phone at (866) WAR-DIET.