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.
1. Blende, Sunny. (2012). Ultrarunning.com. Retrieved from
2. Fleming, J., Sharman, M. J., Avery, N. G., Love, D. M., Gómez, 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.