Yesterday, I wrote about macro balance and the need for carbohydrates relative to physical activity. To better understand the high-fat/low-carbohydrate v. low-fat/high-carbohydrate debate, it is helpful to understand what is referred to as the “crossover effect” in exercise physiology.
The crossover effect refers to the shift in fuel used by the body to produce adenosine triphosphate (ATP; the fuel for muscle contraction) as the intensity (or duration) of exercise increases. At very low intensities of physical activity (<30% VO2max) the primary fuel source is fat. (Conversely, physical efforts that can be sustained for very long durations use fat.) High intensity exercise (>70% VO2max; or short duration) relies primarily on carbohydrate. As the intensity of the exercise increases, dependence of fat decreases and carbohydrate increases. Metabolism shifts somewhere in between, depending on training and the individual. It is believed that, with appropriate diet and training, metabolism can be shifted to rely more heavily on fat. Sports nutritionist, Bob Seebohar*, refers to this as the “metabolic efficiency point”. The caveat is that this is affected by and applicable to endurance exercise performance. In other words, a shift to greater fat metabolism can benefit endurance athletes but will have little benefit to the performance of high-intensity exercise—e.g., sprint, strength, or power performance.
Another important concept for consideration is “lactate threshold”—the point at which the athlete begins to accumulate lactate (the by-product of anaerobic metabolism) in the blood. Interestingly, lactate threshold occurs 50-60% of VO2max in the untrained and 65-80% of VO2max in endurance-trained individuals. The physiology is beyond today’s writing, but suffice it to say that lactate threshold is an important predictor of exercise performance and intensity. (The athlete who can compete at a higher lactate threshold can, thus, outperform a competitor of similar VO2max but lower lactate threshold.) A higher lactate threshold and reduced reliance on carbohydrates for fuel spares muscle glycogen for high-intensity performance.
So, the bottom line? Fat metabolism can be amped up by consuming less carbohydrate and more “healthy” fats, but carbohydrates—specifically, muscle glycogen—is still essential for the performance of activities greater than lactate threshold. Thus, for the sedentary individual (and, please, let’s not be sedentary) or the endurance athlete, the “metabolic efficiency point” can be shifted by eating a lower carbohydrate diet (and simply by doing moderate-to-high intensity endurance exercise). Glycogen remains, however, necessary for the highest intensities.
It was mentioned, yesterday, that creatine supplementation (creatine monohydrate or creating HCl) can help, but there remains the need for adequate glycogen in the muscle.
In the past, “carbohydrate loading” was popular among distance runners. For the endurance athlete, high-carbs are less of a concern for body composition. Traditional carbohydrate loading is less important than simply getting sufficient calories—and making sure that muscle and liver glycogen are topped-off.
While the ketogenic diet is effective for shifting the “metabolic efficiency point”, the extremely low carbohydrate intake will likely deplete glycogen stores with high-intensity exercise. Thus, there is a need for carbohydrate in the diet. If one takes the high-fat/low-carb approach, the practice of “carb cycling” is warranted. This involves cycling high-fat/low-carb and low-fat/high-carb days (there are varying approaches, e.g., 3:1 ratio, but it is up to the individual to determine what is best).
Hopefully, this adds some clarification to the argument that a ketogenic diet is of little value to the athlete who is performing at intensities near or above the lactate threshold. For most nonathletes, this is not of great concern. For intense exercise (>75% maximal performance), carbohydrates are necessary. While a high-fat diet can increase the metabolic efficiency point and, thereby, promote greater reliance on stored body fat, high-performance endurance athletes rarely carry “excess” body fat. As well, consuming sufficient calories tends to be the greater challenge. Weight lifters may wish to cut body fat from time to time, but maintenance of muscle mass and strength is critical—thus, carbs are necessary.
Eat to perform.
Be your best today; be better tomorrow.
Carpe momento!
*http://running.competitor.com/2010/10/nutrition/become-a-fat-burning-machine_8244