BMI can be too low.

More info: A body mass index result below 18.5 is considered underweight.

Health Risks of a Low BMI

Though many people strive to lose weight, being too skinny can lead to health problems just as being overweight can.  Underweight people run the risk of malnourishment and could be at risk for health conditions such as cardiovascular disease, high blood pressure, diabetes, gall bladder disease, osteoarthritis, and certain types of cancer.

Average BMI for Men

The average BMI for men is 18.5-24.9.  BMI is based on your height multiplied by your weight and is based on the metric system.  For a metric conversion,  multiple your weight by .45.  Next multiply your height by .025.  To calculate your BMI, square your metric height. Finally, divide your metric weight by your metric height squared.  Now, you have your BMI number, which can be compared to the BMI category index.

You can peruse the full detailed index at the National Blood, Heart, and Lung institute's site: Body Mass Index Scale 1.

Average BMI for Women

The interesting factor about the body mass index scale is that it is does not differentiate between men and women leading many experts to question its validity.  Since the scale does not differentiate between muscle mass and fat the results in some cases can be misleading as men inherently have more muscle mass than women.  As an example, a heavily muscled athlete could potentially be classified with an overweight woman of the same height and weight.