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Journal of the American College of Nutrition, Vol. 19, No. 3, 345-350 (2000)
Published by the American College of Nutrition

Review

A Perspective on Fat Intake in Athletes

David R. Pendergast, EdD, FACN, John J. Leddy, MD and Jaya T. Venkatraman, PhD

Department of Physiology and Biophysics (D.R.P.), University at Buffalo, Buffalo, New York
Sports Medicine Institute, Department of Orthopedics (J.J.L.), University at Buffalo, Buffalo, New York
School of Medicine and Biomedical Sciences; Nutrition Program, Department of Physical Therapy, Exercise and Nutrition Sciences, School of Health Related Professions (J.T.V.), University at Buffalo, Buffalo, New York

Address reprint requests to: David R. Pendergast, Ed.D., Department of Physiology and Biophysics, School of Medicine and Biomedical Sciences, 124 Sherman Hall, 3435 Main Street, Buffalo, NY 14214.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 BACKGROUND
 CONCLUSIONS
 REFERENCES
 
Performance in endurance events is dependent upon the maximal aerobic power, the percentage of that power that can be sustained and the availability of substrates (carbohydrates [CHO] and fats). The purpose of this paper is to present a perspective of recent studies that demonstrate the role of fat intake and oxidation on endurance performance. Studies have shown that fatigue is associated with reduced muscle glycogen and that increasing muscle glycogen or blood glucose prolongs performance while increasing fat and decreasing CHO decreases performance. This has led to an emphasis on CHO intake in athletes in endurance sports, which quite often leads to low caloric intake. It is well known that trained subjects have higher levels of fat oxidative capacity, which spares glycogen during endurance sports. Data from recent studies in trained athletes, who were fed iso-caloric high-fat diets (42% to 55%) that maintained adequate CHO levels, have shown an increase in endurance in both men and women when compared to diets composed of low fat intake (10% to 15%). The magnitude of the effect on endurance was significant at high percentages of maximal aerobic power and increased as the percentage of maximal aerobic power decreased. Based on this review, a baseline diet comprising 20% protein, 30% CHO and 30% fat, with the remaining 20% of the calories distributed between CHO and fat based on the intensity and duration of the sport, is recommended for discussion and future research.

Key words: dietary lipids, exercise, endurance, fat oxidation, athletes, dietary carbohydrates

Key teaching points:

• Intramuscular glycogen and/or fat depletion result in muscle fatigue.

• A diet that is low in either fats or carbohydrates will result in muscle fatigue.

• Balancing total caloric intake to caloric expenditure is a critical issue in sports nutrition.

• The caloric intake of fats and carbohydrates, above that needed for rest, should be determined by the caloric expenditure of carbohydrates and fats during the exercise.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
BACKGROUND
 CONCLUSIONS
 REFERENCES
 
Athletic performance is determined in part by the energy cost of performing the sport and the ability of the metabolic system to provide the rate and amount of energy needed. For longer-duration activities (over 30 minutes), such as long-distance running, the percentage of sustainable metabolic turnover, the capacity of the available energy sources (stores/reserves) and body composition are the primary factors contributing to performance. The major determinants of fat oxidation are the intensity and duration of the exercise as well as the diet and training status of the athlete.

The role of carbohydrates (CHOs) during exercise has been well evaluated. Based on published data scientists have concluded that CHOs supply most of the energy required during higher intensity endurance exercise and that athletes should keep fat intake to very low levels, sometimes to as low as 10% to 15% of daily calories [1,2]. The role of fats in exercise has not been well studied, is therefore not well understood and deserves further evaluation and study. The purpose of this paper is to present selected recent papers that examine the relative role of fat oxidation in supplying energy during longer duration exercise and contrast them to selected previous studies emphasizing the role carbohydrate metabolism. Hopefully the perspective developed in this paper will stimulate further discussion and research to determine the optimal CHO and fat intake for athletes.


   BACKGROUND
TOP
 ABSTRACT
 INTRODUCTION
 BACKGROUND
CONCLUSIONS
 REFERENCES
 
The time to cover a given distance in endurance runs is dependent upon the energy cost to cover a given distance and the percentage of the maximal aerobic power than can be sustained over the distance. Factors such as training status, hydration status as well as psychological and motivational factors may also modulate performance. The % of o2max that can be sustained for the event distance decreases as the race distance increases and is higher in well-trained subjects. Implicit in this equation is the availability of substrates, particularly carbohydrates and fats, which can be released into the blood after ingestion, released from storage depots or stored intramuscularly. Thus, overall, the stores of CHO and fat, maximal aerobic power and the rate of utilization of CHO and fat are among the factors that determine endurance performance.

Over the past 30 years the emphasis of research has been primarily on CHO metabolism. Studies have demonstrated that, as exercise intensity increases, CHO oxidation increases while the oxidation of fats decreases [1,3,4]. This shift is due to the abundance of glycolytic enzymes, the limited rate of mitochondrial fat oxidation and the shift to fast glycolytic muscle fibers at high exercise intensities [4]. This has been demonstrated by an increase in the expiratory gas exchange ratio RER (co2/o2) with greater exercise intensity so that above about 80% of o2max the substrate providing energy is predominately carbohydrate [4]. Furthermore, as exercise continues at a fixed level of o2, the RER decreases. The low RER reflects muscle glycogen depletion, while the level of fat oxidation remains constant [1,3]. Under these conditions, the sustainable exercise intensity inevitably decreases, and the athlete becomes fatigued. Indeed, previous studies have correlated fatigue in long-duration exercise closely with depletion of muscle glycogen stores [2,5]. In fact, near complete depletion of muscle glycogen [6] has been demonstrated after ultra-endurance exercise. It has been shown that glycogen stores can be increased by increased dietary carbohydrate intake. "Glycogen loading" has been shown to enhance endurance performance [7,8]. Blood-borne glucose and fat cannot enter muscle cells at a great enough rate to supply energy meaningfully at exercise intensities above approximately 40% of o2max [9]. Yet, despite the low flux rate of glucose from the blood to the mitochondrion, it has been shown that consuming carbohydrates during endurance exercise increases capacity, perhaps due to maintaining blood sugar levels to prevent central fatigue [10].

Based on the studies described above, the general consensus is that CHOs, specifically intramuscular glycogen stores, are the primary substrate for endurance exercise and that high-CHO diets (60% to 70%) improve, while high-fat diets (60% to 75%) compromise, endurance performance [2]. Previous studies showed reduced endurance performance when subjects consumed high-fat diets as intramuscular glycogen was compromised [4,5,11–14]. Fats have been considered inconsequential or even detrimental to elite endurance exercise performance. As a result, athletes have been advised to eat very low-fat diets (10% to 15% of daily calories), which is, in fact, below the level recommended for all Americans [15]. A recent review paper concluded that fat supplementation is not only not beneficial for athletes, but in fact is detrimental to athletes’ health and performance [2]. However, due to the long history of concentrated work on CHOs and the lack of a reliable methodology for analyzing and quantifying lipid metabolism, it has not been given careful enough analysis.

In spite of the position presented above, there are several reasons to re-evaluate the role of fats in endurance exercise. A closer examination of these studies reveals that the high-fat diets were also very low CHO diets; thus, the intramuscular levels of glycogen may have been extremely low at the beginning of the endurance exercise [16]. The low glycogen content was either due to a relatively low intake of CHO (% of daily calories) or to a low total caloric intake and thus a low absolute (g/day) CHO intake. It is axiomatic that glycogen depletion will stop exercise if muscles begin exercise with a low glycogen content. Well-trained humans, however, improve fat utilization during exercise [4]. Therefore, if fat intake is increased, while maintaining sufficient CHO intake, it is possible that endurance exercise time could be improved, provided that enhanced fat oxidation allowed the muscle to spare glycogen [17–21]. In this scenario, we envision parallel depletion of intramuscular glycogen and fats; both would therefore last longer and performance would improve.

When exercising at equivalent intensities, highly trained subjects have lower RER values than untrained subjects, suggesting greater fat oxidation during exercise in the trained [1,3]. The rate of reaction of the fat oxidative enzymes and muscle lipoprotein lipase activity (which stores fat in muscle) [22–25] are significantly greater in trained than in untrained subjects [5,26]. Other studies have shown that exercise fat mobilization from stores is similar in fit an unfit subjects, yet fat oxidation is significantly increased in fit subjects [25]. A low-fat diet that reduced intramuscular lipid stores could inhibit optimum performance in highly trained athletes [27–30]. It has been shown that intramuscular fat stores are reduced after endurance exercise [6,22,30–32] and, during ultra-endurance events, intramyocellular lipid stores are almost completely depleted [6]. Furthermore, low intramuscular fat may limit exercise performance [6,14]. Therefore, if the intramuscular triglycerides and the intramyocellular lipids (fats in contact with mitochondria) are depleted by a low-fat diet, one could speculate that endurance exercise performance could be compromised, just as it has been shown for low intramuscular glycogen on a low CHO diet.

Recent studies have shown that the intramuscular triglyceride content can be increased by a high-fat diet that follows an exercise program, even if there was no depletion of intramuscular triglycerides during the endurance exercise [33]. If fatty acids are not oxidized in the mitochondria, they are esterfied in the intramuscular triacylglycerol pool [25]. An examination of these data suggests that a high-fat diet may increase the intramyocellular fatty acid pool (fatty acids in lipid droplets in contact with mitochondria) [34]. One could hypothesize that a diet that was calorically balanced and high in fats, yet does not compromise intramuscular glycogen, would improve maximal aerobic power and prolong endurance exercise capacity. Several recent studies in humans [35–45] have demonstrated significantly improved prolonged exercise time when high-fat diets were compared with low-fat diets.

The dietary caloric intake of athletes should balance their high caloric expenditure in terms of both total calories and calories expended from CHOs and fats. The respiratory quotient (RQ) reflects the balance of fat and CHO use, assuming protein metabolism during exercise is minimal [46]. If carbon dioxide stores are constant, the expiratory gas exchange ratio RER, measured at the mouth, is a good estimation of the respiratory quotient. RER’s of between 0.8 and 0.9 have been measured for exercise intensities of 60% to 75% of o2max [40,35, respectively], and an RER of 0.93 for 80% of o2max [43]. Although the percentage of fat oxidized decreases with increasing intensity, there is still significant fat oxidation at high exercise intensities as the overall rate of oxidation is increased. RER is constant throughout the entire running time (45 to 120 minutes) at all levels of o2 from 60% to 80% o2max [35,36,40,43]. This indicates that the balance between CHO and fat oxidation is set by exercise intensity and not by exercise time and that the intramuscular stores of fat and CHO would determine maximal endurance exercise time at a given % of o2max.

Exercise training is generally associated with an increase in maximal aerobic power and endurance [3]. It is also generally agreed that the maximal potential for aerobic power and endurance is set genetically [3]. Any examination of the effects of dietary fat intake has to be considered in light of potential training effects. In studies of 25 male and female middle-aged endurance runners, with a long history of running, o2max did not significantly improve while running 40 to 50 miles per week for six months [44]. This same group of runners, when put on a high-fat diet, had a 3% to 8% increase in o2max (not significant) without a significant change in the peak heart rate observed in the max o2 test [47]. In a study on young elite endurance runners, a high-fat diet compared with a low-fat diet significantly increased o2max 8% to 12%, without a significant increase in the peak heart rate during the o2max test [35]. It should be noted that the diets in these two studies [35,43] were not randomized and cardiac output was not measured, so further work in this area is needed. Future work may also focus on the role of body fat distribution and the percentage of slow twitch oxidative muscle fibers in determining fat oxidation and endurance performance.

Muscle fiber composition is important, as the potential for increased fat oxidation applies to slow twitch oxidative fiber metabolism. Thus, the higher the percentage of these fibers, the greater the potential benefits from the high-fat diet. It has recently been shown that experienced male runners (n=6, age=35±5 years), training 35 to 65 miles per week, consuming high dietary fat for one month (42%) had significantly increased in the volume density of total lipid in the muscle, without significant changes in the volume density of total mitochondria or total body weight and percent body fat [48]. Many athletes consume low calorie and low fat diets as a mechanism to reduce both fat intake and body weight [23,28]. Low caloric and/or fat intake diets may result in low levels of intramuscular fat stores that compromise performance.

Several recent studies that examined the effect of increased fat intake on endurance performance are presented in Table 1. Subjects with reduced energy intake (500 to 800 kcal/day below estimated expenditure) [35,43,49] have been shown to have reduced endurance exercise time. Increasing the caloric intake to match expenditure using CHO significantly increases the time to exhaustion by approximately 20% at exercise intensities of 70% and 80% of o2max [35,43,49]. Increasing the total caloric intake to meet expenditure using fat [49] by keeping subjects on an isocaloric diet, while increasing the percentage of fat to 30% and to 42% (from 15%) of total calories brings about a further significant increase (40%) in endurance time [35–40]. There is an improved endurance time even at 125% o2max [50], when athletes (with higher levels of fat oxidation) are compared to sedentary subjects (with lower levels of fat oxidation). The greatest improvement in endurance time occurred at 65% of o2max [36,40], with the degree of improvement decreasing as oxygen consumption increased (Table 1).


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  		Table 1. Description of the Protocols Used in Selected Studies that Varied Dietary Fat Intake and Determined the Effects on Performance

 
It is important to note that in the studies demonstrating increased endurance running time on high-fat diets [35–40] that fat accounted for 30% to 45% of calories and protein 15% to 20% of calories. CHOs thus remained at 35% to 40% of total calories, and the caloric intake was balanced to expenditure. The latter fact is important, as the calorically balanced diets (CHO and fat diets) had 500 to 800 more calories per day than the athletes’ normal diet [23,25,43,49].

The experiments cited in Table 1 that studied exercise at 75% and 80% of o2max, used equal numbers of men and women (12 each) [43]. The data at 60% of o2max were collected during running [40] and on a cycle ergometer [36] and the remaining data at 70% to 80% o2max on a treadmill [35,43,49]. The subjects studied at 75%, 70% and 65% of o2max on high-fat diets were young (20s) [35,40,49], while the subjects studied at 80% of o2max on high-fat study were older (30s and 40s) [43]. The subjects in the exercises at 65% [40] or 80% [43] o2max were less trained (lower o2max) than the subjects in the other trials [35,36,43,49]. Although the diets for the 70% and 80% of o2max studies were not randomized [35,43], the 75% o2max study was a crossover design [49], while the diets in the 65% o2max study was randomized [36]. Whether the studies were serial, randomized or crossover designs, all studies reported similar improvements in endurance. The studies shown in Table 1 were short-term (one week) and long term (two to four weeks), and all studies demonstrated similar increased endurance on high-fat compared to low-fat diets. A schematic representation of the potential improvements in endurance exercise from eating a calorically balanced high-fat diet, which maintains CHO and protein levels, is presented in Fig. 1. This figure is a summary of the data from Table 1 for subjects on a "normal" diet, subjects who supplement the diet with CHOs and, finally, subjects who supplement the diet with fats as described in the previous paragraph.



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  		Fig. 1. The time to exhaustion (endurance) is plotted for the percentages of o2max at which the subjects exercised. The data are combinations of the data from selected studies cited in Table 1. The circles represent exercise time for subjects eating a "normal endurance runners diet" consisting of high CHO (60%), but too few calories to meet energy expenditures (25% less than expended). The squares represent subjects who ate an isocaloric diet high in CHOs. The triangles represent data for subjects on an isocaloric diet that consisted of 30% to 65% fat and at least 30% CHOs. The lines through the data were fit by the least squares method. The data for the three diets are significantly different from each other.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 BACKGROUND
 CONCLUSIONS
REFERENCES
 
It has been shown that high CHO (60% to 70%) and low-fat (10% to 15%) diets enhance endurance performance, and high-fat diets (60%) are detrimental to performance. Athletes eating high-carbohydrate (low-fat) diets do not consume as many calories as they expend and may not meet the ADA levels of iron and zinc. Recent data, also, indicate that diets comprising 32% to 55% fat can improve endurance capacity compared to diets with 15% fat. There is evidence that fit subjects have higher fat oxidation due to increased enzyme levels, fatty acid transport and beta oxidation. In addition, intramuscular triglycerides and intracellular fats can be increased by a high-fat diet and can support higher levels of fat oxidation without compromising CHO stores. It is clear that if CHO intake is reduced to below 20% of total calories, or to less than 1.9 g/kg/day, glycogen stores are compromised and therefore performance will be compromised. Similarly, reducing fat intake to less than 20% of total calories compromises fat stores and therefore endurance performance.

It appears that a critical issue regarding the role of diet in exercise is that total caloric intake must be balanced to total caloric expenditure. Furthermore, the substrates consumed should replenish the intramuscular stores of the substrates used during training and competition. In trained athletes eating isocaloric diets that have sufficient levels of fats and CHOs (muscle stores), our data suggest that the blend of fats and CHOs used during exercise is set by the intensity of the exercise and is constant throughout the exercise time. Most scientists agree that a diet containing 15% to 20% protein calories is sufficient to meet the protein demands of most athletes. Thus a general isocaloric diet should comprise 30% to 35% CHOs, 30% fats and 20% protein, with the balance (20%) of total calories supplying the substrates used in training and competition. For competitions requiring exercise intensities of up to 85% of o2max, dietary fats may be more beneficial. For exercise intensities above 100% of o2max, CHOs would be the preferred macronutrient. The ratio of the intake of fats and carbohydrates to optimize performance for exercise between 80% to 100% o2max remains to be investigated.

Received April 1, 1999. Revised February 1, 2000. Accepted February 1, 2000.


   REFERENCES
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 INTRODUCTION
 BACKGROUND
 CONCLUSIONS
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