What metric should you use to identify if you are improving from diet and fitness? Weight doesn't seem to reflect what's happening.

BMI is determined using weight and height.   It is one method to measure the level of health risk associated with obesity, and is used in the medical community because of the ease of calculation.   BMI tends to correlate with body fat, but may overestimate risk in athletes and others who have a muscular build, and may underestimate risk in older persons and others who have lost muscle mass since it is only based on height and weight. A BMI of over 25 is generally considered overweight.  The higher the BMI, the greater the risk for disease.  Since BMI uses total body weight and not estimates of fat and lean body mass, BMI should be used along with other measures such as waist circumference.

Waist circumference is important because excess fat around the abdomen is a greater health risk than excess fat on the hips.  Studies have shown that the “apple” figure vs. the “pear” shape has a much greater risk of cardiovascular disease.  Smoking and drinking too much alcohol tend to increase abdominal fat and therefore, the risk for disease related to obesity. Waist circumference measures increase risk of type 2 diabetes, hypertension, and cardiovascular disease, especially when used along with BMI.  If your waist is over 35 inches if you are a woman, and over 40 inches if you are a man, abdominal fat may indicate an increased risk even if you are at normal weight.

The most accurate measure is BODY COMPOSITION - a reading that will measure the percentage of lean mass and fat mass.  Weight may not change if muscle increases, even though fat mass is decreasing.