"Fat Loading" for Endurance Events

by Scott Kolasinski

Fat Loading for Endurance Performance?

Most sport nutritionists would recommend a high-carbohydrate diet for those performing an endurance event. However, other scientists have challenged this central dogma suggesting that endurance performance may benefit from high-fat diets. Here are the proposed advantages of a high-fat diet:
• Fat has 9 calories per gram versus carbohydrate which has 4 calories per gram; that is more than twice as much energy per gram.
• Fat is utilized more during submaximal work, therefore, the more available, the better one should perform.
• Athletes can adapt to the high-fat, low-carbohydrate diet while maintaining their endurance capacity.
• There is an increase in triglycerides (fat) stores within muscle.
• There is an increase in fat utilization for energy and a decrease in carbohydrate use, thus saving carbohydrates (muscle glycogen) for intense activities.

Compared to a high carbohydrate diet (60-70% energy from carbohydrate), "fat loading" (60-70% energy from fat) increases the contribution of fat for fuel (i.e. called “oxidation”) and spares muscle glycogen during submaximal exercise (<70% of VO2max). While previous studies have used two to seven week periods of fat adaptation, it is not practical for an athlete to maintain such a diet. Athletes on low-carbohydrate diets for this length of time will usually have a decrease in performance.

However, what about a change in diet only for a couple of days to try to optimize fat utilization for energy while still getting the benefits of sparing muscle glycogen that high-carbohydrate diets usually create? Could that create a benefit in performance?

Burke et al. investigated the effects of a five-day fat loading period followed by one day of carbohydrate restoration on fuel usage and performance. Eight well-trained cyclists and triathletes performed two hours of work at 70% of their VO2max followed by a brief time trial (1). A five-day time frame represented a more manageable period for extreme dietary change, allow physiological adaptations and it would minimize the potential health and training disadvantages caused by longer periods of high fat diets.

In this study, the fat loading diet provided 4 g of fat/kg of bodyweight (BW) (65% of energy), 1.7 g of protein/kg BW (13% of energy) and 2.4 g of carbohydrate/ kg BW (19% of energy). The isoenergetic carbohydrate diet (the control group) supplied 0.7 g of fat/kg (13% of energy), 1.7 g of protein/kg BW (13% of energy) and 9.6 g of carbohydrate/kg BW (74% of energy).

All athletes consumed a high carbohydrate diet (10 g of carbohydrate/kg BW) for one day on day 6 following both diets and rested to normalize muscle glycogen stores independent of previous dietary treatment. After an overnight fast on day seven, the athletes performed the 2-hour work protocol and the 30-min time trial.

The results showed that during the two hours of cycling, the fat loading group burned significantly more fat than the control group and less carbohydrate, believed to be sparing muscle glycogen. The time trial performance was 8% faster for the fat loading group than for the control group, and the power output was higher than the carbohydrate group, but neither measurement was statistically significant. This study demonstrated a significant increase in fat metabolism during submaximal exercise following a brief period of fat loading – an adaptation that persisted even after one day on a high carbohydrate diet. However, these authors concluded there was no clear evidence that fat adaptation improved cycling time-trial performance.

It is possible that fat adaptation may be more applicable to ultraendurance athletes by sparing muscle glycogen, since these individuals compete at an intensity (>65% of VO2max) and duration (>4 hours) that significantly reduce the body's carbohydrate stores. Furthermore, fat oxidation (i.e. using fat for fuel) has the potential to meet a large proportion of the fuel requirements for ultraendurance events.

In order to investigate this, Carey et al. evaluated the effects of a six-day fat adaptation period followed by one day of carbohydrate restoration on fuel usage and performance during four hours of submaximal cycling followed by a one hour time trial (2). The researchers also provided carbohydrate feedings before and during exercise to reproduce nutritional strategies commonly used during ultraendurance events and evaluate their influence on metabolism and performance.

The fat loading diet was 69% fat, 16% carbohydrate and 15% protein. The isoenergetic control, high-carbohydrate diet was 70% carbohydrate, 15% fat and 15% protein.

On day eight, all athletes consumed the control diet and rested. On day nine, the athletes consumed a breakfast (similar in size and composition to what they might consume before an ultraendurance event) that provided 3 g of carbohydrate/kg BW. One hour later, the athletes began four hours of cycling at 65% of VO2max. The athletes consumed a sports drink every 30 minutes for an average intake of 100 g of carbohydrate per hour. Following the four-hour ride, the athletes underwent a one-hour time trial in which they rode as fast as possible.

The results of this study showed that during the four-hour ride, the fat loading group used more fat for energy than the control group, and spared muscle glycogen. There was a greater power output and distance covered for the fat loading group, but neither was significantly different.


Despite marked differences in fuel utilization favoring fat oxidation during four hours of submaximal exercise and maximizing carbohydrate availability before and during exercise, fat adaptation failed to enhance subsequent time trial performance compared to a high carbohydrate control diet. The researchers note, however, that the athletes were able to ride the time trial at a power output that was 11% higher after fat adaptation. Although this performance enhancement failed to reach statistical significance, it represented a 4% performance improvement, which would certainly be worthwhile and meaningful for an ultraendurance athlete.

Both of the above studies used small subject sizes. Perhaps this is why a statistical significance could not be detected.

Other follow-up studies have found: no significant effect of fat loading (1,3,4), benefits (5) or impairment (6,7), although the percentage of dietary fat and the length of the protocol varied. However, again, these studies used trained athletes in small subject sizes. These researchers noted that any sort of nutritional strategy that brings about improvements in performance, especially in people who may have reached their genetic potential, should be a worthwhile strategy to try. Therefore, it may work in some but still not others, often labeled “responders” and “non-responders”.

But why the varied results? A variety of explanations have been offered to explain the apparent lack of transfer between metabolic changes (i.e. greater fat utilization) and performance outcomes (8). Theoretically, if an athlete is using more fat for energy, and sparing muscle glycogen for intense activity, then an athlete’s performance should improve.

Some proposed explanations include the failure of scientists to detect small changes in performance that might be worthwhile in real-life sports and the existence of "responders" and "nonresponders" to fat-adaptation strategies. In addition, adaptations to a fat-rich diet have been shown to increase plasma norepinephrine concentrations and heart rate during submaximal exercise, possibly leading to increased perceived effort of exercise training (8).

However, there is a recent paper that suggests what was initially viewed as "glycogen sparing" after adaptations to a fat-rich diet may actually be a downregulation of carbohydrate metabolism or "glycogen impairment."

One study (3) has reported that fat adaptation/carbohydrate restoration strategies are associated with a reduction in the activity of pyruvate dehydrogenase (PDH) throughout the duration of exercise (8). PDH is one of the primary enzymes responsible for efficient energy production. A reduction in PDH would impair rates of glycogen production from fatty acids (i.e. glycogenolysis) at a time when muscle carbohydrate requirements are high (8), such as during an intense sprint uphill. In this way, high-carbohydrate diets are still superior to high-fat diets.

Conclusion
Therefore, the idea of fat adapting or “fat loading” for improved endurance performance may appear worthwhile for activities considered to be continuously submaximal, but few sports are continuously performed at the same pace. The strategic activities that occur in such sports, the breakaway, the surge during an uphill stage, or the sprint to the finish line, are all dependent on the athlete's ability to work at high intensities. With growing evidence that this critical ability is impaired by fat loading strategies, some scientists believe there appears to be little scientific evidence to support recommending fat loading as an effect strategy for improving endurance performance (8).

However, the research is still young. From what we have, the subject sizes have been small, and some studies suggest trends that may find a statistical difference if the sample size were larger.

Would fat loading be beneficial for you? Maybe. Hopefully this article shows what you are dealing with. It may or may not help you. It is clear that a high-carbohydrate diet will offer benefits at some level, but you do not know if you are a “responder” or “non-responder”. I have written out a couple of the nutritional strategies used in two studies, but other fat adaptation strategies may work better for you. Just like a lot in sports nutrition, there is always a bunch of trial-and-error. Good luck if you try any fat adaptation strategy, and please let me know how it works for you.

If you have any questions concerning this article, you can email me at scott@focusedtrainers.com.

Until next time…

References
1. Burke LM, Angus DJ, Cox GR, Cummings NK, Febbraio MA, Gawthorn K, Hawley JA, Minehan M, Martin DT, Hargreaves M.. Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling. Journal of Applied Physiology. 2000;89(6):2413-21.

2. Carey AL, Staudacher HM, Cummings NK, Stepko, NK, Nikolopoulos V, Burke LM, Hawley JA.. Effects of fat adaptation and carbohydrate restoration on prolonged endurance exercise. Journal of Applied Physiology. 2001;91(1):115-22.

3. Stellingwerff T, Spriet LL, Watt MJ, Kimber NE, Hargreaves M, Hawley JA, and Burke LM. Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration. Am J Physiol Endocrinol Metab. 2006 Feb;290(2):E380-8.

4. Burke LM, Hawley JA, Angus DJ, Cox GR, Clark S, Cummings NK, Desbrow B, and Hargreaves M. Adaptations to short-term high-fat diet persist during exercise despite high carbohydrate availability. Med Sci Sports Exerc 34: 83–91, 2002.

5. Lambert EV, Goedecke JH, Van Zyl CG, Murphy K, Hawley JA, Dennis SC, and Noakes TD. High-fat versus habitual diet prior to carbohydrate loading: effects on exercise metabolism and cycling performance. Int J Sport Nutr Exerc Metab 11: 209–225, 2001.

6. Helge JW, Richter EA, and Kiens B. Interaction of training and diet on metabolism and endurance during exercise in man. J Physiol 492: 293–306, 1996.

7. Kiens B and Helge JW. Adaptations to a high fat diet. In: Nutrition in Sport, edited by Maughan RJ. Oxford, UK: Blackwell Science, 2000, p. 192–202.

8. "Fat adaptation" for athletic performance: the nail in the coffin?
Burke LM, Kiens B. J Appl Physiol. 2006 Jan;100(1):7-8.