Sunday, March 25, 2012

Dietary Fat in Endurance Athletes

A lot of us, athlete or not, associate negative thoughts with the word "fat."  However, your body needs fat to a certain extent for physiological processes to run smoothly.  Fat is especially important for endurance athletes, and you may not be getting enough.  That's why I decided to write an article review on the topic of fat for endurance performance.  Here's what my teacher said about this-she agrees!  "Great topic! Also, it is important to note that for endurance athletes who have very high kcal needs, fat is a very concentrated source of energy (9 kcals/g compared to carb of 4 kcals/g), therefore, having 30% of kcals from fat will help to meet total kcals needs without requiring larger volumes of food."  So, get at least 30% of total calories from fat!  (If you're an endurance athlete, the general population is less than that 30% because you're not burning enough off, and it will start to stick inside you and cause damage in a number of ways).

                                                   Dietary Fat in Endurance Athletes
           There is much debate in the athletic and scientific community on whether a high-fat or a low-fat diet improves endurance performance.  Supporters of a high-fat diet state that an increase in dietary fat over time will enhance fat burning by boosting the capability to breakdown and move fat around for energy use (4).  However, too much fat in the diet, with 60% or more of calories coming from fat in the diet, can cause adverse health problems, such as heart disease and high cholesterol (4).  Our book also says that restricting calories from fat to dangerously low levels can weaken endurance exercise performance, and the same thing can happen by eating too many calories from fat.  Our book says that it is safe to eat as much as 50% of total calories from fat without developing a decrease in exercise performance or adverse health symptoms, as long as the athlete is burning a significant amount of calories a day, such as individuals who take part in endurance exercise (4).  This diet is not recommended for sedentary people, and even people who are moderately active. 
I found an article from Ultrarunning.com titled “Fats in the Endurance World,” by Sunny Blende, a sports nutritionist.  She claims that in endurance sports, the importance of fat consumption is greatly misinterpreted.  Some athletes totally avoid it while others eat too much of it because they think it will help them last longer and “spare” carbohydrate (1).  I have always been interested in this subject because I am tired of being told I should avoid foods with a high fat content.  After reading the section in our book labeled "High-fat versus low-fat diets for endurance training and exercise performance,” I was even more interested to read on because I have heard both types of those diets are good for the endurance athlete from many different sources.  This stimulated me to find more literature on the topic of how important fat is in an endurance athlete’s diet, as well as find what information was out there from a popular ultrarunning website.  I also found some published articles from reliable sources online, such as the International Journal of Sport Nutrition and Exercise Metabolism.  One article says that high-carbohydrate diets have prolonged endurance exercise performance more so than high-fat diets in the past (2).  However, today, there is increasing curiosity in high-fat diets because the effects include decreased levels of depleted glycogen during training or competition from metabolic changes that help the body use fat for energy both during exercise and at rest (2).  The body, in turn, will hold off on taking energy from carbohydrates so that the fat is burned first, and then the carbohydrates will be used, leading to a longer period to exhaustion. 
“These adaptations include decreased muscle and liver glycogen storage and rate of breakdown, increased gluconeogenesis, increased muscle triacylglycerol storage and utilization, increased mitochondrial oxidative capacity, increased ketone production, and decreased use of glycolysis-derived acetyl CoA” (2).  However, the effects of a high-fat diet on competition are vague, with some studies that show exercise capacity is pro-longed, while others have shown it is not affected, and yet others say exercise capacity actually is reduced (2).  The differences in study findings might be because of variability in fitness level of participants, exercise tests, the balance of the carbohydrate to fat ratio in calories consumed, or the length of the diet study (2).  One study took two groups of 20 recreationally active men who were either placed on a moderate-to-high carbohydrate diet or a high-fat and moderate protein diet (with 60% calories from fat) for six weeks.  Each group took part in a maximal oxygen consumption test, two 30-s Wingate anaerobic tests, and a 45-minute time trial on a bike before and after the intervention.  There was not a change in relative maximal oxygen consumption between both groups, but it was noted that there was significant reduction in absolute terms (L/min) for the group that consumed a high-fat diet.  The values for peak RER were significantly reduced over the course of the intervention period for those on the high-fat diet, but the high-carbohydrate group did not show any change.  Also, in the high-fat diet group, RPE (ratings of perceived exertion) was much higher at the end of the study at minutes 9 and 11. 
Interestingly, there was no noteworthy variation in RPE in the control group. In both groups, changes in overall time were not observed (2).  For the Wingate test, a test of anaerobic power, an all-out sprint test lasting 30 seconds, the high-fat diet group’s peak and average power significantly declined over the course of 6 weeks, but the high-carbohydrate diet group did not change in their results (2).  “During the 45-min cycling bout, work output for the HFMP group was significantly reduced from week 0 to week 6 at 15, 30, and 45 min (Figure 1). These values corresponded with significantly decreased RER values at 15 and 30 min for this group (Figure 2). No change was noted in either of these variables for the control group. Oxygen consumption was unchanged except for a significant decrease at the 15-min time point in the HFMP group” (2).  Also, both groups did not show a significant change in heart rate (2).  “Nevertheless, our results indicate that adaptation to a HFMP diet in non-highly trained men does not improve exercise performance and may slightly impair maximal aerobic capacity, peak power output, and endurance capacity when associated with a small but significant loss in body mass” (2). 
Supporters of the high-fat diet for endurance athletes say that an increase in fat intake overtime will encourage the burning of lipids by increasing the capacity to mobilize and breakdown fat (4).  In a study done on two groups of males who had the same fitness status, one group was fed a high-carbohydrate diet (65% calories from carbs), and the other group was fed a high-fat diet (62% of total calories from fat).  Both groups exercised 3 days a week at 50-85% of their VO2max (aerobic capacity), for 60-70 minutes during the first 3 weeks, and 4 days a week during the last few weeks.  What the study found was that there was an increase in endurance capacity for those who consumed the high-fat diet (115%), however, the high-carbohydrate group was able to increase their endurance substantially (194%) (4).
     Also, our book states that, "Comprised training capacity and symptoms of lethargy, increased fatigue, and higher ratings of perceived exertion usually accompany exercise when subsisting on a high-fat diet" (4).  It is important to remember that there are negative health hazards associated with a high-fat diet.  What I found interesting, though, was that this risk may not be a hazard for athletes who burn a ton of calories each day (like endurance athletes), and consume a higher-fat diet (around 50% calories from fat, not the 62% like in the study).  This will not increase the risk for heart disease or elevated cholesterol levels, if the athlete maintains a healthy or stable body weight and is able to burn many calories each day (4).  Our book also states that, "Conversely, significant restriction of dietary fat intake below recommended levels also impairs endurance performance" (4).  So, I think it is very important that athletes recognize this, especially if they are burning a lot of calories each day through training.  They need fat, and cannot limit it to dangerously low levels because they will not be able to give 100% in each workout or race.  They are still going to have healthy numbers for blood lipid profiles, triglycerides, etc., as long as they are burning enough calories each day, and maintaining their body weight.  I think it is our job as health professionals to help people understand this.  There is so much false information out there in the media.
            There is one more concern that I have, as well as the scientific community has, with endurance athletes consuming low-fat diets.  There are many endurance athletes who restrict their calories on purpose in order to lose weight to improve race performance.  Some may develop disordered eating patterns because of the desire to restrict energy intake to lose weight to improve performance, and lastly, many just do not eat enough calories because there is not a physiological stimulus to eat enough calories to match the calories they lose through exercise (3).  “Inadvertent low energy availability is more extreme when consuming a low fat, high carbohydrate diet. Low energy availability, reproductive disorders, low bone mineral density
and stress fractures are more common in female than male athletes. Functional menstrual disorders caused by low energy availability should be diagnosed by excluding diseases that also disrupt menstrual cycles” (3).  It is very important to eat enough calories from dietary fat in order to avoid these health risks.
            A study from this research paper by Anne Loucks also found that increasing total calories from fat to at least 30% without decreasing the ingestion of carbohydrates can greatly enhance endurance performance.  “Increasing dietary fat from 17% to 31% increased ad libitum energy intake and availability by =30% without reducing carbohydrate intake, and increased 80% maximal oxygen uptake running endurance time by =18% without increasing bodyweight or body fat” (3).  However, she found another experiment, which agrees with our book, saying that a high-fat diet that contains over 60% of calories from fat will hinder endurance performance.  “Increasing dietary fat from 17% to 68% did not improve 100 km time-trial performance in trained cyclists. However, the cyclists obediently consumed the same energy (=51 kcal/kgFFM/day) on both diets as the investigators told them to do, whereas the runners freely consumed what their appetites told them to eat.  Appetite is not a reliable indicator of energy requirements in endurance athletes” (3).  Ad libitum is just the physiological drive to eat, so when athletes were told to increase their fat intake to up to 30% of total calories, this stimulated them to eat enough calories to match their expenditure. 
            From researching this topic, it seems that including at least 30% of calories from fat in an endurance athlete’s diet can help improve endurance capacity, as long as the athlete does not eat more than 50% of total calories from fat, which will weaken their endurance performance.  I was glad to see that this article from Ultrarunning.com was legitimate with the information on fat, because all too often people think that eating low-fat diets will help them lose the weight they want for competition.  However, this is not the case, and it seems that a healthy balance of all three macronutrients, carbohydrates, protein, and dietary fat, without going overboard in total calories, will help one lose or maintain their weight for optimal endurance performance.
 
 References
1.      Blende, Sunny.  (2012).  Ultrarunning.com.  Retrieved from
http://www.ultrarunning.com/ultra/9/9_2/fats-in-the-endurance-wor.shtml.
2.      Fleming, J., Sharman, M. J., Avery, N. G.,  Love, D. M., G√≥mez, A. L., Scheett, T. P.,
Kraemer, W. J., &Volek, J. S.  (2003).  Endurance capacity and high-intensity exercise performance responses to a high-fat diet.  International Journal of Sport Nutrition and Exercise Metabolism, 13, 466-478.
3.       Loucks, A. B.  (2007).  Low energy availability in the marathon and other endurance
sports.  Sports Medicine, 37, 348-352.
4.      McArdle, W. D., Katch, F. I., &Katch, V. L.  (2009).  Sports and exercise nutrition.  (3rd ed.).  Baltimore, MD:  Lippincott Williams & Wilkins.

Thursday, March 15, 2012

Colorado Adventures


Well, I figured I should blog a bit about my trip to Colorado.  I need to keep a blog/journal/whatever on all my adventures and when I have time, need to write about my backpacking trip in the Smokies a few yrs back!

Anyways, me and Mike (my boyfriend) decided to drive to Colorado for my spring break.  Why drive?  Because I had always wanted to take a road trip out West, and it was cheaper than flying!  We left Friday morning around 7:30am (I don't have classes Fri).  We drove 10hrs to Omaha, Nebraska, where we then unloaded all our active gear and suitcases in our hotel, and then worked out for 30min in the fitness center (It was way too windy and cold for my liking to run outside...lol mainly the wind bothered me).  I just couldn't believe how flat the plains are!  Anyways..we ate dinner at a real good steak house in town because Omaha, Nebraska is known for their steak!  The state is also known for creating KoolAid!

Next morning, up early and drove about another 8 hrs until we arrived in Winter Park, CO!  Gosh, it took about an hour to drive up the mountains, it was fun though, with all the twisty-turns around the mountains.  We met my friend we were staying with in Tabernash, CO (like 15min from the resort), to figure out where her house was.  She works at the Starbucks in Winter Park Resort, pretty cool!  Dropped our bags off after we found her house, then went to the grocery store for food for the week, then went to bed.

Sunday....we woke up pretty early (it was like that the whole week, our bodies were on Eastern time), and went to Monarch Lake, part of Arapaho National Forrest.  I had planned to run 10 miles that day for my Glass City Half Marathon training...but only got in 7 (it was hard because I ran uphill for half of it!)  So then I turned around, ran back to the car, and waited for Mike to get back-he cross country skiied while I ran.  Then, we put on our snowshoes that a friend from work let me borrow, and we snowshoed around that whole lake!  I thought I was going to have trouble adjusting to the snowshoes since I have never tried them before, but I had no problem!  I was even ahead of Mike for awhile (haha) until the end, which was rough for me, my glycogen stores were running low.  It was a beautiful day, too, with the sun shining on us, I just hiked in my snowpants and a long shirt.  We were hiking for about 3 and a half hours!  Plus the 7mi I ran, we were hungry!  Went out for Chinese once we got back.

Monday=Snowboarding!  Mike had a lesson for skiing from 9:30am-3:00pm.  He has only skiied 4 times in his life, and with his shoulder surgery awhile back, I figured it'd be a good idea for him to take a lesson out West.  So, I rode all over the place, the whole mountain haha :)  I felt like I was in heaven!  I love snowboarding, it makes me feel so alive, especially being surrounded by the great outdoors.  (Then again, so does every other sport I do!).  I was riding so fast down some of those hills, it was great to get a lot of speed for a long period of time.  Once I got to the "Jane" side of the mountain (where all the locals ride...the tougher side..) I was wowed even more.  It was challenging, but I handled it well.  I avoided moguls because I'm not that great on them, especially when they're on a black diamond.  So I rode the blues and blue-blacks which are just as steep as our blacks back East.  They closed the highest lift, so I didn't get to experience that on Monday.  It goes up to 12,000 feet!  Anyways, Mike did well in his lesson, and I had a lot of fun snowboarding, and we both got pretty sunburned because I forgot to take the sunscreen with me....

Tuesday=more snowshoeing!!  We decided to experience Rocky Mountain National Park since it was less than an hour's driving distance from where we were staying.  We stopped at the visitor center, and the nice lady at the front desk told us about each trail in the area that we could snowshoe.  There were trails that gained a lot in elevation, and those that were more scenic.  I wanted to hike all day, and I wanted to hike this one trail that was about 7 miles long with over 1,000 feet in elevation gain.  It was through the woods and the woman said it wasn’t too scenic, but we were more likely to see animals, like Moose!  So, naturally, I wanted to do the most difficult and long trail (it was for advanced hikers), so we drove to that one.  We started hiking up with our snowshoes, and once I got into the forest a little bit, the snow got deeper and I kept sinking!  So I let Mike go first so he could make holes for me.  He also had poles, and I did not, which probably would have helped me get up faster, but other than that I didn’t want to hike with poles.  We got to the top, and it was so snowy, we weren’t sure where to go next, and the previous footholes had stopped there, so we turned around and went back to the car.  I was actually relieved, because we had hiked for about 1 hour and 20 minutes, and I was getting tired of falling in holes, and trying to get back up in the deep snow with all my snow gear on!  

Next, we went to the most scenic route the lady at the visitor center told us about.  It was actually a road that you can drive up the mountain on, but since there was so much snow, people hiked or used snowshoes on it.  Well…it was all uphill obviously from the start.  I was already kind of tired from falling through snow from the trail before, but I kept up a good pace until about the last 30 minutes.  My heart rate was getting higher from the elevation and climbing.  Mike was way ahead of me.  There were 6 turn-arounds to get to the top, and by the 5th one, I was actually getting angry.  I wanted him to wait for me (that was selfish of me because I know he was getting a better workout and would have had to slow down for me-this is how I think, haha I was ahead of him on Sunday).  So, I plopped down in the snow and started to pout.  I actually started to cry.  I think I just wanted it to be over.  Then, I got up (because I’m strong-willed, and refuse to let nothing go undone that I’ve started) and climbed to the top.  When I saw Mike returning downhill, I asked how much further.  It wasn’t too much further, so I made him hike back up with me.  (haha..he didn’t mind).  Here, I took a Hammer gel because I was pretty depleted, and drank a bunch of water.  My mood got better after that (I get pretty cranky when I’m depleted and hungry).  Then, it was all downhill from there!  Mike stayed closer to me because he noticed I was pretty weak from the uphill climb, and didn’t want me to fall off the side of a cliff.  

We hiked a total of 5hrs and 20minutes, no wonder I was so tired!  I ate a big lunch, too, but gosh was that climb tough after falling through snow for an hour before!  We reached 10,000 feet that day, pretty cool. 
Wednesday we drove to Colorado Springs.  I had to meet with a USAT coach that some of my teachers knew just to chat with him about what I needed to do if I wanted to become a coach or work at the Olympic training center.  After chatting with him for awhile, he told me to apply to a bunch of the internships.  So, I applied to about 7 or 8 so far J  But who knows, I’m not going to worry about it and I just let life take me wherever from now on.  I have ideas, but it’s too early to make a plan.  Life never works out the way you want it to anyways.  That’s why I have adventure :)
 
Mike rode his bike around Garden of the Gods, which was pretty intense, I hear.  Then, he came back, and we met my friend who is an intern there right now for lunch.  I guess Michael Phelps just left last week, darn!  Other than that, the OTC wasn’t anything too special, just a big place to workout.

We had wanted to drive up Pike’s Peak, but then it was almost 3pm by the time we got to the Manitou Incline, so we just hiked that.  OMG-one of the toughest workouts of my life, it reminded me of backpacking up steep inclines in the Smokies.  It climbed 2000 feet in 1 mile.  Took us an hour.  Then, we got to the top of that, snapped a few photos, then started walking down the mountain.  Now, I saw someone start running down it.  I decided to start walking since the climb was tough enough.  I kept slipping all over the place, then tried running.  Running was actually easier to me, so I handed Mike my jacket and flew down the mountain.  It was fun!  I love trail running, actually, because you really have to concentrate and jump over rocks and uneven terrain.  Didn’t fall or slip once while running.  

We snowboarded Thurs and Fri, and Mike got a lot better at skiing :)  I had a real fun time in Colorado, and didn’t want to go back home.  Now I’m here, wishing I was either in Colorado again or back home where all my favorite parks are :)
  
Wow, this is long, and sorry if there’s any typos, but I’m going to bed!  And I will continue to dream about my goals, like running ultras, climbing more mountains, snowboarding more out West, completing long endurance rides, and more triathlons.  Which, by the way, I still haven’t signed up for Nationals because I have no idea what’s going on this summer.  I can’t sign up for that 10k swim either yet, because I don’t know where I’m going.  Another thing-all these races cost $!  I just want a job that can support my side jobs! Haha.  

One more thing-I sometimes wonder what people think of my long adventures or workouts.  All I can say is, it's FUN to me.  My idea of fun is torturing myself.  I don't know why....on that trip I had alcohol twice...whereas some people might enjoy their whole spring break drinking...I enjoy mine being physically active.  It's my favorite hobbie!  I usually don't get bored when I'm out there hiking, running, or cycling or whatever for hours on end.  What else would I be doing with my life?  Sitting in front of a computer screen (I do that enough) or watching TV?  You can learn a lot about yourself out there in the outdoors.  And about the great outdoors, as well as other people.  I've met some pretty amazing people just through running, snowboarding, and triathlons.  Do something with your life!  I may not be elite, but I always hope to help and inspire others to reach their goals and dreams! 

Thursday, March 1, 2012

Can Female Ultra-Endurance Athletes Compete with the Men?

I think anyone in the endurance community has heard this theory before that females may be able to maintain that high intensity of exercise for a very long time...and perhaps even compete with, and beat some of the males.  Especially when you get into ultra-distance events.  More and more females are winning these things.  I received a perfect score on this paper, so I'm hoping I'm on to something here :)  Teacher said she enjoyed  my topic and would love to see more research done in the area!  Hope it helps anyone who reads it!  Ok-here's the paper:

I found it very interesting while reading the chapter titled, “Macronutrient Metabolism in Exercise and Training,” from our book, “Sports and Exercise Nutrition,” to read that trained muscle has an improved ability to catabolize, or breakdown, carbohydrate aerobically for energy use (McArdle, Katch, & Katch, 2009).  This is due to an increased oxidative capacity of the mitochondria and increased glycogen storage.  Trained muscle uses more fat for energy during submaximal exercise, and relies less on muscle glycogen and blood glucose (McArdle, Katch, & Katch, 2009).  Furthermore, when I read the section on gender differences in the training effects of substrate use, I was intrigued by the fact that at an equal percentage of an athlete’s VO2max, such as 80%, the female gender takes a smaller proportion of energy from carbohydrate oxidation than men during submaximal exercise.  After cardiovascular exercise training, women show a greater shift towards fat catabolism, whereas men do not (McArdle, Katch, & Katch, 2009).  This got me to thinking about women endurance athletes, especially female ultrarunners.  Ultrarunning consists of running anything over a marathon distance, from a 50k (31 miles) to a 100 mile race or beyond.  I have heard by many ultrarunner friends, as well as from a few articles over the past couple years about female ultrarunners and how some of them can outrun the men. 
This phenomenon seems to be largely unexplained by science, however, and I was skeptical for the many reasons articles from ultrarunning sources online had given.  I found an article from the magazine, Runners World, titled, “Why Women Rule,” by Lisa JHung.  She states that, “In ultrarunning, not only do women compete on the same course and at the same time as the men, they really compete—and do well—against them” (JHung, 2010).  The article then goes on to talk about some ultrarunning races where certain women have either won, or placed in the top three overall, against both genders.  Lisa Jhung asks the question of what makes it possible, whether physiologically, psychologically, or both, for women to compete at these high levels with the men.  Jason Coop, whom JHung interviewed, is an ultrarunning coach, and thinks that women have an edge in ultrarunning just because of their smaller body size.  As the terrain in the race gets more challenging, the smaller body size of the female runner is a larger benefit.  He says that the more difficult the course is the more of an advantage the female ultrarunner has.  However, Coop states that on the uphills, it may not be a benefit to be a female because their hearts and lungs are smaller in size than the male.  “But the descents—like the 36,000 feet of descending at Hardrock—take less of a toll on a smaller person's body. There's less eccentric work with the legs. Over the course of time, as descents add up that's a huge advantage” (JHung, 2010).  Hardrock is a challenging 100-mile foot race in the mountains of Silverton, Colorado.  Eccentric work is the part of the run cycle in which the controlled muscle lengthens under strain during downhill running; more muscle damage occurs during this phase of running (Eston, Mickleborough, & Baltzopoulos, 1995).  Eccentric work is responsible for a lot of the soreness in runners after a hard workout or race (Eston, Mickleborough, & Baltzopoulos, 1995). 
Coop thinks that women seem to excel the more challenging the race is, like in the Badwater ultramarathon, where temperatures can often reach 120-130 degrees in the desert of Death Valley, California.  He then goes on to explain that there is also a psychological component in women that allows them to have an advantage over men.  Coop states, "But I firmly believe that good women endurance athletes are also psychologically better than good male endurance athletes on the elite side" (JHung, 2010).  A sports psychotherapist, Bruce Gottlieb, agrees with Coop, stating that the perseverance seen in these ultrarunning females may be due to the fact that they often think with greater willpower and persistence, whereas men are more inclined to think in terms of going harder, stronger, and faster (JHung, 2010).  The article then goes on to mention an argument that is often brought up about these ultrarunning women.   This is the thought that females are able to endure the pain of a challenging race for so long just because it is ingrained into their nature due to the ability to give birth.  Whether some of these proposed physiological and psychological explanations are facts or not, I found an article from the Journal of Applied Physiology that may explain why some females can endure such long running events, in terms of substrate use during different exercise intensities. 
In my opinion, Coop’s idea of women seeming to excel over the course of a race the more challenging it is, is due to more of a physiological explanation (although I do not personally disagree with the proposed psychological explanations).  In accordance with our book, this study found that maximal fat oxidation was significantly greater in women than in men, with men showing lower rates of fat oxidation and a greater shift towards using carbohydrate as the primary fuel earlier in their workout.  The primary fuels, carbohydrate and fat, oxidized by the muscle to produce energy when an individual exercises, is a widely known concept (Venables, Achten, & Jeukendrup, 2004).  This can be influenced by the amount of glycogen in the muscle, one’s diet, intensity and duration of exercise, and fitness level.  The article does say that there is still some debate on whether sex differences in fat oxidation exists.  Many studies have shown that fat breakdown (oxidation) in the female sex plays a greater role in oxidative metabolism than men (Venables, Achten, & Jeukendrup, 2004). 
Venables, Achten, & Jeukendrup (2004), gathered 300 volunteers (143 females and 157 males) to participate in their study to assess which substrate was being used during a maximal oxygen consumption test to exhaustion.  The test consisted of participants jogging or running on a treadmill, in which the percent grade and speed were increased every few minutes.  During this time, substrate (carbohydrate and fat) use was measured through indirect calorimetry, which uses a metabolic cart to measure oxygen consumption and carbon dioxide breathed out.  For each participant, the maximal fat oxidation and corresponding intensity at which maximal fat oxidation appeared was measured.  On average, maximal fat oxidation was seen at 48+/- 1% of VO2max, which corresponded to exercising at 62+/- 1% of maximal heart rate.  At about 62% of an individual’s maximum heart rate is when the substrate use transitions over from fat use to mostly carbohydrate use to fuel the working muscles (Venables, Achten, & Jeukendrup, 2004).  When exercise intensity was increased even more, a greater decrease in fat oxidation occurred, at about 84% of VO2max and 89% of maximal heart rate, and a RER (respiratory exchange ratio) value of 1 was reached, meaning the exercising individual was burning mainly carbohydrates (Venables, Achten, & Jeukendrup, 2004). 
In contrast with the women, however, the men who participated in the study burned significantly less fat and notably more carbohydrates.  This is due to their greater fat-free mass and lesser body fat mass as opposed to women (Venables, Achten, & Jeukendrup, 2004).  What I found interesting was that the exercise intensity at which maximal fat oxidation was observed was considerably lower in the males than the females.  “When the oxidation data were expressed as a percentage of total energy expenditure, the contribution of fat oxidation to total energy expenditure was greater in women than the men with a concomitant lower contribution of CHO oxidation to total.  This effect was equal across all exercise intensities” (Venables, Achten, & Jeukendrup, 2004).  What this means is that at any given exercise intensity, women actually use more energy fuel from fat than carbohydrates, whereas men use more carbohydrates for fuel rather than fat.  However, females are still burning more carbohydrates at a high intensity of exercise than fat, but what this article and our book is showing is that they actually are able to burn a greater amount of fat  than men even at higher intensities.  This means that for a long endurance event, the exercising female is able to hold off on depleting their glycogen stores; they use more energy from fat metabolism for a longer period of time than males. 
However, this study did not control for menstrual phase or diet, so it really is unknown how much of the discrepancy in fat oxidation contributes to these elements.  Even though there are studies that found the menstrual phase and oral contraceptives both do not contribute to fat oxidation during moderate-intensity exercise, some studies have found conflicting evidence (Venables, Achten, & Jeukendrup, 2004).  Nevertheless, this study has shown that even though those elements were not controlled for, during exercise, there are sex differences in the type of fuel being burned.  “Also, it is highly likely that a degree of the variation found in fat oxidation could be accounted for by diet because it has been shown that altering the diet, either to a high fat/low-CHO or low-fat/high-CHO diet, can increase or decrease fat oxidation, respectively” (Venables, Achten, & Jeukendrup, 2004).  This also means that as the more trained the individual is to exercise at these moderate and higher intensities for a long duration, the more efficient they are going to be at using fat or carbohydrates for fuel during a certain exercise intensity, whether male or female.  However, the female gender is able to hold off carbohydrate oxidation for a longer period of time.  From reading and learning all of this, I am going to assume that some of these female ultrarunners who can run for so long and beat the men, or come close to it, eat a higher fat diet so they can spare their muscle glycogen for later use when they have burned through all of their fat stores for energy use.  Women have also benefited from the training factor, in which trained muscle uses more fat from energy during submaximal exercise (such as running a very long run).  Men can do this, as well, however not to the same extent as women, as shown by this study and previous literature.  Perhaps this is why some female ultra-distance runners have an advantage over the men. 
In conclusion, this study says that when maximal fat oxidation was corrected for fat-free mass in both genders (since some women have less fat relative to their body weight than their higher fat counterparts), a fraction of the inconsistency across individuals may be due to training status and sex, but not percent body fat.  “Although gender accounts for only a small fraction of the observed total variability in fat oxidation, it is apparent from this study that a gender dimorphism does exist in that women have higher maximal rates of fat oxidation and that lipid remains the dominant fuel at higher exercise intensities than in the men” (Venables, Achten, & Jeukendrup, 2004).  Lastly, the article states that in this study, the explanation for many of the differences across individuals in substrate use at a given exercise intensity cannot be rationalized with the variables that were tested (Venables, Achten, & Jeukendrup, 2004).  One other article I found stated this, “A recent study, however, once again suggests that women may in fact have some way, not yet understood, to burn fatty acids better than men do.  If that’s the case, and you combine that ability with women’s greater body fat reserves, the implications are obvious. If some glycogen-sparing goes on along the way, women might be able to get more out of that premium fuel than men do” (Donaldson, 2011).  More research needs to be done in order to figure out what factors across both genders contribute to an increase in fat oxidation, which could be due to hormones or body fat percentage.  We do know this, however:  women can use more energy from fat metabolism for a greater period of time, sparing muscle glycogen across a long endurance race, which may give them an advantage. 

References

Donaldson, Jamie.  (2011).  The secret feminism of “Born to Run” pt. 1: women and

            ultrarunning.  Retrieved from

            http://fitandfeminist.wordpress.com/2011/09/01/the-secret-feminism-of-born-to-run-pt-1-

            women-and-ultrarunning/

Eston, R. G., Mickleborough, J., & Baltzopoulos, V.  (1995).  Eccentric activation and muscle
            damage: biomechanical and physiological considerations during downhill running. 
            British Journal of Sports Medicine.  29, 89-94. 

JHung, Lisa.  (2010).  Why women rule:  In the sport of ultrarunning, women compete shoulder-

            to-shoulder with the men, and sometimes beat them to the finish line.  Retrieved from

            2,00.html
McArdle, W. D., Katch, F. I., & Katch, V. L.  (2009).  Sports and exercise nutrition.  (3rd ed.).
            Batimore, MD:  Lippincott Williams & Wilkins.