During each triathlon season there is always one nutrition topic that really grips me. This year has been no exception and it hit me early: Fat loading. Yes, that’s right, fat loading. A couple of weeks ago, a client of mine was getting ready for a very long cycling session and he asked me to develop a pre-race nutrition plan for him to trial. We dialed in his hydration and fuel regimen and I detailed a carbohydrate loading schedule for him to follow to the gram. Later that day he emailed me the following question: What about fat loading?
In the past, I had approached fat loading with a high degree of skepticism. This time I took a different angle. Could fat loading actually work? Could it somehow improve performance? Under what circumstances would it make sense to fat load? With fervor, I began a quest to review the current research on record to see if I could find even partial answers to these questions.
Could fat loading actually work?
Trained muscles have higher levels of enzymes that promote the potential use of fat as a part of the energy mix during exercise than do untrained muscles. This enzymatic difference is particularly enhanced in the endurance athlete. Intramuscular triacylglycerol (IMTG), or stored fat, in working muscle is a critical energy supply during endurance exercise lasting greater than 90 minutes at a sustained intensity of 70% V02 max or less. Trained muscles of the endurance athlete have more stored fat. This is an adaptive mechanism in order to conserve the limited supply of carbohydrate stores. Perhaps increasing dietary fat could promote a greater carbohydrate sparing effect! It could actually work if there was a “fat adaptation” to the fat loading diet regimen.
In the initial studies on this topic, researchers compared a high fat diet to a high carbohydrate diet. Both diets were equal in calories and the subjects consumed the respective diets for 5 days. On Day 6, subjects consumed a high carbohydrate diet and fasted overnight. The next day, the subjects cycled for two hours at 70% V02 max and then completed an exhaustive 30-minute time trial. Only water was consumed during the exercise session. The researchers found that the fat loading diet had a glycogen sparing effect. Although there was not a statistically significant difference in the time trial performances between the two diets, there was a trend toward improved performance on the fat loading diet.
To further examine the topic, the same group of researchers did a similar study, but the only difference was that they had subjects consume a high carbohydrate pre-exercise meal and gave subjects a carbohydrate/electrolyte drink every 20 minutes during the exercise session. Relative to the first study, the amount of fat burned during exercise was less, but still more than when compared to the carbohydrate-loaded diet. Other studies evaluating the effect of short-term fat loading came to the same conclusion: there is a relative fat adaptation that produces a greater utilization of fat as a fuel during endurance exercise.
Could fat loading improve performance?
It seems as though the difference in the effect on performance is dependent on the duration and intensity of activity. The majority of research studies to date on fat loading conclude that it does not translate into a performance advantage when compared to carbohydrate-loading regimens. Again, what the research literature consistently found was that fat loading failed to provide an improvement in performance. In fact, it had a negative effect on performance, and subjects had increased ratings of perceived exertion for exercise at high intensities for shorter time periods when compared to carbohydrate loading regimens. However, and of greater interest to triathletes, the performance results for exercise bouts at submaximal efforts greater than 2 hours showed a similar result to the carbohydrate loading regimens. In other words, it did not hinder performance. To me, this is noteworthy and worthy of further research efforts.
Under what circumstances would it make sense to fat load?
Perhaps during certain endurance events such as an Ironman distance triathlon and ultra-endurance events; I say “perhaps” because the research is limited in this area. Nonetheless, it is quite intriguing.
For example, a recent case study of 2 trained rowers competing in an ultra-endurance event (40-90 day event covering 3,000 nautical miles) found an improvement in performance when the subjects followed a high-fat diet compared to a high carbohydrate diet. A similar result was found on subjects cycling for 5 hours at 65-79% V02 max. This result is particularly interesting as it relates to the Ironman cycling distance at similar intensities.
Giving specific recommendations on how to fat load is too premature at this time. Fat loading appears to have promise for certain endurance events (i.e. Ironman distance) and ultra-endurance events. Further research is needed to better define the optimal parameters of fat loading for improved, sport-specific performance. I believe the early research on fat loading has shown that, as a practice, it could hold potential. Stay tuned as additional research will shed more light on this very exciting sports nutrition topic.
Vogt M, et al. Effects of dietary fat on muscle substrates, metabolism, and performance in athletes. Med Sci Sports Exerc. 2003 Jun;35(6):952-60
Burke LM, et al. Carbohydrates and fat for training and recovery. J Sports Sci. 2004 Jan;22(1):15-30
Stepto NK, et al. Effect of short-term fat adaptation on high-intensity training. Med Sci Sports Exerc. 2002 Mar;34(3):449-55
Carey AL et al. Effects of fat adaptation and carbohydrate restoration on prolonged endurance exercise. J Appl Physiol. 2001 Jul;91(1):115-22
Hawley JA, et al. Effect of altering substrate availability on metabolism and performance during intense exercise. Br J Nutr. 2000 Dec;84(6):829-38
Rowlands DS, Hopkins WG. Effects of high-fat and high-carbohydrate diets on metabolism and performance in cycling. Metabolism 2002 Jun;51(6):678-90
Robins AL, et al. The effect of nutritional manipulation on ultra-endurance performance: a case study. Res Sports Med. 2005 Jul-Sep;13(3):199-215