You surely already know that there is no point in weighing yourself every 5 minutes! You should weigh yourself when you get up in the morning, before breakfast and after having urinated. This should be done several mornings in succession in order to get a reliable average reading.
What's more, weighing yourself just after a run is not advisable:you will probably not want the unexpected surprise of a weight gain when you have rehydrated yourself! Indeed, a run can result in a moderate loss of water of between 0.5 and 1 L per hour, whereas this fluid loss can exceed 2 L depending on the intensity of effort and the external temperature.
Another factor that can result in a temporary weight loss:energy consumption. Your body draws on its glycogen reserves during any exercise that requires endurance. This results in a slight loss of weight that is equally temporary because the weight is regained as soon as your reserves are built up again…
HOW SHOULD YOU MEASURE YOUR FITNESS WEIGHT?
The well-known body mass index (BMI) can give you an initial indication for estimating your fitness weight. Using a calculation that combines your height and your weight, you can work out whether your body shape matches the standard or if you are overweight.
The calculation is as follows: weight in kg/(height in metres squared).Example:72/(1.81x1.81)= 21.98. If, as is the case here, the result lies between 18.5 and 25, your BMI is considered to be "normal".If it exceeds 25, you are overweight and, if it is below 18.5, you are underweight.
If you are new to running, this measure can be helpful to motivate you to train and get back in physical shape. This being said, in order to obtain a more reliable and precise measurement, you need to know the proportion of fat and muscle mass of your body by consulting a specialist.