One of the most commonly asked questions in the gym regards the exercise intensity a person should train, if their main goal is to lose weight, or more precisely, burn fat.
How hard should you train to lose weight?
Exercise intensity and duration are important factors when considering the total, and type of calories (carbohydrate or fat) burnt during an exercise session. The higher the intensity, the more carbohydrate will be used to fuel the exercise. This is because the process by which the human body extracts energy from carbohydrate is a lot faster than that of fat. In other words, using fat alone for energy is just too sluggish to keep up with the demands of high intensity exercise, and can therefore only partly contribute to the overall demand. However, a higher proportion or percentage of fat is used as energy compared with carbohydrate at lower exercise intensities. On this basis it would appear prudent to exercise at lower intensities, as this would appear to burn off more body fat. However, it is more important to look at the calorie burning process a bit more closely. We will look at two hypothetical exercise sessions of either walking or jogging for 20 minutes:
Walking for 20 mins:
Calories burnt per minute = 5 Kcal
Total calories burnt = 100 Kcal
Percentage of calories burnt from fat = 80% = 80 Kcal
Percentage of calories burnt from carbohydrate = 20% = 20 Kcal
Jogging for 20 mins:
Calories burnt per minute = 10 Kcal
Total calories burnt = 200 Kcal
Percentage of calories burnt from fat = 50% = 100 Kcal
Percentage of calories burnt from carbohydrate = 50% = 100 Kcal
From this we can see that even though a higher percentage of fat was burnt during walking (80%) compared to jogging (50%), the total calories burnt in the same time scale was twice as high during jogging. The total fat calories burnt during jogging were also higher than that of walking. For weight loss purposes, the most important thing you can do is to create a negative energy balance:
Energy IN < Energy OUT = Weight Loss
Energy IN refers to the intake of food and drink, and Energy OUT refers to an individual's metabolism and physical activity level.
Although the equation is over simplistic, it does demonstrate that it is the total calories burnt during physical activity that is of primary importance to weight loss. This does not mean that low intensity exercise does not have a place in a weight loss program, but if time is a constraint then choosing an aerobic exercise of a slightly higher intensity may incur greater caloric burning benefits. Low intensity exercise like walking has the benefit of being less stressful and can be partaken in much more frequently and should be cycled in with higher intensity exercise if you are training more than three times per week. Remember to always train within your own boundaries, build up slowly, and enjoy yourself. It is of more benefit for you to participate in exercise at an intensity you feel you can stick at and turn into a positive habit. On the other hand, push yourself too hard and you may end up exhausted, sore or injured, consequently deciding that exercise isn't for you.