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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
During 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 intensity exercise, 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 per month.

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?
From low to moderate intensities of exercise the absolute rate of fat oxidation 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 644%VO2max, corresponding to 743%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 553% and 724%VO2max; corresponding to between 683% and 793%HRmax).

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 influenced.

Exercise training
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 exercise.

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: 43-49.

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