The emphasis in exercise is usually on carbohydrates, but fats are also important.

Carbohydrate (CHO) and fat serve as the two main substrates for the

production of energy during prolonged muscle contraction. Although CHO

and fat are oxidised simultaneously, the relative contribution of these

substrates to oxidative metabolism during exercise varies, and is

dependent on a variety of factors, including exercise intensity and

duration and substrate availability and training status, among other


Endurance (>90 minutes) and ultra-endurance exercise (>5

hours) are typically undertaken at a moderate-to-high exercise intensity

during which muscle glycogen is the predominant fuel, especially during

the earlier stages of exercise. The ingestion of a high-CHO diet (7-10 g

CHO/kg body mass) 1-3 days prior to an endurance event can maximise

pre-exercise muscle glycogen stores and improve prolonged endurance time

to fatigue. As a result, endurance athletes are often advised to ingest

a high-CHO diet or ‘carbohydrate-load’ during the few days

prior to an endurance event. However, the body’s endogenous

glycogen stores are restricted to a maximum of approximately 350-500 g

and are significantly depleted after 2-3 hours of moderate intensity

(65-75% VO2peak) exercise in the fasted state. The potential of

endogenous carbohydrate stores alone to fuel endurance events lasting

longer than 2 – 3 hours is therefore limited.

In contrast to the limited CHO stores, the body’s fat stores

are virtually unlimited. Even the leanest athletes have more than 80 000

kCal of potential energy stored as triglycerides in adipose tissue and

intramuscular triglyceride stores. This is >40 times more than the

energy stored as glycogen in skeletal muscle, and sufficient energy to

fuel more than 25 marathon races. However, although fat is potentially

an excellent energy source for prolonged exercise, the capacity to

oxidise fat is limited, especially during higher-intensity exercise. It

has been suggested that an increased ability to utilise fat during

exercise may be of particular benefit to performance during longer

duration endurance and ultra-endurance events. Therefore, more recent

nutritional strategies have not only focused on optimising pre-exercise

muscle glycogen stores, but also on increasing fat oxidation during

exercise, in an attempt to ‘spare’ muscle glycogen and improve

endurance exercise performance.

Strategies to increase fat oxidation during exercise

Fat ingestion before and during exercise

A high-fat meal containing predominantly long-chain triglycerides

is emptied from the stomach very slowly and is therefore not recommended

prior to endurance and ultra-endurance events. In contrast to long-chain

triglycerides, medium-chain triglycerides (MCTs) contain shorter fatty

acid chains (6-12 carbons) and can enter the circulation directly

through the portal vein. In addition, MCTs do not require the long-chain

acylcarnitine transferase system for transport into the mitochondria,

the major rate-limiting step of fat oxidation, and are readily oxidised,

especially when co-ingested with carbohydrates. MCTs therefore appear to

be an ideal energy source during exercise.

Numerous studies examining the effects of MCT ingestion, either

prior to or during exercise, on endurance exercise performance have been

undertaken. The majority of these studies failed to demonstrate any

improvement in fat oxidation or time-trial performance following MCT

ingestion. In fact, a recent study demonstrated that the ingestion of

MCTs actually compromised ultra-endurance (5 hours) cycling performance.

(1) The decrement in performance with MCT ingestion might relate to the

gastrointestinal side-effects associated with MCT ingestion, namely

nausea and diarrhoea. Therefore MCT ingestion is not recommended prior

to or during endurance and ultra-endurance events.

‘Fat loading’

A strategy that has been associated with an increase in fat

oxidation and a reduction in muscle glycogen utilisation during exercise

is the ingestion of a prolonged high-fat diet. This strategy, also known

as ‘fat-loading’ or ‘fat adaptation’, is a

nutritional strategy whereby well-trained athletes adapt to a high-fat

diet for at least 5 days. An example of a high-fat meal plan is given in

Table I.

While the ingestion of a high-fat, low-CHO diet for 1-4 days

reduces resting muscle glycogen stores and compromises the capacity to

perform prolonged sub-maximal exercise, there is evidence to suggest

that prolonged (>5 days) high fat intakes induce metabolic and

hormonal adaptations that ‘retool’ the muscle to enhance rates

of fat oxidation and reduce rates of CHO oxidation during exercise and,

to a large extent, compensate for the reduced CHO availability. (2-4)

The suggested duration of the fat-loading phase (at least 5 days) is

based on the findings from Goedecke et al., (5) who demonstrated that

high fat intake for as little as 5-10 days is sufficient to maximise fat

oxidation during exercise. Most of the early studies used longer time

periods, which is unnecessary.

Mechanisms underlying the adaptations to a high-fat diet

Increase intramuscular triglyceride stores and oxidation

A number of studies have consistently demonstrated a substantial

increase (37-130%) in intramuscular triglyceride stores with diets

containing 41-65% fat for periods ranging from 2 days to 7 weeks. An

increase in intramuscular triglycerides following a high-fat diet may

provide an additional available substrate pool, which could account, in

part, for the enhanced rates of fat oxidation in response to a high fat


Changes in skeletal muscle proteins involved in FFA transport and


It was previously thought that free fatty acid (FFA) diffused

freely into the muscle cell across the lipid membrane. However, more

recent studies have demonstrated that FFA uptake into the muscle is

facilitated by transport proteins and is highly regulated. Adaptation to

a high-fat diet increases the number of FFA transporter proteins,

possibly facilitating an increased FFA uptake into the muscle cell for

oxidation. Furthermore, the increase in fat oxidation following fat

adaptation has also been attributed, in part, to changes in skeletal

muscle enzyme activities that increase the uptake and oxidation of fatty

acids in the mitochondria (increase CPT-1 and 3-HAD), and reduce CHO

oxidation (decrease hexokinase and pyruvate dehydrogenase).

Changes in glucose tolerance and insulin sensitivity/resistance

A high-fat diet has also been shown to induce insulin resistance,

suppressing CHO metabolism and increasing fat oxidation. The ingestion

of a high-fat diet for only 5 days reduced glucose tolerance, as

demonstrated by a significant increase in 30-minute plasma glucose

concentrations during an oral glucose tolerance test. It is well

documented that increased FFA availability and elevated intramuscular

triglyceride stores induce insulin resistance. However, this does not

hold true for endurance-trained athletes who are able to fully oxidise

the available intramuscular triglyceride stores, and are markedly

insulin sensitive. Therefore, the greater intramuscular triglyceride

storage in the trained athlete as opposed to obese and/or type 2

diabetics, represents an adaptive response to endurance training,

allowing a greater contribution of the intramuscular triglyceride pool

as a substrate source during exercise.

Effects of fat adaptation on performance

Despite adaptations that favour fat oxidation, and

‘spare’ muscle glycogen stores, the performance effect of fat

loading is not clear. A number of studies have manipulated dietary fat

intake over longer periods (>5 days) in trained and untrained

subjects. The majority of these investigated the effect of a high-fat

diet on moderate-intensity (60-85% [VO.sub.2peak]) endurance capacity,

measured as exercise time to fatigue.4,6-9 The findings of these studies

are not consistent, with nearly equal proportions of studies showing an

improved (3 studies), (4,7,8) no change (2 studies), (6,9) or a

decrement in endurance capacity (2 studies) in response to a high-fat

diet compared with a moderate-to-high CHO diet. Together these results

suggest that the ingestion of a high-fat diet (>55% fat energy) for

7-14 days can potentially improve moderate-intensity endurance capacity

in trained athletes. However, exercise performance is not typically

measured as time to fatigue, questioning the applicability of these

results to athletes taking part in endurance races that are typically

measured by the time to complete a set distance (time trial).

A few studies have examined the effects of fat loading on

time-trial performance. They include aspects of a ‘real-life’

race situation such as sprinting and pacing and allow subjects to

‘compete’ in a situation that mimics the demands of a

‘real-life’ race situation. (5,10,11) However, none of the 3

studies demonstrated a significant change in time-trial performance

following 2-5 weeks of a high-fat diet (53-69% fat energy) compared with

moderate5 or high-CHO diet. (10,11) The failure to demonstrate a

time-trial performance benefit might relate to low muscle glycogen

levels associated with the diet, as time trials are undertaken at a

higher intensity and include sprinting bouts during which muscle

glycogen is the predominant fuel. Hence, fat loading is not recommended

as a nutritional strategy of choice prior to endurance time-trial


Effects of fat adaptation followed by CHO loading on substrate

metabolism and exercise performance

The restoration of muscle glycogen stores following a period of fat

adaptation could, theoretically, provide an athlete with the opportunity

to enhance fuel provision during exercise from both glycolytic and

lipolytic pathways. Therefore researchers have examined the effects of a

high-fat diet followed by CHO loading on substrate utilisation and

exercise performance.

Studies investigating 5-10 days of fat adaptation, followed by 1-3

days of high CHO intake in competitive athletes demonstrated reduced

muscle glycogen stores following the fat-adaptation period compared with

the high-CHO diet. However, 1 day of rest in combination with a high-CHO

diet was sufficient to super-compensate muscle glycogen concentrations

to similar levels, independent of the preceding diet. (2) Despite the

restoration of muscle glycogen levels prior to exercise, this dietary

strategy was associated with significantly higher rates of fat oxidation

and a ‘sparing’ of muscle glycogen stores during sub-maximal

exercise. (1,3,12,13) However, the available evidence for a potential

ergogenic effect of this particular dietary strategy on prolonged

endurance exercise is not clear-cut. Of the 6 studies that have examined

the effects of fat loading followed by CHO restoration on prolonged

(2.5-7.5 hours) exercise performance, only 1 study demonstrated an

improved time-trial performance following the high-fat, CHO-restoration

diet compared with a high-CHO diet. (14) The remaining studies

demonstrated no change in overall performance. (1,3,12,13) One of these

studies did, however, demonstrate that a fat-loading diet compromised

high-intensity sprint performance compared with a high-CHO diet. (13)

However, all 6 studies demonstrated an individual variability in

response to the experimental diets. Some athletes improved performance,

others demonstrated a decrease in performance and in some athletes

performance remained unchanged in response to the high-fat,

CHO-restoration dietary strategy. The results suggest that some athletes

respond to certain diets, and show true performance benefits, whereas

others do not respond. Indeed, there is evidence to suggest that some

athletes (i.e. athletes with a ‘fat burner’ metabolic

phenotype) are capable of using fat more effectively than others during

exercise, (15) and hence may respond better to a fat-loading diet than

those who preferentially burn CHO (i.e. athletes with a ‘CHO

burner’ metabolic phenotype). Indeed, preliminary research from our

laboratory demonstrated that the ‘fat burners’ performed a 200

km time trial faster on a fat-adaptation, CHO-restoration diet compared

with a high-CHO diet (data unpublished). In contrast, the ‘CHO

burners’ performed the 200 km cycling time trial faster on the

high-CHO diet compared with the fat-loading diet.

Although there is evidence to suggest that a high-fat diet induces

adaptations that allow the body to use fat more efficiently during rest

and exercise, even after 1 day of CHO loading more evidence is needed to

confirm the efficacy of a fat-loading diet prior to ultra-endurance

events in athletes with different metabolic phenotypes.

In a nutshell

* Fat adaptation for 5-6 days followed by 1 day of CHO loading

should only be considered for ultra-endurance events (>4-5 hours)

during which muscle glycogen is indeed limiting.

* Fat adaptation should not be considered for shorter events,

particularly those that include high-intensity (>85% [VO.sub.2peak])


* Since the diet is very high in fat and saturated fat, and results

in insulin resistance, it has health consequences and is only

recommended for well-trained non-diabetic athletes and athletes without

lipid problems.

* The fat-adaptation diet should only be followed for a short

period (5-6 days) and used a few times per year as a specific pre-race

dietary strategy to enhance ultra-endurance performance.

* It is important to realise that not everyone may respond

similarly to different dietary strategies, including a fat- adaptation

diet. Athletes are therefore encouraged to experiment with different

dietary strategies to determine which strategy is most effective for

them and the specific event they are participating in.

* Athletes should be encouraged to seek the expertise of sports

dieticians who will assist in manipulating their diet according to their

individual specified requirements. This is particularly important when

an athlete is attempting to fat load, due to the complexity in devising

a diet of this nature and the potential health implications.


(1.) Goedecke JH, Clark VR, Noakes TD, et al. The effects of

medium-chain triacylglycerol and carbohydrate ingestion on

ultra-endurance exercise performance. Int J Sports Nutr Exerc Metab

2005; 15: 15-27.

(2.) Burke LM, Angus DJ, Cox GR, et al. Effect of fat adaptation

and carbohydrate restoration on metabolism and performance during

prolonged cycling. J Appl Physiol 2000; 89: 2413-2421.

(3.) Carey AL, Staudacher HM, Cummings NK, et al. Effects of fat

adaptation and carbohydrate restoration on prolonged endurance exercise.

J Appl Physiol 2001; 91: 115-122.

(4.) Lambert EV, Speechly DP, Dennis SC, et al. Enhanced endurance

in trained cyclists during moderate intensity exercise following 2 weeks

adaptation to a high-fat diet. Eur J Appl Physiol Occup Physiol 1994;

69: 287-293.

(5.) Goedecke JH, Christie C, Wilson G, et al. Metabolic

adaptations to a high-fat diet in endurance cyclists. Metabolism 1999;

48: 1509-1517.

(6.) Helge JW, Wulff B, Kiens B. Impact of a fat-rich diet on

endurance in man: role of the dietary period. Med Sci Sports Exerc 1998;

30: 456-461.

(7.) Hoppeler H, Billeter R, Horvath PJ, et al. Muscle structure

with low- and high-fat diets in well-trained male runners. Int J Sports

Med 1999; 20: 522-526.

(8.) Muoio DM, Leddy JJ, Horvath PJ, et al. Effect of dietary-fat

on metabolic adjustments to maximal Vo2 and endurance in runners. Med

Sci Sports Exerc 1994; 26: 81-88.

(9.) Pogliaghi S, Veicsteinas A. Influence of low and high dietary

fat on physical performance in untrained males. Med Sci Sports Exerc

1999; 31: 149-155.

(10.) Rowlands DS, Hopkins WG. Effects of high-fat and

high-carbohydrate diets on metabolism and performance in cycling.

Metabolism 2002; 51: 678-690.

(11.) Vogt M, Puntschart A, Howald H, Mueller B, et al. Effects of

dietary fat on muscle substrates, metabolism, and performance in

athletes. Med Sci Sports Exerc 2003; 35: 952-960.

(12.) Burke LM, Hawley JA, Angus DJ, et al. Adaptations to

short-term high-fat diet persist during exercise despite high

carbohydrate availability. Med Sci Sports Exerc 2002; 34: 83-91.

(13.) Havemann L, West SJ, Goedecke JH, et al. Fat adaptation

followed by carbohydrate loading compromises high-intensity sprint

performance. J Appl Physiol 2006; 100: 194-202.

(14.) Lambert EV, Goedecke JH, Zyle C, et al. High-fat diet versus

habitual diet prior to carbohydrate loading: effects of exercise

metabolism and cycling performance. Int J Sports Nutr Exerc Metab 2001;

11: 209-225.

(15.) Goedecke JH, St Clair Gibson A, Grobler L, et al.

Determinants of the variability in respiratory exchange ratio at rest

and during exercise in trained athletes. Am J Physiol (Endocrinol Metab)

2000; 279: E1325-E1334.

JULIA GOEDECKE, BSc (Med) Hons Nutrition and Dietetics, PhD

Exercise Science

Registered Dietician, University of Cape Town

Julia Goedecke is a registered dietician, employed by the MRC as a

Specialist Scientist at UCT. Her research focuses on the regulation of

fuel metabolism at rest and during exercise. In addition her research

explores the mechanisms underlying the association between obesity and

disease risk in South African women.

LIZE HAVEMANN, B Dietetics, PhD Exercise Science

Sports Dietician, Cape Town

Lize Havemann is a registered dietician who was recently awarded

her PhD in Exercise Science from UCT. Her thesis examined the effect of

different nutritional strategies on ultra-endurance cycling. She is

currently employed as a Postdoctoral Fellow at UCT, continuing her work

in this area. She is also currently practising as a sports dietician in

Cape Town.

Table I. Example of a high-fat meal plan (~68% fat)


* High-fat muesli with added sesame and sunflower seeds

* Full-cream yoghurt with added cream


* 50 g peanuts


* 6 high-fat salt biscuits with butter, cheese, salami and avocado


* 40 g crisps


* High-fat lasagne (high-fat meat with cream and cheese sauce)

* Salad with olives, feta cheese, avocado and salad dressing

* Full-cream chocolate milkshake

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