Want To Lose Some Body Fat? What exercise burns fat?
Carrying excess fat is unhealthy. The only effective way to lose body fat is to
increase energy expenditure by exercising more and/or decrease energy intake by
eating less. A combination of the two in moderation is probably most effective,
since dieting alone is often accompanied by an adaptive reduction in metabolic
rate (resting energy expenditure). Many people exercise in order to lose fat,
enhance muscle tone and improve their body shape. But what exercise intensity
should be performed in order to optimise the burning of fat?
Exercise Expends Energy And So Requires Fuel
dynamic aerobic exercise your muscles use oxygen to burn fuel in the form of
both fat and carbohydrate, which releases energy that can be used to perform
muscular work. The precise fuel mixture that the muscles use for exercise
depends mostly on the exercise intensity and duration. During short-term high
intensity exercise, the muscles predominantly use carbohydrate in the form of
muscle glycogen (a storage form of sugar) and glucose that is absorbed from the
blood. For exercise of low-moderate intensity the muscles oxidise mostly fat.
Even for moderate
in the first few minutes we tend to use mostly carbohydrate, but as exercise
duration increases, the contribution of fat becomes greater.
At rest we expend about 1 kilocalorie (or 4 kilojoules) of energy per minute.
Moderate intensity exercise will typically raise the rate of energy expenditure
to around 6 kilocalories (25 kilojoules) per minute. So if you exercised for one
hour at this intensity you would expend about 360 kilocalories (1500
kilojoules), burning up to 40 grams (2 ounces) of fat in the process. Although
this doesn't sound a lot, when exercise is performed on a regular basis (e.g.
one hour every day) this mounts up to over 1 kilogram (2.2 pounds) of fat loss
How Is Exercise Intensity Quantified?
Exercise requires energy to fuel the contracting muscles. For sustained
activity, oxygen is also required and is pumped from the lungs to the muscles
via the red blood cells in the circulation. The heart is the pump and the heart
rate can increase from around 70 beats per minute at rest up to around 200 beats
per minute during
exercise, depending on
age. You can estimate your own maximal heart rate (HRmax) by subtracting your
age in years from 220 (e.g. a typical 40 year-old person will have a predicted
maximum heart rate of 220 - 40 = 180 beats per minute). With increasing exercise
intensity, oxygen uptake increases proportionally until a plateau is reached. At
this point further increases in work rate do not elicit further increases in
oxygen uptake and the plateau value is called the maximal oxygen uptake or
VO2max. This can also be referred to as the aerobic capacity and is strongly
associated with a person's capacity to perform prolonged exercise.
At rest, the rate of oxygen uptake of an adult is about 0.25 litres per minute.
During exercise this can increase up to about 10-20 times, though this is
dependent on fitness and genetic factors. An elite endurance runner or cyclist
can have a VO2max of over 5 litres per minute, whereas a typical middle-aged
couch potato might only have a VO2max of less than 2 litres per minute. Because
different people have different maximal oxygen uptakes, one way of quantifying
relative exercise intensity is to express the work rate as a percentage of a
particular person's maximal oxygen uptake (%VO2max).
Measuring oxygen uptake requires rather sophisticated and expensive scientific
equipment, but the heart rate can be measured by counting the pulse at the wrist
or with the use of relatively inexpensive heart rate monitors. Another way of
easily quantifying relative exercise intensity is to measure the heart rate
during exercise and express it as a percentage of the maximal heart rate (%HRmax).
The HRmax can either be determined in an incremental exercise test to fatigue or
predicted (with ±10% accuracy) from the person's age as described above.
What Is The Optimal Exercise Intensity To Burn Fat?
low to moderate intensities of exercise the absolute rate of
increases and then declines as exercise becomes even more intense. Generally the
highest rates of fat oxidation are found at low to moderate exercise intensities
(range 33-65% VO2max). Most published scientific studies, however, have measured
fat oxidation at
only two or three different exercise intensities. This has made it difficult to
accurately determine the exercise intensity that elicits maximal fat oxidation.
Hence, very recently in our human performance laboratory at the University of
Birmingham we have systematically studied fat oxidation over a large range of
exercise intensities in order to identify the exercise intensity at which fat
oxidation is maximal, which we have called "Fatmax".
In our study we used an incremental cycling exercise protocol with 5-minute
stages and 35-watt work rate increments to determine Fatmax. We found that
Fatmax was located at 64±4%VO2max, corresponding to 74±3%HRmax. In addition to
Fatmax, a Fatmax-zone was also determined. This zone was defined as a range of
exercise intensities with fat oxidation rates within 10% of fat oxidation rates
at Fatmax. The results of this study are shown in Figure 1 and indicate that fat
oxidation rates are within 10% of the peak rate over a relatively large range of
intensities (between 55±3% and 72±4%VO2max; corresponding to between 68±3% and
So this is the intensity of exercise that people should aim to do in order to
maximise their fat burning. It would be advisable for those unaccustomed to
exercise to do dynamic exercise (e.g. cycling, running, aerobics or swimming) at
the low end of this range, as of course the other important factor is how long
you exercise for. No point in trying to exercise at the upper end of this range
if you are going to have to stop due to fatigue after only a few minutes!
Data from the University of Birmingham Fatmax study. Fat oxidation rates (in
grams per minute) are plotted against exercise intensity expressed as percentage
of VO2max (%VO2max). Data are based on 11 male subjects of moderate fitness.
It must be noted that the absolute rates of fat oxidation are dependent on
carbohydrate intake. It has been shown in numerous studies that ingestion of
carbohydrate (e.g. starchy foods such as potatoes, cereals, rice, bread and
pasta, sweets and sports drinks) in the hours before exercise, reduces the rate
of fat oxidation in a subsequent exercise bout. To prevent a
carbohydrate-induced decrease in fat oxidation rates, all exercise tests in our
study were performed after an overnight fast. However, although it is known that
carbohydrate intake can influence the absolute rate of fat oxidation during
exercise, it is not known whether the intensity at which this occurs is also
Endurance training increases the capability for fat oxidation. In the trained
muscles there are more mitochondria (the powerhouses of the cell where oxidation
of fuel takes place) and increased concentrations of enzymes involved in fat
oxidation. There is a greater reliance on fat as a source of energy during
submaximal exercise at the same absolute work rate in trained individuals
compared with the untrained. Adipose fat tissue becomes more sensitive to the
effects of hormones such as adrenaline which mobilise fat from its storage sites
when needed for exercise. Training also increases the activities of both muscle
and adipose tissue lipoprotein lipase, an enzyme that facilitates the use of
circulating triglyceride fat as a fuel source for the trained muscles and
promotes the clearance of circulating triglycerides even at rest. Hence, one
advantage of becoming fitter is that you can burn even more fat when you
References and further reading
Achen J, Gleeson M and Jeukendrup AE. (2001).
Determination of the optimal exercise intensity that elicits maximal fat
oxidation. Medicine and Science in Sports and Exercise, 33 Supplement: S52.
Arnos PM, Sowash J and Andres FF. (1997). Fat oxidation at varied work
intensities using different exercise modes. Medicine and Science in Sports and
Exercise, 29 Supplement: S199.
Horowitz J, Mora-Rodriguez R, Byerley L and Coyle E. (1997). Lipolytic
suppression following carbohydrate ingestion limits fat oxidation during
exercise. American Journal of Physiology, 273: E768-E775.
Maughan RJ, Gleeson M and Greenhaff PL (1997). Biochemistry of exercise and
training. Oxford: Oxford University Press.
Thompson DL, Townsend KM, Boughey R, Patterson K and Basset DR. (1998).
Substrate use during and following moderate- and low-intensity exercise:
Implications for weight control. European Journal of Applied Physiology, 78:
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