Fats are an important energy source for the endurance cyclist, providing more than 50% of the Calories for activities performed at less than 50% VO2 max. As the level of exertion increases above 50% VO2 max, the percentage of Calories provided by triglycerides decreases to the point that they play only a minor role as an energy source for short distance, maximum performance sprint events (90 to 100% VO2 max).
Almost all fat digestion occurs in the small intestine where these triglycerides are cleaved into their component molecules - glycerol and fatty acids. The fatty acid molecules are then transported through the blood, diffuse through cell membranes throughout the body where they are either directly metabolised as an energy source or reconstituted into triglycerides for storage, mainly in fat cells. (Excess carbohydrates in the diet are also converted into triglycerides for storage in the same cells).
Barry Sears in The Zone has suggested that a diet composed of at least 30% fat Calories, 30% protein Calories, and only 40% carbohydrate Calories will improve competitive aerobic performance compared to the athlete's traditional high carbohydrate diet - which is relatively fat restricted with less than 20 to 25% total Calories as fat.
Although various arguments have been put forward, such as:
There have been two well controlled studies demonstrating that high fat diets (70% fat in one, 38% in the other) increased the exercise to exhaustion time for activities performed at a moderate rate of 50% VO2 max (80 vs 42 minutes of cycling in one, 76 vs 70 minutes of running in the other). Glycogen sparing effects were studied to determine if there was a preferential shift to fat metabolism during exercise, but none were found. A third study tracked Calorie replacement after exercise in two groups (one on a low fat diet and the other eating normal and high fat foods) and found that those on the low fat diet did NOT replace the Calories they had expended during their training program while those on a more liberal fat diet did, suggesting that poorer performance on a multi day low fat diet might be the result of a cumulative Caloric deficit (during the training program) leading to limited muscle glycogen stores at the start of the event rather than to an intrinsic advantage of fat over carbohydrates as a primary energy source for the exercising muscle. The message to be carried away is, once again, one of moderation. Fats are OK, and indeed useful if they are eaten in a balanced diet that encourages adequate Caloric intake for the athlete in training. But there is no proof that, carbohydrate intakes being equal, pushing a high fat diet offers any additional performance advantages.
A number of physiologic studies have proven that fat CANNOT sustain high level (80 - 100% VO2 max.) aerobic and anaerobic activity (remember that the cause of the "bonk" is the shift to fat metabolism as glycogen stores are depleted), and that a high carbohydrate diet is best for replacing glycogen stores post exercise (a chronic deficit in replacing carbohydrates has been proven to lead to chronic fatigue).
Recently a very nice study once again demonstrated that CHO, not fat, is necessary for maximum performance. Seven trained athletes (in a cross over study) rode for 2 hours at 65% VO2 max to deplete muscle glycogen stores (proven by biopsy before and after the 2 hour ride). They then ate an equal Caloric diet which was high CHO (83% CHO, 5% fat) or high fat (16% CHO, 68% fat) for the next 24 hours. Muscle biopsies were again done at 24 hours and demonstrated that the high carbohydrate diet had replenished 93% of the muscle glycogen vs only 13% for the high fat diet, and also that muscle triglycerides were 60% higher in those on the high fat diet. THEN they all cycled at their maximum self paced rate (time trial level of 75 - 80% VO2max) until they had completed a set amount of work (1600 kJoules). The high fat group could not maintain their VO2 and slowly dropped to 55% VO2max while the high carohydrate group maintained at 75 - 80% throughout the ride. And the high carbohydrate group finished at 117min vs 139min for the high fat group - almost 20% better in terms of time.
This study documents that fats do NOT replete muscle glycogen, and it is muscle glycogen that limits maximum performance. Thus fats cannot replace CHO in rebuilding glycogen stores during a training program, and as fats are quite effective in quelling hunger and replacing carbohydrate Calories in the diet, athletes on a high fat training diet run the risk of chronic muscle glycogen depletion and poor performance.
A final question has to do with the role of fats in the 4 hour period immediately preceeding the event. If the training diet has maximized muscle glycogen stores, it appears that a 4 hour pre race meal high in fat is equivalent to one that is high in carbohydrates for endurance activity at 50 to 60% VO2max. This has not been studied for high VO2max events, but at this time there is no evidence that fats offer any advantage to carbohydrates in the 4 hour prerace interval.
What can one take away about fats for training and endurance?
Consider replacing as much saturated fat as possible with mono-unsaturated fats. You also need to be sure to get adequate omega-3 and omega-6 fatty acids -- the essential fatty acids found in many foods, including fish, flaxseed, walnuts, and wheat germ. Use nuts such as almonds and walnuts as a topping in your cereal or add them to salads. Substitute monounsaturated (olive oil, canola oil, and avocado) for saturated oils and eliminate trans-fatty acids.
As the negative effects of an extremely high fat diet on health are well accepted, carbohydrates are clearly superior to fats for high intensity events (both for training and on event day), and fats may, EVEN AT THEIR BEST, only be equal to carbohydrates for lower intensity, endurance events, there is no reason to emphasize fats in a training or day of event dietary program. And for those who still aren't convinced, it should be remembered that even the leanest athlete has plenty of stored fat available (approximately 100,000 Calories worth in a 70 kg male) without any need for diet supplements.
A variation on this theme is reflected in fat containing energy bars which are alleged to improve performance by SELECTIVELY increasing fat metabolism. While there has been some evidence that an occasional long slow recovery ride in your training program MIGHT improve the ability to metabolize or use stored fat Calories for muscle energy, there is no scientific basis for the claims made by these products that eating any particular food or food type (i.e. fat) will enhance fat metabolism.
An healthier alternative to eating more fat would be to focus on a training program that stresses more miles at a relatively slow pace (60% VO2 max.) to improve the muscle cells ability to use internal fat stores. Another variation on this theme is to avoid carbohydrates in the pre ride meal, and minimize carbohydrate snacks while on that long slow ride to "force" the development of metabolic pathways that use fat energy (a planned "bonk' if you will). The idea, or so goes the theory, is that when it comes time for that sprint at the end of a competitive event, having used more of you fat stores for muscle energy along the way, there will be more muscle glycogen remaining to give you the edge. Friends have told me that the agony of training on a low CHO diet is not worth it - so you will have to make up your own mind as to the benefits of this type of "metabolic training".
TYPES OF FAT (vegetable vs animal - saturated vs non saturated)
READER'S QUESTIONS
Q.I am trying to chase nutritional information down to primary references. For example, you and many others state that one pound of body fat = 3,500 calories, but give no reference. I calculate 9 calories/gram of fat x 453 grams/pound = 4046 calories per gram of fat.
A.I did find this footnote in Understanding Nutrition 4th edition, 1987, by Whitney and Hamilton; West Publishing Company (a text on nutrition) on page 90. "The reader who knows that 1 pound = 454 grams and that 1 gram of fat = 9 Calories, may wonder why a pound of body fat doesn't equal 9 x 454 Calories. The reason is that body fat contains some cell water and other materials; it's not quite pure fat."