A Perspective on Fat Intake in Athletes
David R. Pendergast, EdD, FACN, John J. Leddy, MD, Jaya T. Venkatraman, PhD
Department of Physiology and Biophysics (D.R.P.), Sports Medicine Institute, Department of Orthopedics (J.J.L.), 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
Key words: dietary lipids, exercise, endurance, fat oxidation, athletes, dietary carbohydrates
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 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.
Athletic performance is determined in part by the energy well evaluated. Based on published data scientists have con-
cost of performing the sport and the ability of the metabolic cluded that CHOs supply most of the energy required during
system to provide the rate and amount of energy needed. For higher intensity endurance exercise and that athletes should
longer-duration activities (over 30 minutes), such as long- keep fat intake to very low levels, sometimes to as low as 10%
distance running, the percentage of sustainable metabolic turn- to 15% of daily calories [1,2]. The role of fats in exercise has
over, the capacity of the available energy sources (stores/ not been well studied, is therefore not well understood and
reserves) and body composition are the primary factors deserves further evaluation and study. The purpose of this
contributing to performance. The major determinants of fat paper is to present selected recent papers that examine the
oxidation are the intensity and duration of the exercise as well relative role of fat oxidation in supplying energy during longer
as the diet and training status of the athlete. duration exercise and contrast them to selected previous studies
The role of carbohydrates (CHOs) during exercise has been emphasizing the role carbohydrate metabolism. Hopefully the
This paper was presented at the 38th Annual Meeting of the American College of Nutrition: Nutrition in Sports Medicine, September 26 –28, New York, 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.
Journal of the American College of Nutrition, Vol. 19, No. 3, 345–350 (2000)
Published by the American College of Nutrition
Fat Intake in Athletes
perspective developed in this paper will stimulate further dis- the primary substrate for endurance exercise and that high-
cussion and research to determine the optimal CHO and fat CHO diets (60% to 70%) improve, while high-fat diets (60% to
intake for athletes. 75%) compromise, endurance performance . Previous stud-
ies showed reduced endurance performance when subjects con-
sumed high-fat diets as intramuscular glycogen was compro-
mised [4,5,11–14]. Fats have been considered inconsequential
BACKGROUND or even detrimental to elite endurance exercise performance. As
a result, athletes have been advised to eat very low-fat diets
The time to cover a given distance in endurance runs is (10% to 15% of daily calories), which is, in fact, below the
dependent upon the energy cost to cover a given distance and level recommended for all Americans . A recent review
the percentage of the maximal aerobic power than can be paper concluded that fat supplementation is not only not ben-
sustained over the distance. Factors such as training status, eficial for athletes, but in fact is detrimental to athletes’ health
hydration status as well as psychological and motivational and performance . However, due to the long history of
factors may also modulate performance. The % of VO2max that concentrated work on CHOs and the lack of a reliable meth-
can be sustained for the event distance decreases as the race odology for analyzing and quantifying lipid metabolism, it has
distance increases and is higher in well-trained subjects. Im- not been given careful enough analysis.
plicit in this equation is the availability of substrates, particu- In spite of the position presented above, there are several
larly carbohydrates and fats, which can be released into the reasons to re-evaluate the role of fats in endurance exercise. A
blood after ingestion, released from storage depots or stored closer examination of these studies reveals that the high-fat
intramuscularly. Thus, overall, the stores of CHO and fat, diets were also very low CHO diets; thus, the intramuscular
maximal aerobic power and the rate of utilization of CHO and levels of glycogen may have been extremely low at the begin-
fat are among the factors that determine endurance perfor- ning of the endurance exercise . The low glycogen content
mance. was either due to a relatively low intake of CHO (% of daily
Over the past 30 years the emphasis of research has been calories) or to a low total caloric intake and thus a low absolute
primarily on CHO metabolism. Studies have demonstrated that, (g/day) CHO intake. It is axiomatic that glycogen depletion will
as exercise intensity increases, CHO oxidation increases while stop exercise if muscles begin exercise with a low glycogen
the oxidation of fats decreases [1,3,4]. This shift is due to the content. Well-trained humans, however, improve fat utilization
abundance of glycolytic enzymes, the limited rate of mitochon- during exercise . Therefore, if fat intake is increased, while
drial fat oxidation and the shift to fast glycolytic muscle fibers maintaining sufficient CHO intake, it is possible that endurance
at high exercise intensities . This has been demonstrated by exercise time could be improved, provided that enhanced fat
an increase in the expiratory gas exchange ratio RER (VCO2/ ˙ oxidation allowed the muscle to spare glycogen [17–21]. In this
V˙ O2) with greater exercise intensity so that above about 80% of scenario, we envision parallel depletion of intramuscular gly-
VO2max the substrate providing energy is predominately carbo- cogen and fats; both would therefore last longer and perfor-
hydrate . Furthermore, as exercise continues at a fixed level mance would improve.
of VO2, the RER decreases. The low RER reflects muscle When exercising at equivalent intensities, highly trained
glycogen depletion, while the level of fat oxidation remains subjects have lower RER values than untrained subjects, sug-
constant [1,3]. Under these conditions, the sustainable exercise gesting greater fat oxidation during exercise in the trained [1,3].
intensity inevitably decreases, and the athlete becomes fa- The rate of reaction of the fat oxidative enzymes and muscle
tigued. Indeed, previous studies have correlated fatigue in lipoprotein lipase activity (which stores fat in muscle) [22–25]
long-duration exercise closely with depletion of muscle glyco- are significantly greater in trained than in untrained subjects
gen stores [2,5]. In fact, near complete depletion of muscle [5,26]. Other studies have shown that exercise fat mobilization
glycogen  has been demonstrated after ultra-endurance ex- from stores is similar in fit an unfit subjects, yet fat oxidation
ercise. It has been shown that glycogen stores can be increased is significantly increased in fit subjects . A low-fat diet that
by increased dietary carbohydrate intake. “Glycogen loading” reduced intramuscular lipid stores could inhibit optimum per-
has been shown to enhance endurance performance [7,8]. formance in highly trained athletes [27–30]. It has been shown
Blood-borne glucose and fat cannot enter muscle cells at a great that intramuscular fat stores are reduced after endurance exer-
enough rate to supply energy meaningfully at exercise intensi- cise [6,22,30 –32] and, during ultra-endurance events, in-
ties above approximately 40% of VO2max . Yet, despite the tramyocellular lipid stores are almost completely depleted .
low flux rate of glucose from the blood to the mitochondrion, Furthermore, low intramuscular fat may limit exercise perfor-
it has been shown that consuming carbohydrates during endur- mance [6,14]. Therefore, if the intramuscular triglycerides and
ance exercise increases capacity, perhaps due to maintaining the intramyocellular lipids (fats in contact with mitochondria)
blood sugar levels to prevent central fatigue . are depleted by a low-fat diet, one could speculate that endur-
Based on the studies described above, the general consensus ance exercise performance could be compromised, just as it has
is that CHOs, specifically intramuscular glycogen stores, are been shown for low intramuscular glycogen on a low CHO diet.
346 VOL. 19, NO. 3
Fat Intake in Athletes
Recent studies have shown that the intramuscular triglycer- twitch oxidative muscle fibers in determining fat oxidation and
ide content can be increased by a high-fat diet that follows an endurance performance.
exercise program, even if there was no depletion of intramus- Muscle fiber composition is important, as the potential for
cular triglycerides during the endurance exercise . If fatty increased fat oxidation applies to slow twitch oxidative fiber
acids are not oxidized in the mitochondria, they are esterfied in metabolism. Thus, the higher the percentage of these fibers, the
the intramuscular triacylglycerol pool . An examination of greater the potential benefits from the high-fat diet. It has
these data suggests that a high-fat diet may increase the in- recently been shown that experienced male runners (n 6,
tramyocellular fatty acid pool (fatty acids in lipid droplets in age 35 5 years), training 35 to 65 miles per week, consuming
contact with mitochondria) . One could hypothesize that a high dietary fat for one month (42%) had significantly in-
diet that was calorically balanced and high in fats, yet does not creased in the volume density of total lipid in the muscle,
compromise intramuscular glycogen, would improve maximal without significant changes in the volume density of total
aerobic power and prolong endurance exercise capacity. Sev- mitochondria or total body weight and percent body fat .
eral recent studies in humans [35– 45] have demonstrated sig- Many athletes consume low calorie and low fat diets as a
nificantly improved prolonged exercise time when high-fat mechanism to reduce both fat intake and body weight [23,28].
diets were compared with low-fat diets. Low caloric and/or fat intake diets may result in low levels of
The dietary caloric intake of athletes should balance their intramuscular fat stores that compromise performance.
high caloric expenditure in terms of both total calories and Several recent studies that examined the effect of increased
calories expended from CHOs and fats. The respiratory quo- fat intake on endurance performance are presented in Table 1.
tient (RQ) reflects the balance of fat and CHO use, assuming Subjects with reduced energy intake (500 to 800 kcal/day
protein metabolism during exercise is minimal . If carbon below estimated expenditure) [35,43,49] have been shown to
dioxide stores are constant, the expiratory gas exchange ratio have reduced endurance exercise time. Increasing the caloric
RER, measured at the mouth, is a good estimation of the intake to match expenditure using CHO significantly increases
respiratory quotient. RER’s of between 0.8 and 0.9 have been the time to exhaustion by approximately 20% at exercise in-
measured for exercise intensities of 60% to 75% of VO2max ˙ ˙
tensities of 70% and 80% of VO2max [35,43,49]. Increasing the
[40,35, respectively], and an RER of 0.93 for 80% of V ˙ O2max total caloric intake to meet expenditure using fat  by
. Although the percentage of fat oxidized decreases with keeping subjects on an isocaloric diet, while increasing the
increasing intensity, there is still significant fat oxidation at percentage of fat to 30% and to 42% (from 15%) of total
high exercise intensities as the overall rate of oxidation is calories brings about a further significant increase (40%) in
increased. RER is constant throughout the entire running time endurance time [35– 40]. There is an improved endurance time
(45 to 120 minutes) at all levels of VO2 from 60% to 80% ˙
even at 125% VO2max , when athletes (with higher levels of
V˙ O2max [35,36,40,43]. This indicates that the balance between fat oxidation) are compared to sedentary subjects (with lower
CHO and fat oxidation is set by exercise intensity and not by levels of fat oxidation). The greatest improvement in endurance
exercise time and that the intramuscular stores of fat and CHO ˙
time occurred at 65% of VO2max [36,40], with the degree of
would determine maximal endurance exercise time at a given improvement decreasing as oxygen consumption increased
% of VO2max. (Table 1).
Exercise training is generally associated with an increase in It is important to note that in the studies demonstrating
maximal aerobic power and endurance . It is also generally increased endurance running time on high-fat diets [35– 40]
agreed that the maximal potential for aerobic power and en- that fat accounted for 30% to 45% of calories and protein 15%
durance is set genetically . Any examination of the effects of to 20% of calories. CHOs thus remained at 35% to 40% of total
dietary fat intake has to be considered in light of potential calories, and the caloric intake was balanced to expenditure.
training effects. In studies of 25 male and female middle-aged The latter fact is important, as the calorically balanced diets
endurance runners, with a long history of running, VO2max did (CHO and fat diets) had 500 to 800 more calories per day than
not significantly improve while running 40 to 50 miles per the athletes’ normal diet [23,25,43,49].
week for six months . This same group of runners, when The experiments cited in Table 1 that studied exercise at
put on a high-fat diet, had a 3% to 8% increase in VO2max (not ˙
75% and 80% of VO2max, used equal numbers of men and
significant) without a significant change in the peak heart rate women (12 each) . The data at 60% of VO2max were ˙
observed in the max VO2 test . In a study on young elite collected during running  and on a cycle ergometer 
endurance runners, a high-fat diet compared with a low-fat diet ˙
and the remaining data at 70% to 80% VO2max on a treadmill
significantly increased VO2max 8% to 12%, without a signifi- [35,43,49]. The subjects studied at 75%, 70% and 65% of
cant increase in the peak heart rate during the VO2max test . ˙
VO2max on high-fat diets were young (20s) [35,40,49], while
It should be noted that the diets in these two studies [35,43] ˙
the subjects studied at 80% of VO2max on high-fat study were
were not randomized and cardiac output was not measured, so older (30s and 40s) . The subjects in the exercises at 65%
further work in this area is needed. Future work may also focus ˙ ˙
 or 80%  VO2max were less trained (lower VO2max) than the
on the role of body fat distribution and the percentage of slow subjects in the other trials [35,36,43,49]. Although the diets for the
JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION 347
Fat Intake in Athletes
Table 1. Description of the Protocols Used in Selected Studies that Varied Dietary Fat Intake and Determined the Effects on
Total Calories % Fat
Study n TR Gender Age Time Des %max %Imp.
Low Med High Low Med High
Ref. #35 6 A M 21 700 15 24 38 1 N/H/L 75 10–32
Ref. #36 5 A M 22 12 70 2 rand 60 87
Ref. #40 10 S M 19–41 38 74 3 N/H 65 35
Ref. #43 25 RA M/F 34 650 16 32 42 4 L/M/H 80 20–40
Ref. #48 8 A M 20 500 29 32 1 X-over 70 25
Ref # number of the study in the reference list.
Number of subjects studied in each protocol n.
Training status of subjects TR: A athletes, RA recreational athletes, S sedentary.
Gender: M males, F females.
Total caloric intake compared to caloric expenditure: is balanced, a deficit in intake to expenditure.
Time the number of weeks on each diet investigated.
Experimental Design Des: rand random; X-over cross-over; order of diets N normal, L low-, M medium-, H high-fat.
The percentage of VO2max used for the endurance test % max.
Percent improvement in running time on the experimental diet % imp.
70% and 80% of VO2max studies were not randomized [35,43], schematic representation of the potential improvements in en-
the 75% VO2max study was a crossover design , while the durance exercise from eating a calorically balanced high-fat
diets in the 65% VO2max study was randomized . Whether diet, which maintains CHO and protein levels, is presented in
the studies were serial, randomized or crossover designs, all Fig. 1. This figure is a summary of the data from Table 1 for
studies reported similar improvements in endurance. The stud- subjects on a “normal” diet, subjects who supplement the diet
ies shown in Table 1 were short-term (one week) and long term with CHOs and, finally, subjects who supplement the diet with
(two to four weeks), and all studies demonstrated similar in- fats as described in the previous paragraph.
creased endurance on high-fat compared to low-fat diets. A
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 triglyc-
erides and intracellular fats can be increased by a high-fat diet
and can support higher levels of fat oxidation without compro-
mising 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,
Fig. 1. The time to exhaustion (endurance) is plotted for the percent- glycogen stores are compromised and therefore performance
ages of VO2max at which the subjects exercised. The data are combi- will be compromised. Similarly, reducing fat intake to less than
nations of the data from selected studies cited in Table 1. The circles
20% of total calories compromises fat stores and therefore
represent exercise time for subjects eating a “normal endurance runners
diet” consisting of high CHO (60%), but too few calories to meet
It appears that a critical issue regarding the role of diet in
energy expenditures (25% less than expended). The squares represent
subjects who ate an isocaloric diet high in CHOs. The triangles repre- exercise is that total caloric intake must be balanced to total
sent data for subjects on an isocaloric diet that consisted of 30% to 65% caloric expenditure. Furthermore, the substrates consumed
fat and at least 30% CHOs. The lines through the data were fit by the should replenish the intramuscular stores of the substrates used
least squares method. The data for the three diets are significantly during training and competition. In trained athletes eating iso-
different from each other. caloric diets that have sufficient levels of fats and CHOs
348 VOL. 19, NO. 3
Fat Intake in Athletes
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