I know the usual guidelines about what constitutes safe weight loss (1/2 to 1kg per week), but is there any research out there that supports this rate as being optimal? Also, is there any research about what proportions of this are fat versus muscle versus water under various training states?
The American College of Sports Medicine has issued a Position Stand on this issue. ACSM talks about how changes in energy intake play a signficant role in reducing body weight. Simply put, when energy intake is lowered below energy needs of the body, weight loss will occur. The reason for the .5-1 kg per week rationale is because many of these diets will reduce energy intake to 1200-1800 kcals per day. If you go lower than 1200 kcals per day, you end up putting your body into a starvation mode where it holds onto its 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. This is also a realistic goal for many people to attain, which will help keep them motivated in the long run.
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, you will be using glycogen and ATP-CP to fuel short-term bouts of exercise.
The training status of an individual 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.
- ACSM Position Stand (2001). Appropriate Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults. Medicine and Science in Sports and Exercise. 2145-2156.