Journal Title (Medline/Pubmed accepted abbreviation): Appl. Physiol. Nutr. Metab.
Page numbers: 12-22
doi (if applicable): 10.1139/H10-089
Summary of article:
Low intensity endurance exercise (>90 min) depletes short term energy storage, i.e. muscle glycogen. There are nutritional approaches to increase and preserve glycogen stores, and increase the efficiency of fat utilization in order to increase performance at endurance events. High-fat, low carbohydrate (CHO) diets have been shown to increase the rate of fat oxidation. “Fat adaptation” is a method by which endurance athletes consume a high-fat, low CHO diet for 14 days before the event. During this time, they maintain normal high and low intensity training. Some athletes prefer to then consume a high CHO diet for 1-3 days before the event (“CHO restoration”). Both fat adaptation and fat adaptation-CHO restoration have been shown to increase rates of fat oxidation and prolong stores of glycogen during the endurance event.
Endurance training is effective in part because it increases mitochondrial volume and therefore the rate of ATP (energy) synthesis. However, it has been shown that fat adaptation does not increase mitochondrial volume and therefore works via a different mechanism than training to improve endurance.
Athletes must keep in mind several factors when implementing the fat adaptation strategy:
- Training on a low CHO diet will be impaired initially due to reduced glycogen stores.
- Regardless of the state of glycogen in the days up to the event, maximal glycogen stores will yield optimal performance the day of the event. It has been shown that at least one day of high CHO intake is required to synthesize muscle glycogen.
- Some athletes do not benefit from the fat adaptation-CHO restoration protocol. In fact, some have noted a decline in performance.
- Most of the research presented in this article was completed on highly trained endurance athletes. It may not apply to moderately-trained or untrained athletes.
There are several proposed mechanisms that may account for the increase in fat oxidation:
- The fatty acid must first enter the cell and then the mitochondrion to be oxidized. There is evidence that the amount of FAT/CD36, a protein that translocates fatty acids from the outside to the inside of the muscle cell, is increased with the fat adaptation diet. In addition, there may be increased activity of carnitine palmitoyl transferase (CPT1), the protein that shuttles fatty acids into the mitochondrion, with the fat adaptation diet.
- The increase in the availability of free fatty acids in the blood “primes” the fat oxidation pathway during rest, therefore having immediate effects at the onset of exercise.
- Fat is stored as triglycerides (3 fatty acids in one molecule) in both adipose (fat) tissue as well as in muscle tissue. Higher quantities of intramuscular triglycerides have been observed after fat adaptation. It is therefore readily available for usage by muscles during exercise.
- Pyruvate dehydrogenase, an enzyme that is key to the CHO oxidation pathway, is downregulated in the fat adaptation diet. Therefore, CHOs are burned at a lower rate and are therefore available for a longer amount of time during exercise. The changes in the rate of CHO oxidation is maintained after CHO restoration.
Interpretation of findings/Key practice applications:
The fat adaptation-CHO protocol appears to be able to modify various indices of both fat and carbohydrate metabolism that, in theory, should benefit endurance exercise performance. However, despite the biochemical changes observed thus far, evidence for actual performance benefits is generally lacking. One potential advantage of this “train low, compete high” approach is that, because no exogenous supplements are required, there is no risk for testing positive for any controlled substances. However, execution of this strategy requires high amounts of self-discipline and it is common for athletes to experience increased tiredness and lethargy during training. It is unknown what effects, if any, moderate adherence to the diet might show. Because some athletes have not reported success with this protocol, it is recommended that athletes try this approach at practice or at a relatively less important event in case it leads to a worse performance. The exact time period for optimal fat adaptation and CHO restoration has not been worked out and may be different for different athletes. It may take several attempts to discover what works best for an individual.