Body Mass Index (BMI), used to calculate obesity, is only a guide as this only takes into account height and weight.
Weight on its own does not reflect true health or changes in health by adopting exercise and a healthy diet. By exercising muscle mass can increase and fat can reduce but your weight may not change much but you are healthier as a result of the lifestyle change.
Measuring fat, muscle mass in each limb as well as the visceral fat around the trunk is a better indicator of health. Visceral fat is the fat around the vital organs that has a significant impact on health.
On many diets, muscle mass can decrease as well as fat. This is because muscle can be used as fuel to produce energy when the body does not have enough calories. This results in a lower metabolic rate, and the risk of rebound weight gain after the diet.