Even if three team members perform at their full potential, one runner "bonking" can ruin an otherwise strong team showing. How fast athletes start, how fast they press or vary their pace, and how often they rest, determines not only how well they finish, but if they finish at all.

Therefore, pacing is essential to the success of the team. The maximum rate of pace in this extreme endurance event is proportionately related to the efficiency in which an athlete converts his or her body fat into the energy cycle. Too fast a pace makes body fat inaccessible. Too slow a pace reduces the maximal performance.

The Role of Body Fat
The relationship between the ideal body fat level and endurance fitness varies with each sport. Runners and cyclists prefer to minimize their percentage of body fat. Muscle is twice as dense as fat, weighing almost twice as much as the same volume of fat.

For endurance athletes, fat is essentially "dead" weight, requiring energy to move it, but providing little immediate contribution the body's power output. Therefore, the same power output generated from the legs will allow an athlete to jump higher or go further with each stride, provided he or she has less fat.

The Body Mass Index
Body Mass Index (BMI) is a formula for evaluating height and weight values to examine risk for weight-related health problems. The lower a body's BMI, the lower the risk for overall health problems. Federal guidelines on the identification, evaluation, and treatment of overweight and obesity in adults define "overweight" as a body mass index value between 25 and 29.9; and "obesity" as a body mass index value greater than or equal to 30.

BMI is a predictive measure of performance. In high intensity aerobic sports where weight is a factor - such as cycling, running or swimming - athletes attempt to reduce their body fat. Among elite pro-cyclists, the best climbers are in the 19 to 20 BMI range, though the best sprinters with higher muscle mass tend to register BMI's slightly higher in the 23 range. The last 3 winners of the Tour De France recorded BMI's in the 21 to 22 range. Elite marathon runners gravitate toward BMI's between 18 and 19.

Track and field sprinters register toward higher BMI's than distance runners. In one study of 174 female Olympic runners, endurance runners (3000 meters up to the marathon) were measured at 11 pounds lighter - controlled for height - than sprinters (200 meters and 400 meters).

A consistent trend of decreasing Body Mass Index with distance running was also observed in the study. Adjusting for height, marathon runners were over 15 pounds lighter than 100-meter sprinters. [1] Though less than the population average, elite swimmers tend to carry higher fat levels than most athletes, which increases their buoyancy. Reducing the amount of work needed to keep the swimmer afloat, outweighs the extra metabolic cost of maintaining too much fat tissue.

There is a threshold of too much body fat in the better swimmers. Football lineman or sumo wrestlers use their higher body fat levels in specific role in their sport, but they too must observe ideal body fat to muscle mass balance for sport specific tasks.

The Role of Body Fat in a 300-mile Jungle Expedition
The endurance Eco-Challenge athlete is eager to burn more fat during exercise, hoping to conserve muscle-glycogen carbohydrate stores. Why? Compared to our body's limited carbohydrate stores, fat-fuel reserves are plentiful. Fat tissue is a fuel requiring both oxygen and blood circulation.

However, with less fat, the oxygen and blood demands that the fat tissue stay alive and redirect to the muscles, to facilitate working at a higher power output. Performance in aerobic-dependent sports is limited by each athletes' maximal rate of oxygen uptake (V02-max).

Every healthy athlete stores between 70,000 and 100,000 kcal of energy in body fat tissues, mainly in the peripheral adipocytes. Highly trained athletes - who have little adipose tissue - have fat stores that far exceed their athletic needs. Although most fat is stored in adipose tissue, trained endurance athletes have small important amounts of triglycerides within their muscle cells. Active muscle mass may contain up to 300 grams of fat, most stored within the myocyte as small lipid droplets.

As a stored source of energy, fat has an advantage over carbohydrate because the energy density is higher while the relative weight is lower. Fatty acids provide more Adenosine Triphosphate (ATP) per molecule than glucose. However, to produce the equivalent amount of ATP, the complete oxidation of fatty acids requires more oxygen than the oxidation of carbohydrate. [2] Theoretically, even the skinniest endurance athlete has enough body fat to sufficiently generate muscle movement for completing a 700-mile walk - enough body fat to ride a bike from Los Angeles to Dallas on water and electrolytes.

Slow Pace is Key to Success
Low-intensity walking strongly stimulates lipolysis (fat breakdown for energy cycle) from peripheral adipocytes (fat stores), while intramuscular triglycerides contribute little or nothing to total energy expenditure [3]. The rate of carbohydrate use is low: carbohydrate needs are met predominantly by circulating blood glucose, with little or no muscle glycogen breakdown.

The rate of appearance of fatty acids into the plasma peaks during low-intensity exercise (25 to 30 percent of VO2-max), then declines if exercise intensity increases. In contrast, the rate of fat oxidation is highest during moderate activity such as easy jogging (65 percent of VO2-max). As low-intensity expeditionary exercise continues beyond 90 minutes, the pattern of substrate metabolism changes little relative to the first 20 to 30 minutes of exercise.

The rate and fuel selection is roughly 60 percent body fat, 30 percent muscle glycogen and 10 percent lean muscle mass aminos at an easy 25 to 65 percent VO2-max rate. Carbohydrate from muscle glycogen stores is nearly all-spent during the first 90-120 minutes of aerobic exercise, while lean muscle mass protein donations are limited and minimal, signifying body fat stores as the prevailing endogenous fuel required for the remaining 9,960 minutes.

The caloric expense for traversing the Eco-Challenge 2000 course ranges from 37,000 to 45,000 calories. How fast the athletes push themselves determines the overall performance efficiency. The primary determinant of performance in all endurance sports is the ability to produce and efficiently utilize large amounts of energy at an extraordinary fast rate for prolonged periods of time.

With 70,000 to 100,000 body fat fuel calories available, the rate of exercise ranks at least as high as sleep-enhanced navigational accuracy or sport-specific training the body for maximal performance.

To access and metabolize the body fat stores, a minimal amount of circulating blood glucose or muscle stored glycogen must be available. If a team pushes too fast, muscle glycogen levels may dip so dangerously low that metabolism of fat fuels is inhibited, especially those in the slowest athlete. By contrast, going too slow may permit optimum use of carbohydrate muscle glycogen with fat fuels, but it may also prolong the team's finish time. Like going out too fast in the quarter mile footrace, the rate of pace dramatically effects individual energy endurance performance.

Faster Versus Slower and the Cause of the Dreaded "Bonk"
Each team needs to know and hold to a pace that minimizes the time required to go from point A to point B. The faster the four athletes go, the more calories they burn. The more calories they burn, the sooner the slowest team member "bonks." When increasing the pace, a proportionate increase in caloric expense is required fuel muscle movement in time and space:

Swimming - (20 yards/min) = 0.032 calories/lb./min

Swimming - (50 yards/min) = 0.070 calories/lb./min

Bicycling - (15 mph) = 0.049 calories/lb./min

Bicycling - (25 mph) = 0.139 calories/lb./min

Running - (5 mph) = 0.061 calories/lb./min

Running - (10 mph) = 0.114 calories/lb./min

Each athlete progressively trains his or her body to adapt to peaked-performance demands from an extraordinary fuel depletion event. Endurance exercise performance is directly related to the ability to apply a high fraction of VO2-max (individual inherent & trained oxygen capacity generated into the energy cycle) with minimal accumulation of blood lactate. The maximum VO2-max percent sustainable in endurance events is dependent on the race distance and individual lactate threshold (LT) limits.

The lactate threshold is defined as the highest exercise level or level of oxygen uptake that is not associated with an elevation in blood lactate concentration above the pre-exercise level. Exercise at an intensity above LT reduces endurance time due to metabolic acidosis and accelerated glycogen depletion, therefore the successful endurance athlete is often characterized by the ability to perform high amounts of work at or just below LT.

The LT reflects the degree of muscular stress, glycogenolysis and fatigue, and is specific to the mode and rate of exercise. Training-induced increases in capillary density, size and number of mitochondria, as well as the levels of various enzymes and transfer agents enhance aerobic metabolism and theoretically have the potential to alter LT. Because these adaptations occur peripherally, training to increase LT must be performed separately for each type of exercise.[4, 5, 6, 7]

The Conversation Pace Formula
Body Fat helps performance when it is burned in a sparing fashion. In contrast, dead body fat weight lugged and tugged over 300 miles of forbidding terrain is a hindrance when the pace is too fast for fat fuel transfer into the energy cycle. The term "conversation pace" has been coined for marathon runners, suggesting if they can talk in complete sentences without gasping for a breath of air, access to burning fat at an ideal rate permits completion of a 26-mile marathon in the best possible time.

With the additional 274 miles, one must modify the "conversation pace" formula to fit the Eco-Challenge athletes. Ideal fat for fuel physiology indicates that one must lengthen talk time to complete chapter-length paragraphs; this standard includes meals and sleep breaks for tempering metabolic fuel imbalances while negotiating the upper limits of the ultimate endurance experience.

Save it, don't slave it; train it, don't strain it; spare it, don't pare it; pace it, don't race it; finish it, don't diminish it -- these are the key dictums in optimal fat fuel metabolism, factors that may forecast the finish.

REFERENCES [1]-Age, height and weight of female Olympic finalists in running events. Khosla T, Br J Sports Med 1985 Dec 19:4 214-6. [2]-Fat burning during exercise: Can ergogenics change the balance? Hawley JA, The Physician & Sportsmedicine, 1998, 26;9. [3]-Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration, Romijn JA, Coyle EF, Sidossis LS, et al., Am J Physiol 1993;265(3 pt 1):E380-E389. [4]-Coyle EF (1995): Integration of the physiological factors determining endurance performance ability. Exercise and SportScience Review 23: 25-62.

[5]-O�Toole ML and Douglas PS (1995): Applied physiology of triathlon. Sports Medicine 19: 251-267. [6]-Sleivert GG and Rowlands DS (1996): Physical and physiological factors associated with success in the triathlon, Sports Medicine 22: 8-18. [7]-Can Endurance Running Training be Transferred to Cycling Performance, or Vice Versa? JHW Chau, Exercise Physiology 652 (1999) Topic for Critical Evaluation.