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European Journal of Sport Science, vol. 1, issue 5
©2001 by Human Kinetics Publishers and the European College of Sport Science
Fatmax: A New Concept
to Optimize Fat Oxidation During Exercise?
Asker E. Jeukendrup and Juul Achten
It has been recognized that facilitation of fat metabolism is important for both
performance and the health-related aspects of exercise. Although several studies have
described the relationship between exercise intensity and fat oxidation, few studies have
studied this relationship over a wide range of intensities. In absolute terms, carbohydrate
oxidation will increase proportionally with exercise intensity, whereas the rate of fat
oxidation will initially increase but will decrease again at high exercise intensities. Here
we have defined the exercise intensity at which maximal fat oxidation is observed as
Fatmax. This exercise intensity may have importance for weight loss programs, health-
related exercise programs, and endurance training. Future research should focus on
developing an exercise test with which Fatmax can be accurately determined. Further
research should investigate the efficacy of training at Fatmax in a variety of conditions.
Key Words: exercise intensity, fuel oxidation, fat metabolism
• It has been recognized that facilitation of fat metabolism is important for both
performance and the health-related aspects of exercise.
• In absolute terms, carbohydrate oxidation will increase proportionally with exercise
intensity, whereas the rate of fat oxidation will initially increase but will decrease again at
high exercise intensities.
• Here we have defined the exercise intensity at which maximal fat oxidation is observed
• Fatmax may have importance for weight loss programs, health-related exercise
programs, and endurance training.
Much of the emphasis of exercise programs for obese patients, and endurance training for
athletes, is on cardiovascular and respiratory benefits, and increasing the oxidative capacity of
skeletal muscle (3, 11). Fat metabolism, however, is an area that has received less attention, even
though its importance has been recognized for both performance and the health-related aspects of
exercise (14–17, 25).
Endurance performance is largely determined by the ability of the athlete to mobilize and oxidize
fat to spare reserves of carbohydrates, and this is most likely susceptible to specific training (13).
In weight control programs, exercise is used alone or in conjunction with a dietary restriction.
When exercise is the only intervention, the main goal is to reduce body fat (9, 22, 30). When
combined with a diet program, it is mainly used to counteract the decrease in fat oxidation often
2 Jeukendrup and Achten
seen after weight loss (2, 7, 23). In a variety of clinical conditions, exercise may be beneficial
through changes in fat metabolism. For example, an increased capacity to oxidize fat as a fuel in
diabetic patients will make them less dependent on glucose as a source of energy (29). Although
it is generally agreed that exercise programs will increase the capacity to oxidize fat, there is
controversy as to what exercise intensity or training program should be employed in order to
achieve the aims of all three groups, and guidelines are generally inconsistent.
The purpose of this short review is not to describe new factual information but to provide new
ideas, introduce a new concept, and expand the boundaries of our thinking about exercise
training in relation to fat metabolism.
Fat Oxidation During Exercise
Effect of Exercise Intensity on Fat Oxidation
Already in 1939, Christensen and Hansen (4) observed that changes in the exercise intensity
induced changes in substrate utilization. With increasing exercise intensity, the relative
contribution of carbohydrate as a fuel will increase; concomitantly, the relative contribution of
fat oxidation decreases. However, in absolute terms, carbohydrate oxidation will increase
proportionally with the exercise intensity, whereas the rate of fat oxidation will initially increase
but will decrease again at high exercise intensities (21). At intensities where the rate of
glycolysis has increased considerably (at exercise intensities above lactate threshold), relatively
less fatty acids will be used as a fuel, and although the energy expenditure is increasing, absolute
rates of fat oxidation will actually decrease. In Figure 1, fat oxidation is depicted versus the
exercise intensity expressed as VO2. The data are based on the theoretical calculation based on
stoichiometric equations (8), where the respiratory exchange ratio (RER) increases linearly from
0.86 (an average resting RER) to 1.00 as VO2 increases linearly from 1.2 to 4.0 L/min. The
exercise intensity at which fat oxidation is optimal will thus be somewhere between the low and
the high intensity exercise. Here, we would like to introduce the term Fatmax to describe the
exercise intensity at which fat oxidation is maximal. To our knowledge, at present, no studies
have attempted to identify this exercise intensity systematically and accurately.
Figure 1 — Theoretical curve indicating fat oxidation as a function of exercise intensity (VO2). The
exercise intensity at which the highest rate of fat oxidation is observed is referred to as Fatmax.
Fatmax: A New Concept? 3
Mechanisms Behind Fatmax
Most likely, Fatmax will be just below the intensity where the rate of glycolysis commences to
increase markedly. Recently, we and others have shown that increasing the rate of glycolysis by
feeding subjects high carbohydrate pre-exercise feedings (6) or by increasing the exercise
intensity from 40 to 70% VO2max (24) decreases the oxidation of long chain fatty acids but not of
medium chain fatty acids. This suggested that increasing rates of glycolysis inhibit fat oxidation
by inhibiting the CPT-1 and carnitine-mediated transport of fatty acids into mitochondria. In
addition, several studies reported a decreased rate of appearance of fatty acids into the circulation
during intense exercise, suggesting that the mobilization of fatty acids is decreased. This could
be linked to reduced rates of lipolysis or reduced blood flow to the adipose tissue (for review, see
14–17). Because fat oxidation is directly dependent on the rate of glycolysis, Fatmax might
coincide with the onset of blood lactate accumulation. This hypothesis has not been tested.
Potential Applications of Fatmax
Exercise training programs at an intensity that elicits maximal rates of fat oxidation (i.e.,
Fatmax) may be useful in a variety of conditions and for a variety of populations. For example, it
could have implications for exercise programs to treat and prevent cardiovascular diseases,
obesity, and non-insulin-dependent Diabetes Mellitus (NIDDM). It could also be used in general
weight reduction programs and for athletes as a means to increase one’s capacity to oxidize fat.
Training Programs for Athletes. One of the main adaptations to endurance training is a shift
from carbohydrate towards fat metabolism (12, 13, 28). The ability to oxidize fat as a fuel is also
highly correlated with exercise performance (12, 13). Training of endurance athletes therefore
often includes training sessions that specifically aim to improve fat metabolism. Training at an
exercise intensity at which metabolic pathways involved in fat metabolism are maximally
activated could, at least theoretically, result in optimal adaptations in these pathways. This
however has not yet been investigated.
Exercise Programs to Lose Weight or Body Fat. A recent meta-analytical review reported that
a decrease in bodyweight of 0.2 kg/week can be achieved by following an exercise program (19).
The studies showed great variety in the exercise prescription for the obese population. The same
review also suggested that exercise is critical in weight maintenance. A smaller percentage of the
weight loss induced by a restricted diet will be regained when an exercise program is followed
during and after the diet period. However, the best exercise type, intensity, and duration are still
unclear. The latest recommendation of an expert panel from the Centers for Disease Control and
the American College of Sports Medicine consist of 30 min or more of moderate-intensity
physical activity on most days of the week (1). The objective of this recommendation is to
encourage more participation in physical activity. For obese individuals, the frequency, intensity,
and duration of exercise are more focussed on increasing energy expenditure than on increasing
fat oxidation. Finding the optimal intensity for fat oxidation might increase weight loss and
support weight maintenance.
Exercise Programs to Treat or Prevent Welfare Diseases. There is considerable interest in
finding treatments and preventive measures to avoid overweight, obesity, and associated diseases
like NIDDM, arteriosclerosis, and hypertension (26). Exercise has been shown to significantly
decrease the risk of developing such diseases (18, 27). However, there is controversy as to what
4 Jeukendrup and Achten
type of exercise and what exercise intensity would be most effective in reducing this risk. Since
many of these disease states are linked to high levels of circulating triglycerides and disturbance
in fat metabolism (5, 10, 20, 29), it is tempting to think that exercise intensities at which a
maximal amount of fat is oxidized (Fatmax) would be a preferred intensity. Therefore,
determination of Fatmax could also be a useful tool to design exercise programs in order to
obtain optimal health benefits.
Conclusions and Directions for Future Research
The concept of Fatmax seems promising and may be an efficient exercise intensity for weight
loss programs, health-related exercise programs, and endurance training. Future research should
focus on developing an exercise test with which Fatmax can be accurately determined, and such
a test needs to be validated and tested for reliability. Further research should investigate the
efficacy of training at Fatmax under a variety of conditions, and methods should be developed to
monitor exercise intensity during training.
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About the Authors
A.E. Jeukendrup and J. Achten are with the Human Performance Laboratory in the School of
Sport and Exercise Sciences at the University of Birmingham, Edgbaston, Birmingham, UK.
Asker Jeukendrup <A.E.Jeukendrup@bham.ac.uk> is a lecturer at the School of Sport and
Exercise Sciences at the University of Birmingham (UK) and Academic Director of the Human
Performance Laboratory. He completed his Ph.D. in 1997 at Maastricht University in the
Netherlands and has published over 50 peer-reviewed journal articles and book chapters.
Following an M.Sc. in Biological Health Sciences at Maastricht University in the Netherlands,
Juul Achten <J.Achten@bham.ac.uk> started her Ph.D. at the University of Birmingham in the
UK in 1999. Her research in the School of Sport and Exercise Sciences focuses on fat
metabolism and heart rate monitoring.