Most people are aware that doing some form of exercise is beneficial, not only for losing unwanted body fat, but also for improving overall health. However, many are unaware of certain factors that can significantly increase the fat burning potential while performing exercise and during recovery (rest). Four of the most important factors to consider are: time interval from last meal, type of pre-exercise meal, intensity of aerobic exercise and type of exercise (aerobic versus resistance training).
Time Interval from Last Meal to Exercise Performance
How long should someone wait before performing exercise in order to maximize the amount of fat burned during exercise and while resting? There are a number of studies that address this very question and have shown that fat burning is greater when exercise is performed after an overnight fast or in the morning before having breakfast.
One such study (1) looked at eleven overweight and untrained men over a 4-month course of doing aerobic exercise performed after either an overnight fast or 3-hours after consuming a standard meal. The authors noted that the amount of fat burned during exercise was significantly higher following an overnight fast than after 3-hours post-meal. They also observed that fat burning was even higher during the recovery (resting) phase in the fasting group. In conclusion, the amount of fat burned during exercise and while resting was more pronounced when exercise was performed after an overnight fast as opposed to 3-hours after eating a meal.
But, what if you cannot exercise first thing in the morning due to time restraints, and working out later in the afternoon or evening is more practical – what are your options? The timing of your last meal is still very important as to the amount of fat burned during your exercise session. Researches (2) studied the effects of exercise on fat burning in eight overweight and obese women in two trials of exercise: one performed 1-hour after a meal and the other performed 3-hours after the consuming the same meal. The amount of fat burned was greater during exercise and during rest in the 3-hour post-meal group.
Bottom Line: If your goal is to lose body fat, then the longer you wait after eating a meal the more fat you will burn not only during exercise, but afterwards while resting.
What you eat prior to performing exercise is also very important regarding the amount of fat burned during exercise and afterwards during recovery. Individuals who consume carbohydrates before exercising, especially alone, in large amounts or high in glycemic rating, inhibit their ability to burn body fat during exercise and afterwards at rest. Eight healthy sedentary women were fed either a high-glycemic or low-glycemic breakfast 3-hours before walking for 60-minutes(3). Each of the meals had the same amounts of carbohydrate, protein, fat and total calories, but differed in glycemic index rating and total fiber: the low-glycemic index meal was higher in fiber. The researches noted that the amount of fat burned during exercise was twice as much after the low-glycemic meal than the high-glycemic meal both consumed 3-hours before exercise. The amount of fat burned during post-exercise resting was also higher in the low-glycemic index group.
The glycemic index rating reflects the speed at which carbohydrates are digested and absorbed into the bloodstream resulting in elevations in blood sugar and insulin. The higher the glycemic index rating the more rapid the absorption and subsequent blood sugar and insulin elevations. Insulin, a hormone produced by the pancreas gland, regulates sugar and fat utilization. When elevated, fats release from fat cells is inhibited.
Some common high-glycemic foods consumed at breakfast are cereals (corn, rice, wheat), instant oatmeal, white flour baked goods (toast, bagels, croissants, doughnuts), sweetened jams, and white potatoes (hash browns).
Bottom Line: If your goal is to burn body fat, then consume low-glycemic carbohydrates in combination with quality proteins and fats at least 3-hours prior to performing exercise. Do not consume sports drinks, sugary fruit juices or high-glycemic carbohydrates before regular exercise – doing so will inhibit your fat burning potential.
Intensity of Exercise
Generally, the amount of fat burned during exercise is inversely proportional to the intensity level. In other words, the higher the exercise intensity the less fat is burned (while exercising) with sugar (glucose) becoming the dominate fuel source. Of course, this is a very simplistic interpretation – the actual intensity level of exercise is relevant to your fitness status. The better shape you are in, the more efficiently your muscles will burn fat during exercise and while at rest.
Aerobic fitness is determined by an individual’s ability (heart and lungs) to supply the tissues with oxygen during exercise, and is known as VO2max. The higher someone’s VO2max, the greater his or her aerobic fitness. The goal is to increase your ability to burn fat during exercise while increasing overall aerobic fitness. Research (4) has shown us that exercising at 40% VO2max is the optimal for burning fat during exercise and greater aerobic capacity is achieved when exercising at 60-80% VO2max. How do you translate VO2max in a more realistic or easy way to understand and use? Without fancy lab equipment or exercise testing, an easy way to determine your desired exercise intensity range is to calculate your maximum heart rate – all you need is your age and a simple formula (5).
Maximum Heart Rate (HRmax) = 205.8 – (0.685 x age)
- Example Age = 45
- 205.8 – (0.685 x 45)
- 205.8 – 30.83 = 175
- HRmax = 175 beats per minute
Now that you know how to calculate your maximum heart rate, all you have to do is take a percentage of that number based on your desired intensity level for maximizing your fat burning potential: 40% VO2max is equivalent to 63% of maximum heart rate (6). To improve aerobic capacity and fitness, increasing to and 60% VO2max is equivalent to 75% maximum heart rate, and 80% VO2max is equivalent to 88% maximum heart rate. Using the maximum heart rate example above, you can easily determine your optimum fat burning and aerobic fitness heart rate zone.
- 175 maximum heart rate x 63% = 110 beats per minute
- 175 maximum heart rate x 75% = 131 beats per minute
- 175 maximum heart rate x 88% = 154 beats per minute
In the above example, the optimum heart rate during exercise for fat burning is 110 beats per minute, and to increase aerobic fitness the heart rate range is 131-154 beats per minutes. However, since burning fat is best achieved at lower intensities and improved aerobic fitness is accomplished when exercising at higher intensities – how does one manage to accomplish both?
One solution is interval aerobic training. This type of aerobic exercise incorporates both low-intensity and high-intensity in one workout session. An example of this type of exercise: warm up for a few minutes at a low intensity then increase your intensity to 63% of your maximum heart rate or in the above example, 110 beats per minute. Exercise at this intensity for 5-minutes then increase your intensity to 75%-88% of your maximum heart rate for 1-minute (131-154 beats per minute in the above example). Next, slow your intensity returning to your 63% maximum heart rate for another 5-minute session, repeat this 5 to 1, 5 to 1 minute interval until your desired time of exercise is finished. At the end of the session, cool down for a few minutes at a lower intensity before stopping.
Bottom Line: If your goal is to burn fat, implement an interval aerobic training program. Exercise at 63% of your maximum heart rate with periodic increases in intensity to between 75 to 88% of your maximum heart rate. Of course, only exercise if you are a physically able and have been cleared to do so by your doctor.
Type of Exercise (aerobic versus resistance training)
Both aerobic and resistance training (weight-lifting) are important forms of exercise that develop different systems and require various forms of fuels to sustain. As explained above, aerobic exercise performed at lower intensities burns more fat during the exercise, but when the intensity in increased more sugar is utilized. Resistance training is a high-intensity form of exercise and consequently requires the utilization of sugar rather than fat while performing the exercise, but during post-exercise recovery the amount of fat burned increases. So the question becomes how should one incorporate these types of exercises?
Is doing aerobic exercise combined with resistance training (weightlifting) performed in the same session (concurrent training) better than doing them separately on alternating days? Studies show, that doing resistance training alone is superior to aerobic exercise in burning fat calories within two hours post-exercise. This is known as excess post-exercise oxygen consumption (EPOC) and represents the amount of oxygen utilized by the body to return to pre-exercise status – during this time period, body fat is broken down to supply the energy needed.
Researchers (7) also compared two exercise sessions: aerobic immediately (within 5-minutes) followed by resistance training and then resistance training immediately (within 5-minutes) followed by aerobic exercise. They concluded that doing aerobic immediately followed by resistance training was comparable to doing resistance training alone regarding EPOC. In other words, performing resistance training alone (separate session) and doing aerobic immediately followed by resistance were comparably effective in burning body fat, with a slight edge to resistance training alone. Regarding overall performance as measured by the physiological intended effects of aerobic exercise and resistance exercise, performing each type of exercise alone is superior than combining them into the same session. In other words, maximum results are achieved from resistance training when performed separately and on alternating days, and the same is true of aerobic exercise.
Resistance training routines should be designed based on individual ability, medical and fitness status, and should be adjusted and/or changed every 6 to 8 weeks reflecting progress. Incremental adjustments to exercise type, sequence, frequency, intensity and duration are critical in preventing muscle adaptation and “burnout,” and allows for continued overall improvement.
Bottom Line: Overall, the only real advantage of doing concurrent exercise is time-efficiency. Otherwise, doing aerobic and resistance training separately and on alternating days is superior when considering fat burning (EPOC) potential and maximum exercise performance. Periodic adjustments based on individual progress are important to avoid muscle adaptation and to allow for continued metabolic and fitness improvement.
- Crampes et al, Effects of a longitudinal training program on responses to exercise in overweight men, Obesity Research (2003); 11(2):244-256
- Dumortier et al, Substrate oxidation during exercise: impact of time interval from last meal in obese women, International Journal of Obesity (2005); 29:966-974
- Stevenson et al, Fat oxidation during exercise and satiety during recovery are increased following a low-glycemic index breakfast in sedentary women, Journal of Nutrition (2009); 139:890-897
- Jeukendrop et al, Fat Metabolism during Exercise: A Review Part II: Regulation of Metabolism and the effects of training, International Journal of Sports Medicine (1998); 19:293-302
- Inbar et al, Normal cardiopulmonary responses during incremental exercise in 20-70 year old men, Medicine & Science in Sports & Exercise (1994); 26(5):538-546
- Swain et al, Target HR for the development of CV fitness, Medicine & Science in Sports & Exercise (1994); 26(1):112-116
- Drummond et al, Aerobic and resistance exercise sequence affects excess post-exercise oxygen consumption, Journal of Strength and Conditioning Research (2005); 19(2):332-337