That's the advice that some exercise physiologists are giving female athletes, based on the theory that women may be better-suited than men for ultra-distance competitions.

To many athletes, the concept of female ultra-distance superiority makes sense. Since female athletes are often fatter than their male counterparts and since fat tends to be the crucial fuel during ultra-distance competitions, people tend to believe that females are less likely to run low on energy during such competitions. The idea is that with more fat in their fuel tanks, women should be able to exercise for longer periods of time without succumbing to fatigue.

However, this 'fat is better' hypothesis falls apart as soon as it is examined closely. True, female athletes tend to be slightly fatter than males and thus have more fat fuel available per pound of body weight. However, a fair amount of female fat is concentrated in the hips and thighs, where it is difficult to mobilise. In addition, even extremely skinny males have enough fat in their bodies to make it through most ultra-marathons. For example, a 133-pound male with just 3-per cent body fat still has enough lard to run 140 miles using fat as the sole source of fuel! Also, some fat can be ingested orally during an ultra-event, alleviating the need to rely on body lipid stores alone.

In addition, the presence of higher amounts of body fat creates a key disadvantage: even in slow-moving ultra-events, fat still acts as a 'dead weight' which must be carried along while running or cycling - weight which increases an athlete's energy demands but provides no direct propulsive force. The bottom line is that increased body fatness does not seem to offer a direct performance advantage in ultra-distance competitions.

However, having more body fat could assist females in an interesting way: extra fat might mean that female blood-fat levels are higher than those of males during exercise. This surplus blood fat would mean that female leg muscles would have larger 'energy servings' upon which to dine, compared to male muscles, which would be on a comparatively low-fat budget.

Alternatively, blood-fat levels might be equivalent between the sexes, but female muscles might be better able to process the fat brought to them in the bloodstream or might be superior at breaking down the fat which is naturally stored inside muscles (this latter fat is called intramuscular triglyceride). If either scenario - higher blood-fat levels or a greater fat-processing ability - were true, females might enjoy an advantage over males during ultra-distance competitions, because their greater reliance on fat could save precious leg-muscle glycogen.

What the savings could be
During an ultra-marathon, for example, if female fat contributed 60 per cent of the energy needed to run while male fat chipped in just 50 per cent, females would whittle away at their leg-muscle glycogen stores much more slowly. In fact, females could save about three grams of glycogen per mile of running. Over a 60-mile race, this would add up to a savings of over 700 carbohydrate calories, which could be quite significant. In a 100-mile race, the glycogen conservation would be extensive enough to run over 12 miles using only carbohydrate as fuel. Since low glycogen levels are strongly associated with extreme fatigue, superior glycogen preservation in female runners would help them postpone fatigue more effectively than males.

Actual race results support the idea that females might be equal to or better than males at very long distances. Although females seldom win 10Ks or marathons outright, females have won many ultramarathons in competition with males. In 1989, for example, a Californian woman named Ann Trason finished over four miles ahead of the nearest man as she won the USA 24-hour Championships. Trason averaged a not-too-shabby 7:30 per mile for the first 100 miles of the race, and her victory was no isolated fluke; last year women won 12 (out of 225) ultra-races held in the United States (given the lower participation of women in such races, that's pretty impressive)
In addition, informal surveys conducted by Runner's World USA magazine suggest that, compared to male runners, females slow down less as they move from 5-K to marathon competitions, again suggesting that somehow females are better suited for longer distances. It's reasonable to think that women's performances in ultra-events will become even stronger over time, as increasing numbers of women participate in such events.

However, if women are equal to or better than men in ultra-marathons, they must possess some physiological or psychological factor which allows them to overcome the key advantage which males hold - a higher oxygen-carrying capacity in the blood. The key male sex hormone, testosterone, is a potent stimulator of red blood cell production, which is why males end up with higher haematocrits and a greater ability to keep their muscles bathed in oxygen. What might that female-favoring factor be? Does scientific research suggest females are indeed better 'butter-burners?'

The Costill findings
Surprisingly, only a very small quantity of research has been done on the topic. The first high-quality study was carried out in the late 1970s by Dave Costill and his colleagues at the famous Ball State University Human Performance Laboratory. Costill and co-workers studied 13 male and 12 female distance runners who had similar V02max values (60-62 ml/kg/min) and similar weekly training mileages (about 50-60 miles). Both male and female runners had been training for about seven years at the time of the study and had comparable muscle-fibre compositions. Muscle-glycogen concentrations were also identical between the groups.

When the Ball State athletes ran on a treadmill for 60 minutes at an intensity of 70% V02max (about 80 per cent of maximal heart rate), there was wide variability in fat utilization. For some runners, fat supplied only 26 per cent of the energy needed to keep going, while others lit a veritable bonfire of fat, getting 74 per cent of their energy from it. On average, though, fat supplied about 39 per cent of energy at the beginning of the 60-minute ramble and 53 per cent at the end.

However, these differences weren't linked to gender. Males and females burned fat at almost exactly equal rates. Surprisingly, males tended to have somewhat higher blood-fat concentrations than females, although the differences weren't statistically significant. Two problems with the Costill research, though, were that there was no control of menstrual status in the women (menstrual-cycle stage can have a strong impact on fat oxidation) and also no regulation of food intake (eating more fat can raise one's rate of fat metabolism).

What later researchers found
In the 15 years since Costill's initial effort, there have really been just two high-quality studies concerning the effects of gender on fat metabolism during exercise. In research carried out at the University of Tennessee, eight experienced runners (four males and four females) ran for 90 minutes on a treadmill at an intensity of 65% V02max (about 75 per cent of maximal heart rate). Training (number of miles per week) and V02max values were equivalent between the sexes.

Again, fat's contribution to the energy pie increased as exercise proceeded. There was a trend for women to rely on fat more heavily; after 90 minutes of exercise the female runners were getting about 7-per cent more energy from fat than the males.

The best study on the subject was carried out at McMaster University in Canada a few years ago. In that work, menstrual status and dietary intake were tightly controlled, and six males and six equally trained females ran 15.5 kilometres (9.6 miles) on a treadmill at an intensity of 65% V02max (75 per cent of max heart rate) while their rates of fat burning were measured. All of the women were in the follicular stages of their menstrual cycles.

During the run, which lasted from 90-101 minutes, male and female runners had equal levels of blood fat. However, females used significantly more fat and considerably less carbohydrate during the long-duration jaunts. Overall, females burned 428 calories of fat, while males broke down only 242 calories, and females oxidised 548 calories of carbohydrate, while males used up a whopping 960 carbohydrate calories. Muscle glycogen, which was similar between the sexes before the run, was 25 per cent lower in males after the effort. If the test had been a marathon race in which the participants were not allowed to take in carbohydrate as they ran, the females probably would have won.

The females' higher rate of fat burning is particularly impressive when you consider that the McMaster females were in the follicular phases of their menstrual cycles. The follicular stage is actually a time when fat burning during exercise reaches a low ebb. If the females had been evaluated during their luteal phases, their fat-burning advantage over males probably would have increased considerably. In addition to degrading more fat, the females also had higher blood-sugar levels than the males during the exertions, an effect that Dave Costill also detected in his study. Also, there was some evidence that males broke down muscle protein at higher rates. The brain gets into the act
This latter effect is of special interest, because there's some evidence that enhanced protein metabolism during exercise may increase mental fatigue. Basically, as protein is metabolised during long-term exercise, increased quantities of an amino acid called tryptophan enter the brain, where they are converted into another chemical called serotonin, a nerve-cell messenger which increases one's feelings of tiredness. If males routinely break down more protein during ultra-distance efforts, females might be able to sustain higher-quality exercise in the late stages of such races because of reduced mental fatigue.

Some exercise psychologists have also suggested that in general females can handle the mental rigours of ultra-distance competition more effectively than males, because females have a higher ability to cope with extremely frustrating situations. It's clear that the final stage of an ultra-event is almost always frustrating: fatigue mounts to nearly insurmountable levels, it's impossible to exercise at a quality pace, nearly every movement is painful, yet there still may be almost 20-30 miles left to. Females may simply be able to display more perseverance in such situations.

The bottom line is that ultra-distance competitions are events in which females can out-compete men who would beat them rather handily over shorter distances. 'The longer the distance, the closer are the performances of male and female athletes,' says Tracy Horton, Ph.D., of the University of Colorado's Center for Human Nutrition. Horton is currently working on a study which compares fat metabolism in male and female cyclists.

When everything is taken into account, the ability of women to compete effectively with men in ultramarathon running makes good physiological sense. After all, a high oxygen consumption rate of 70-75 ml/kg/min might be needed to win a major race like the Peachtree IOK in Atlanta. Since female runners seldom have maximal oxygen consumption rates above 70 ml/kg/min, their chances of winning such an event are nil. On the other hand, an oxygen consumption rate of 55-60 ml/kg/min might be all that's required to capture a 100-mile competition, and well-trained women can reach that level without too much difficulty The males they're competing with might still have higher maximal oxygen-usage rates, so females would have to work a little harder (at a higher fraction of V02max), but they could do it, especially if their more intense fat fires allowed them to cling to their glycogen longer.

True, if males judiciously used a sports drink or some other carbohydrate source during an ultra-event, it would take away some of the females' carbohydrate-sparing advantage. However, it's important to bear in mind that ingesting carbohydrate will simply delay the negative impact of glycogen depletion; it won't prevent it completely. If male ultra-athletes and female ultra-athletes are both swallowing sports drinks during a 10- to 24-hour race, the males will still hit glycogen rock-bottom sooner.

Owen Anderson