“Everyone knows” athletes need to fuel with lots of carbs right? There’s a reason why just about every marathon century cycling ride and other event that requires pushing the boundaries of human endurance is preceded by a pasta party the evening before. Participants want to make sure their glycogen stores are filled to the brim in preparation for tackling feats that require being on their feet in the water or on a bike nonstop for several hours.

But once upon a time “everyone knew” that eating egg yolks and butter were bad for heart health and “everyone knew” red meat was bad for you. But these nutrition myths have been dispelled in recent years. One by one many of our long-held nutrition and health tenets have crumbled. A recent study on athletic performance comparing fuel usage in high-carb athletes and low-carb keto-adapted athletes suggests that perhaps the time has come to put the notion of carb loading on the chopping block too.

The study led by Jeff Vokek PhD RD and Stephen Phinney MD PhD who have spearheaded much of the research regarding low-carbohydrate diets and athletics and who co-authored the book The Art and Science of Low Carbohydrate Performance produced results that may require entire sections of physiology and exercise science textbooks to be rewritten. According to Volek their data “represents a real paradigm shift in sports nutrition and I don't use that term lightly. Maybe we've got it all backwards and we need to re-examine everything we've been telling athletes for the last 40 years about loading up on carbs.” If so maybe “hitting the wall” will be a thing of the past.

The study cohort consisted of twenty elite ultra-marathoners and ironman distance triathletes all males between the ages of 21 and 45. All participants were in the top 10% of finalists competing in running events ≥50 km and/or triathlons of at least half iron-man distance (113 km). These were highly trained athletes accustomed to pushing the limits of human performance. Half the participants followed a traditional high-carb diet (approximate macronutrient ratios: 59% CHO 14% PRO 25% FAT) and the other half had been following a low-carb/high-fat diet for an average of 20 months (range 9-36 months macronutrient ratios: 10% CHO 19% PRO 70% FAT). Mean intakes of fat and carbs in the high-carb (HC) group were 91g and 486g/day respectively compared to 226g fat and 82g carb/day in the low-carb (LC) group. 

The fact that the low-carb athletes had been following their unconventional diet for several months is an important feature of this study that is often lacking in studies that compare carb-fueled and fat-fueled athletic performance. There is typically an adjustment period when the body transitions from being a “sugar-burner” to a “fat-burner” and upon first adopting a low-carb high-fat diet there may be an initial decline in athletic performance but after the body has become adapted to fueling primarily with fat rather than glucose physical performance typically returns to where it was and then goes on to surpass previous personal bests. Studies that suggest performance suffers from reduced carbohydrate intake too often fail to account for this adjustment period which even in elite athletes may take several weeks and sometimes months. (This study’s authors specified that LC participants needed to have been following their diet for a minimum of 6 months.)

It has long been known that fuel utilization in the body is activity- and tissue-dependent. At rest and between meals cardiac muscle cells use fatty acids for 90% of their ATP needs but this proportion can drop to 60% depending on nutritional status and the intensity of contractions. Skeletal muscle cells also predominantly use fat for energy. Fatty acids are the main source of ATP for skeletal muscles during rest and mild-intensity exercise. For intense activity glucose is used more than fat but these relative contributions to total energy may differ based on exercise duration and the gender and training status of an individual. It may also be that fuel utilization is greatly influenced by the habitual diet the relative proportions of fuel substrate availability and the types of fuel someone has adapted to use.

The small cohort of participants performed a maximal graded exercise test and a 3-hour submaximal run at 64% VO₂ max on a treadmill to determine their metabolic responses. Based on measurements of respiratory quotient and other assessments fat-burning rates during the run were nearly twice as high in the LC group compared to the HC group. Peak fat oxidation was 2.3-fold higher in the LC group (1.54 ± 0.18 vs 0.67 ± 0.14 g/min; P=0.000); mean fat oxidation during the submaximal run was 59% higher in the LC group (1.21 ± 0.02 vs 0.76 ± 0.11 g/min; P=0.000) corresponding to a greater relative contribution of fat (88% ± 2 vs 56 ± 8%; P=0.000). That’s quite a difference in the amount of fat being used as fuel: 1.5g versus 0.67g/min. These were of course elite athletes. But these findings might have effective application in weekend warriors and individuals who are struggling with fat loss—particularly if they’re consuming a low-fat high-carbohydrate diet and they find their disciplined efforts at the gym are getting them nowhere.

Despite the marked differences in fuel use between LC and HC athletes the groups showed no significant differences in the level of glycogen depletion after the 3-hour run nor after 120 minutes of recovery. The study authors speculate that LC-adapted athletes’ bodies are highly efficient at using lactate to serve as a building block for gluconeogenesis and glycogenesis which would account for glycogen level recovery after the effort (since they were not refueling/replenishing with large amounts of carbs). As for the glycogen broken down during the run it may seem odd that the LC and HC athletes had nearly the same amount of glycogen loss. The researchers speculate that while the LC athletes used far less glucose as a direct source of fuel glycogenolysis provides a source of glucose and pyruvate to form oxaloacetate which is crucial for maintaining TCA cycle activity to enable the continued oxidation of fat for fuel.

One thing is for sure: fat is a far more abundant fuel than carbohydrate. Even a lean person has several thousand calories of stored energy all over their body in the form of adipose tissue. Compare this to the limited supply of glycogen in the skeletal muscles. (Liver glycogen contributes only to maintaining the blood glucose level; it is not available to fuel working muscles.) If athletes could tap into more of this fat for fuel rather than carbohydrate they might fare better during long endurance efforts and possibly also even with shorter bursts of intense activity. Many elite athletes have already embraced the low-carb fueling strategy to great success. A low-carb runner won the Western States 100-mile Endurance Run in 2012 and successful triathletes are singing the praises of using fat to fuel their physical endeavors. Mainstream running publications have been featuring articles on this unconventional approach as well as more people become interested in changing the way they prime their bodies for performance.

Perhaps this research will influence the kinds of foods offered at refueling stations along the course of endurance events. Participants may choose to fuel themselves primarily from their own stored body fat or perhaps we’ll see glucose-rich gels and drink powders replaced by packets of coconut butter and macadamia nuts.