In Indoor Rowing News #94, I explained how to use the body mass index (BMI) and your basal metabolic rate (BMR) to a) Determine whether you are overweight and b) To determine your calorie needs. You can find a copy of this article, along with a BMR calculator, at http://www.therowingcompany.com/training/bmr.php There are two groups for whom this information is important: someone who is overweight and using exercise as part of a reduction plan and an athlete training to take part in a weight category competition.For someone using exercise as part of a weight reduction plan, the aim is simply to increase the number of calories expended to a level above those consumed. As a result of exercise and weight loss, the fitness level will increase and a healthier lifestyle will follow. An athlete training for an event in a weight category already has a healthy lifestyle and is using exercise as a means of maximising his physical condition for a performance. What are the obstacles both will encounter in trying to achieve their goal? If the weight watchers over-estimate the number of calories they consume during the day they will not lose the weight expected. If lightweight athletes underestimate the number of calories needed in a day they will under-perform.This is why when you reduce your calorie intake you should not go below the calories required to meet the demands of the BMR. The way lightweight athletes prepare is that at the beginning of a training programme, the emphasis is placed on building endurance. This is achieved with long low intensity sessions where fat provides a high percentage of the energy needs. Glycogen is also used but because initially there are huge stores, if they are not fully replenished between sessions it can go undetected. It is like a small leak in the petrol tank of your car, the fuel gauge will hardly register it. As the competition period approaches the type of training becomes more intensive and glycogen becomes the major sources of energy. The small leak becomes a haemorrhage and so eventually you will run out. The body has a self-defence mechanism that will not allow the complete depletion of the glycogen stores. How this works is that once the stores fall to a critical level, the body will limit the rate of energy consumption and thereby the level of performance of the athlete. There are no obvious outward signs that you would see as in a physical injury but if you get to race day, there is just not the energy to meet demand and the result is disappointment.A similar situation faces the weight watcher should they, instead of overestimating the calories needed during the day, underestimate them. As in the case of the athlete, the lack of fuel slows down the metabolism, which stops weight loss or may even lead to a weight gain. If the lightweight athlete overestimates the daily calorie requirement then quite simply they will not meet their target weight and so will not be able to take part in the event.There is one area that is overlooked when we talk about diet and that is appetite. Apart from the total calories, and a balance between the different foods that we eat, there is also the question of volume.Often I see diets where it says, you can eat as much as you like of this particular food. Is this really true? One of the reasons that people become overweight in the first place is the increase in the size of portions. This creates an insatiable appetite and is a big contributory factor in why so many people fail to stick to a sensible eating plan. Fruit is good for you, you can eat as much as you like of that; yes fruit is good for you but if you eat as much as you like, what about reducing your appetite so the next time you feel peckish and there is no fruit, you don't dive into the biscuits.