Safe Weight Loss Guidelines
ACSM (American College of Sports Medicine) talks about how changes in energy intake play a significant role in reducing body weight.
Simply put, when energy intake is lowered compared to the energy expended by the body, weight loss will occur. The reason for the 0.5-1 kg per week weight loss theory is because many of these diets will reduce energy intake to 1200-1800 kcals per day. This is also a realistic goal for many people to attain, which will help keep them motivated in the long run.
If you go lower than 1200 kcals per day, your body ends up entering into starvation mode where it holds onto its precious energy stores (fat!!). The other way you can lose more weight is by losing water weight, which if not replenished, can lead to more serious problems.
In regards to fuels used during exercise, that is dependent on what energy system you are using for that exercise. The more oxygen a person utilizes, the more fat and glucose they can breakdown for energy for a long period of time.
In fact, endurance athletes use a minor amount of protein for fuel in addition to fat and glucose. If you are training anaerobically (Short-burst, high intensity), you will be using more glycogen to fuel short-term bouts of exercise.
How conditioned an individual is will determine how well he or she is able to use fats as a fuel source in an aerobic state. The trained individual is able to use fat more efficiently than an untrained individual.
Do remember to consult your doctor before embarking on any kind of weight loss programs. Sudden weight gain or loss might be symptoms of a more serious underlying medical illness. It is also vital to determine your cardiovascular health prior to any form of exercise. See your doctor for a safe route towards a healthier you.
Fitness & Lifestyle Consultant