Turning forty isn’t an excuse to accept middle-age spread; instead, it’s a time to get fit and shed some fat to avoid weight gain from a decreased metabolism.During exercise, the body burns fat and glucose (carbohydrate), but the proportion of each that is used up depends on the intensity of exercise. The body will burn more fat with lower intensity exercise, but because less energy is required, the total burned will be less. Also, as the body continues to burn fat after exercising, higher intensity exercise will keep the process going for a longer duration.For example, during a 30-minute walk, 60 per cent of energy will come from fat, compared with only 40 per cent for a 30-minute jog. However, as jogging uses more energy overall, the resultant fat burn is about the same. The added bonus of jogging is that the greater energy expenditure causes the body to use more of its glucose (stored as glycogen), which the body will then work on restocking from food consumed during the hours following exercise. This, combined with the extended burn after exercise, will result in less chance of gaining weight as long as food is eaten in moderation.In practical terms, exercising at a lower intensity will still burn fat and keep you fit, but it must be carried out for a longer duration to be effective. Busy lives usually mean there is limited time to exercise, so if only a short time frame is available, the exercise must be as intense as possible to gain maximum benefit.The best way to get fit and fight fat is to do a combination of cardio, strength and core exercises to raise the heart rate, increase muscle mass and provide inner strength to help combat weight gain.If you have young children, you have a built Personal Coach! Take every opportunity to get free exercise with them. Put them on your shoulders and strengthen your core, play ball to run, kick and lunge.Image: Shutterstock