BMI is widely used as a convenient indicator of a healthy weight range.  It is also generally accepted that it has its limitations and will produce incorrect results for muscular people and at the other end of the scale TOFIs (Thin on the Outside, Fat on the Inside), people who have too little muscle.

The amount of muscle we have tends to decline with age, although regular exercise can help retain and build muscle even as we age.

One issue with weight loss is that we will tend to also lose muscle.  So stepping on the scales and seeing a 2 or 5kg weight loss is not the whole story.

By being able to accurately measure how much fat and muscle you  have and where it is distributed, says a lot about how well the weight loss programme is working.  e.g. Rapid weight loss accompanied by rapid muscle loss, would indicate that you are eating too few calories.  We would then suggest slightly increasing your food intake but focus the balance of food types more towards protein to preserve the muscle.

Muscle consumes energy even when resting, so preserving or increasing muscle can help with weight control.  Our ultimate goal is fit and healthy, not stick thin.