While body mass index is typically a practical way to determine if a person's weight is healthy for his height, it often is not a reliable measurement of obesity and health risks in the elderly. BMI should not be used as a single predictor of health risks, because most individuals actually gain fat mass and lose muscle mass as they age. These shifts in fat composition can have a significant impact when interpreting BMI statistics.
Calculating BMI
Your body is composed of fat mass and lean body mass. Because adipose tissue stores calories as triglycerides, BMI is used as a predictor of cardiovascular disease in the general population. You can calculate BMI by dividing your body weight in pounds by your height in inches squared. Multiply this number by 703. The Centers for Disease Control and Prevention point out that while the correlation between your BMI number and the amount of body fat is strong, there can be some differences based on gender and age. For instance, even when they have the same BMI, women tend to have more body fat than men. Likewise, older adults generally have more body fat than younger adults at the same BMI.
Average BMI
Typically, a person with a BMI lower than 18.5 is considered to be underweight. If your BMI falls between 25.0 and 29.9, most doctors would consider you to be overweight. A healthy BMI range is between 18.5 and 24.9. When it comes to the elderly, however, it's often better to have a BMI between 25 and 27, rather than under 25. One benefit is that a slightly higher BMI may help to protect you from osteoporosis. According to a study published in 2008 in the journal "Nephrology Dialysis Transplantation," an elderly adult with a BMI of 25 to 29 would then be considered normal and not obese. Similarly, an elderly individual with a BMI of less than 20 could be malnourished.
More Body Fat
BMI may not be an accurate indicator of body composition in the elderly, as it does not distinguish muscle from fat. Since body size, shape and composition vary with age, these changes may alter the practical significance of BMI at different ages, suggesting that the BMI limits for the elderly population be adjusted to account for these basic differences. To get a more clear idea of an individual's baseline health status for comparing health risks in the general population, it's important to understand that at the same BMI, most older adults usually have a higher percentage of body fat. Waist circumference, or amount of abdominal fat, may be a better predictor of risk for developing diseases related to being overweight when it comes to the elderly, the PhysOrg website explains.
High BMI
Some of the chronic diseases and other health problems associated with high BMI include coronary heart disease, stroke, hypertension, gall bladder disease, type 2 diabetes, and high levels of LDL --- or "bad" --- cholesterol and triglycerides. Low levels of HDL --- or "good" --- cholesterol, osteoarthritis, sleep apnea and certain cancers are other health conditions to which high BMI and obesity contribute. Health professionals use BMI as a screening tool to identify those adults who might be at higher risk for certain health problems because of too much weight. However, the conclusions of a study published in a 2009 issue of the journal "Annals of Epidemiology" show that because of the physical changes in body proportions that often occur as a person gets older, waist size may matter more than BMI when assessing health risks for individuals age 70 and older.
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