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Posted: October 22, 2004
Multisport: Fat As Fuel - What value are medium-chain triglycerides as an energy source for endurance athletes?
Most endurance athletes and coaches, especially if they read Peak Performance, know how important it is to maintain the body's carbohydrate (glycogen) stores. They are also probably aware that, although the depletion of these stores may be delayed to a degree by consuming carbohydrate during activity, the body's glycogen stores can only provide enough energy for about 90-180 minutes before fatigue sets in. It is when these stores are exhausted that runners 'hit the wall' or cyclists 'bonk' and are forced to slow down. Thus it is clear that during prolonged exercise, such as cycle road racing, energy has to be derived from an additional source.
The other main source of energy during exercise is fat. As a fuel, fat could be considered to have several advantages over carbohydrate, but from an exercise perspective it also has several disadvantages. It advantages are that it yields over twice as much energy as carbohydrate (9 kcal v 4 kcal) and that the body potentially has an abundant supply of it. Typically, the body has approximately 50-60,000 kcal of energy stored as fat compared to only about 1500 kcal of glycogen. Given this huge source of potential energy, is there any point in considering using fat as an ergogenic aid?
One of the effects of endurance training is to make the body more efficient at using fat as fuel. However, despite the abundant store of this substrate, it is not totally clear where the fat used in exercise comes from. Most fat in the body is stored in the form of triglycerides in adipose tissue. However, some fat, known as intramuscular triglyceride, is stored directly within the muscle fibres. It is now believed that the greater utilization of fat by trained endurance athletes is from this source. This has a very important implication, because the amount of energy which can be derived from intramuscular triglyceride is thought to amount to only 2000-3000 kcal.
In low-intensity exercise, it is believed that almost all the fat utilized as fuel is fat stored in adipose tissue, of which, as I've mentioned, there is a plentiful supply. This is slowly broken down and transported in the blood (plasma FFA) to the working muscle. As the intensity of exercise increases to competition levels, the contribution from plasma FFA decreases, and the contribution from intramuscular triglycerides increases, to a point where it accounts for about half of the total fat used.
Depletion of the intramuscular triglyceride stores is now thought to limit endurance performance in much the same way as glycogen depletion. It has been shown that the replenishment of glycogen stores alone is not enough to restore maximum work capacity. Therefore, it may be just as important to restore, or strive to spare, the body's intramuscular triglyceride. This may be of little consequence to athletes completing in single-day events, but for cyclists competing in stage races over several days or weeks, it has considerable implications. Unfortunately, it is not totally clear how to preserve or restore these fat stores.
1 extreme diets consisting predominantly of fats, a nutritional strategy which has been successful with rats but has shown no performance benefit in humans
This last practice, where cyclists are fed 'intravenous triglyceride emulsions', is potentially harmful and needs to be carefully monitored. In 1991, an entire team quit the Tour de France after ingesting a 'contaminated' batch of fat emulsion. Although ingesting fat intravenously during rest periods in stage races may well speed the replenishment of the intramuscular triglyceride stores, this technique would be considered somewhat extreme for most athletes.
The body's limited use of carbs
This leaves fat as another potential ergogenic aid, but ingesting 'normal' fat has several major disadvantages from an exercise point of view. For one thing, it can only be converted into energy slowly, taking about 3-4 hours to pass even into the bloodstream. For another, it also has an inhibitory effect on gastric emptying, and thus will delay fluid and carbohydrate uptake.
This brings us to the comparatively new substance, medium-chain triglycerides (first discussed in PP three years ago), which has been claimed will spare glycogen and by implication spare the body's intramuscular triglyceride stores as well. MCTs have been advocated as a fuel that could be used as an alternative to carbohydrate.
What exactly are MCTs?
1 They mix easily with liquid and have a smaller molecular size than ordinary fat. This means they are absorbed rapidly into the intestine
Do they actually work?
1 MCTs are rapidly emptied from the stomach and available for use as fuel during exercise
Though our first report in September 1994 suggested that MCTs might prove to be the 'perfect dietary fat for endurance athletes', current opinion is still that carbohydrate supplementation is the best policy during endurance events. The use of MCTs is sound in theory, and they may yet prove to be an important source of fuel during exercise when combined with carbohydrate. Although the benefits of MCT supplementation in tests during simulated competition have not proved statistically significant, it must be remembered that many athletes are looking for gains in performance which may be smaller than this level of difference. Because MCT use is limited by the amount that can be tolerated without causing stomach upsets, it has been found that it can only contribute between 3 and 7 per cent of the total energy expenditure during endurance exercise. Yet it could be that even this small contribution may be enough to sometimes make a difference. For instance, could such a contribution lead to the saving of enough glycogen to power a winning burst for the finish line?
Although carbohydrate looks set to remain the king of (legal) ergogenic supplements, the jury is still out on whether MCTs will have an important role to play in the future.
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