A Perspective on Fat Intake in Athletes

Document Sample
A Perspective on Fat Intake in Athletes Powered By Docstoc

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 [2]. 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 [15]. 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 [2]. 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 [16]. 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 [4]. 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 [4]. 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 [4]. 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 [6] 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 [25]. 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 [9]. Yet, despite the          tramyocellular lipid stores are almost completely depleted [6].
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 [10].                   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 [33]. 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 [25]. 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) [34]. 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 [48].
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 [46]. 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 [49] by
[43]. 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 [50], 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 [3]. 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 [3]. 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 [44]. 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) [43]. The data at 60% of VO2max were   ˙
observed in the max VO2 test [47]. In a study on young elite        collected during running [40] and on a cycle ergometer [36]
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 [35].    ˙
                                                                    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) [43]. The subjects in the exercises at 65%
further work in this area is needed. Future work may also focus                        ˙                                ˙
                                                                    [40] or 80% [43] 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 [49], while the                          durance exercise from eating a calorically balanced high-fat
diets in the 65% VO2max study was randomized [36]. 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
                                                                                     endurance performance.
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

(muscle stores), our data suggest that the blend of fats and                     loss, dietary carbohydrate modifications, and high intensity phys-
CHOs used during exercise is set by the intensity of the                         ical performance. Med Sci Sports Exerc 22:470–476, 1990.
exercise and is constant throughout the exercise time. Most                14.   McMurray RG, Ben-Ezra V, Forsythe WA, Smith AT: Responses
scientists agree that a diet containing 15% to 20% protein                       of endurance-trained subjects to caloric deficits induced by diet or
calories is sufficient to meet the protein demands of most                       exercise. Med Sci Sports Exerc 17:574–579, 1985.
athletes. Thus a general isocaloric diet should comprise 30% to            15.   Connor WE, Connor SL: Should a low-fat, high-carbohydrate diet
35% CHOs, 30% fats and 20% protein, with the balance (20%)                       be recommended for everyone? The case for a low-fat high-
                                                                                 carbohydrate diet. N Engl J Med 337:563–563, 1997.
of total calories supplying the substrates used in training and
                                                                           16.   Marliss EB, Simmantirakis E, Miles PDG, Hunt B, Gougeon R,
competition. For competitions requiring exercise intensities of
                                                                                 Purdon C, Halter JB, Vranic M: Glucose turnover and its regula-
up to 85% of VO2max, dietary fats may be more beneficial. For
                                                                                 tion during intense exercise and recovery in normal male subject.
exercise intensities above 100% of VO2max, CHOs would be the
                                                                                 Clin Invest Med 15:406–419, 1992.
preferred macronutrient. The ratio of the intake of fats and               17.   Van Baak MA, Mooij JMV, Wijnen JA: Effect of increased plasma
carbohydrates to optimize performance for exercise between                       non-esterified fatty acid concentrations on endurance performance
80% to 100% VO2max remains to be investigated.                                   during beta-adreoreceptor blockade. Int J Sports Med 14:2–8,
                                                                           18.   Treblay A, Plourde G, Despres JP, Bouchard C: Impact of dietary
                                                                                 fat content and fat oxidation on energy intake in humans. Am J
REFERENCES                                                                       Clin Butr 49:799–805, 1989.
                                                                           19.   Despres JP, Bouchard C, Savard R, Tremblay A, Marcotte M,
 1. Costill DL: Carbohydrates for exercise: dietary demands for opti-            Theriault G: The effect of a 20-week endurance training program
    mal performance. Intl J Sports Med 9:1–18, 1988.                             on adipose-tissue morphology and lipolysis in men and women.
 2. Sherman WM and Leenders N: Fat loading: the next magic bullet?               Metabolism 33:235–39, 1984.
    Int J Sports Nutr 5:S1–12, 1995.                                       20.   Hickson RC, Rennie MJ, Conlee RK, Winder WW, Holloszy JO:
 3. McArdle WD, Katch FI, Katch VI: Human energy expenditure                     Effects of increased plasma fatty acids on glycogen utilization and
    during rest and physical activity. In Harris JM, Stead L, Lukens R           endurance. J Appl Physiol 43:829–833, 1997.
    (eds): “Exercise Physiology, Energy Nutrition, and Human Perfor-       21.   Van Zyl CG, Lambert EV, Hawley JA, Noakes TD, Denis SC:
    mance,” 3rd ed. Philadelphia: Lea and Febiger, pp. 158–173, 1991.            Effects of medium-chain triglyceride ingestion on fuel metabolism
 4. Brooks G: Importance of the “crossover” concept in exercise                  and cycling performance. J Appl Physiol 80:2217–25, 1996.
    metabolism. Clin Exer Pharm Physiol 124:889–895, 1997.                 22.   Jacobs I, Lithell H, Karlsson J: Dietary effects on lipoprotein lipase
 5. Bergstrom J, Hermansen L, Hultman E, Saltin B: Diet, muscle                  activity in skeletal muscle in man. Acta Physiol Scand 115:85–90,
    glycogen and physical performance. Acta Physiol Scand 71:140–                1982.
    150, 1967.                                                             23.   Thompson JL, Manore MM, Skinner JS, Ravussin E, Spraul: Daily
 6. Oberholzer F, Claassen H, Moesch H, Howald H: Untrastructural,               energy expenditure in male endurance athletes with differing en-
    biochemical and energy analysis of extreme duration performance              ergy intakes. Med Sci Sports Exerc 27:347–354, 1995.
    (100km run). Schweizerische Zeitschrift fur Sportmedizin 24:71–        24.   Jansson E, Kaijser L: Substrate utilization and enzymes in skeletal
    98, 1976.
                                                                                 muscle of extremely endurance-trained men. J Appl Physiol 62:
 7. Madsen K, Pedersen PK, Rose P, Richter EA: Carbohydrate su-
                                                                                 999–1005, 1987.
    percompensation and muscle glycogen utilization during exhaus-
                                                                           25.   Kiens B, Essen-Gustavsson B, Gad P, Lithell H: Lipoprotein lipase
    tive running in highly trained athletes. Eur J Appl Physiol 61:467–
                                                                                 activity and intramuscular triglyceride stores after long term high-
    472, 1990.
                                                                                 fat and high-carbohydrate diets in physically trained men. Clin
 8. Roedde S, MacDougall JD, Sutton JR, Green HJ: Supercomposi-
                                                                                 Physiol 7:1–9, 1987.
    tion of muscle glycogen in trained and untrained subjects. Can
                                                                           26.   Dyck DJ, Putman CT, Heigenhouser GJF, Hultman E, Spriet LL:
    J Appl Sport Sci 11:42–6, 1986.
                                                                                 Regulation of fat-carbohydrate interaction in skeletal muscle dur-
 9. Roberts TJ, Weber JM, Hoppeler H, Weibel ER, Taylor CR:
                                                                                 ing intense aerobic cycling. Am J Physiol 265:E852–E859, 1993.
    Design of the oxygen and substrate pathways: II. Defining the
    upper limit of carbohydrate and fat oxidation. J Exp Bio 199:1651–     27.   Jones PJ, Ridgen JE, Phang PT, Birmingham CL: Influence of
    1658, 1996.                                                                  dietary fat polyunsaturated to saturated ration on energy substrate
10. Coyle EF, Coggan AR, Hemmert MK, Ivy JL: Muscle glycogen                     utilization in obesity. Metabolism 41:396–401, 1992.
    utilization during prolonged strenuous exercise when fed carbohy-      28.   Moffatt RJ: Dietary status of elite female high school gymnasts:
    drate. J Appl Physiol 61:165–72, 1984.                                       inadequacy of vitamin and mineral intake. J Am Diet Assoc
11. Galbo H, Holst JJ, Christensen NJ: The effect of different diets and         84:1361–1363, 1984.
    of insulin on the hormonal response to prolonged exercise. Acta        29.   Hurley BF, Nemeth PM, Martin WH III, Hagberg JM, Dalsky GP,
    Physiol Scand 107:19–32, 1979.                                               Holloszy JO: Muscle triglyceride utilization during exercise: effect
12. Helge JW, Richter EA, Kiens B: Interaction of training and diet on           of training. J Appl Physiol 60:562–567, 1986.
    metabolism and endurance during exercise in man. J Physiol             30.   Staron RS, Hikida RS, Murray TF, Hagerman FC, Hagerman MT:
    492:293–306, 1996.                                                           Lipid depletion and repletion in skeletal muscle following a mar-
13. Horswill CA, Hickner RC, Scott JR, Costill DL, Gould D: Weight               athon. J Neurol Sci 94:29–40, 1989.

JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION                                                                                                      349
Fat Intake in Athletes

31. Nieman DC, Carlson KA, Brandstater ME, Naegele RT, Blanken-                 loading on fuel substrate turnover and oxidation during prolonged
    ship JW: Running endurance in 27-h-fasted humans. J Appl                    exercise. J Appl Physiol 74:1921–1927, 1993.
    Physiol 63:2502–2509, 1987.                                           42.   Calles-Escandon J, Driscoll P: Free fatty acid metabolism in aer-
32. Kiens B: Effect of endurance training on fatty acid metabolism:             obically fit individuals. J Appl Physiol 77:2374–2379, 1994.
    local adaptations. Med Sci Sports Exerc 29:640–645, 1997.             43.   Horvath PJ, Eagan CK, Leddy JJ, Pendergast DR: Effect of dietary
33. Starling RD, Trappe TA, Parcell AC, Kerr CG, Fink WJ, Costill               fat level on performance and metabolism in trained male and
    DL: Effects of diet on muscle triglyceride and endurance perfor-            female runners. FASEB J 10:A288, 1996.
    mance. J Appl Physiol 82:1185–1189, 1997.                             44.   Venkatraman JT, Rowland JA, Denardin E, Horvath PJ, Pender-
34. Vock R, Hoppeler H, Claassen H, Wu DXY, Billeter R, Weber JM,               gast DR: Influence of the level of dietary lipid intake exercise on
    Taylor CR, Weibel ER: Design of the oxygen and substrate path-              the immune status in runners. J Med Sci Sports Exerc 29:333–344,
    ways: VI. Structural basis of intracellular substrate supply to             1997.
    mitochondria in muscle cells. J Exp Bio 199:1689–1697, 1996.          45.   Venkatraman JT, Pendergast DR: Effects of the level of dietary fat
35. Muoio DM, Leddy JJ, Horvath PJ, Awad AB, Pendergast DR:                     intake and endurance exercise on plasma cytokines in runners.
    Effect of dietary fat on metabolic adjustments to maximal VO2 and           J Med Sci Sports Exerc, in press, 1998.
    endurance in runners. Med Sci Sports Exerc 26:81–88, 1994.            46.   Rennie MJ, Bowtell JL, Millward DA: Physical activity and pro-
36. Lambert EV, Speechly DP, Dennis SC, Noakes TD: Enhanced                     tein metabolism. In Bouchard C et al (eds): “Exercise, Fitness and
    endurance in trained cyclists during moderate intensity exercise            Health Consensus of Current Knowledge.” New York: Human
    following 2 weeks adaptation to a high fat diet. Eur J Appl Physiol         Kinetics, pp. 423–450, 1994.
    69:287–293, 1994.                                                     47.   Leddy JJ, Horvath P, Rowland J, Pendergast DR: Effect of a high
37. Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL:                 or a low fat diet on cardiovascular risk factors in male and female
    The human metabolic response to chronic ketosis without caloric             runners. Med Sci Sports Exerc 29:17–25, 1997.
    restriction: preservation of submaximal exercise capability with      48.   Hoppler H, Billeter R, Horvath PJ, Leddy JJ, Pendergast DR:
    reduced carbohydrate oxidation. Metabolism 32:769–776, 1983.                Muscle structure with low and high fat diets in well trained male
38. Simi B, Sempore B, Mayet MH, Fravier RJ: Additive effects of                runners. Int J Sports Med, in press, 1999.
    training and high-fat diet on energy metabolism during exercise.      49.   Horvath PJ, Besch SR, Kilanowski C, Leddy J, Pendergast DR:
    J Appl Physiol 71:197–203, 1991.                                            Endurance of male college age long distance runners with energy
39. Soza M, Karpati G: Skeletal muscle endurance: the effect of                 supplementation using peanusts or a high carbohydrate energy bar.
    increased availability of endogenous long-chain fatty acid fuel.            Am Coll of Nutr 39th Annual Mtg Albqf NM p 98, 1998.
    Exp Neuro 91:449–462, 1986.                                           50.   Pendergast DR, Horvath PJ, Leddy JJ, Venkatraman JT: The role
40. Gray CG, Kolterman OG, Cutler DC: The Effects of a Three-Week               of dietary fat on performance, metabolism, and health. Am J Sports
    Adaptation to a Low Carbohydrate/High Fat Diet on Metabolism                Med 24:S53–58, 1996.
    and Cognitive Performance. Washington, DC: Naval Medical Re-
    search and Development Command Research Report No. 90-20,
41. Bosch AN, Dennis SC, Noakes TD: Influence of carbohydrate             Received April 1999; revision accepted February 2000.

350                                                                                                                            VOL. 19, NO. 3

Shared By: