The body mass index, or BMI, a formula derived from your height and weight, is the dominant indicator in the medical community of a healthy weight. BMI has its critics, though, because it doesn't account for waist girth or age. In the elderly, for example, studies are mixed on the impact of a slightly elevated BMI on senior health and mortality, indicating that the guidelines may be too restrictive for people over 60.
The BMI is a reliable indicator of body fatness when compared to more direct measures like underwater weighing, according to the U.S. Centers for Disease Control and Prevention, or CDC. However, the CDC cautions that BMI is not a diagnostic tool, and a person deemed obese based on his BMI should undergo further testing to determine his actual body fat percentage and overall health risk.
Your BMI is the result of a calculation using your height and current weight. In standard measures, divide your weight in pounds by the square of your height in inches multiplied by 703: current weight/(height X height) X 703. The acceptable range of BMI is 18.5 to 24.9. A BMI under 18.5 is considered underweight, while a BMI between 25 and 29.9 is considered overweight. BMIs over 30 are labeled obese.
BMI in Seniors
The overweight designation -- a BMI between 25 and 29.9 -- is an indicator of all-cause and cardiovascular mortality risk in younger adults, but in seniors, that trend appears to reverse. A Yale study conducted in 2001 using the National Heart, Lung and Blood Institute's U.S. Clinical Guidelines on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults found no correlation between seniors who were mildly overweight and increased mortality rates. In fact, study participants labeled overweight had lower mortality rates than those in the acceptable range, suggesting that the ideal weight for seniors, as measured by BMI, is perhaps too restrictive.
As you age, you lose muscle mass. It's called sarcopenia, and as muscle mass decreases, fat tissue centralizes, filling in the gaps, so to speak. You also tend to store more fat as age progresses, thus maintaining steady or slightly increasing weight. Sarcopenia in obese elderly patients is associated with functional impairment and disability, so the upper ranges of BMI indicating obesity may indicate poor health outcomes in seniors as well as younger adults. That suggests that an ideal weight range for seniors is somewhere between the upper limit for good health for younger adults and the obesity range that is detrimental to patients of all ages. The Yale study concludes that 27, and not 25, should perhaps be the ideal upper limit for BMI in older persons.
Weight Loss in Seniors
Treating seniors with a BMI in the mildly overweight range like younger adults may have a negative impact on overall health and mortality rates. Restricted calorie diets in older patients who are not obese are correlated with negative health outcomes, says Harlan M. Krumholz, M.D., who led the Yale study. The potential harm to older adults brought on by diet-induced weight loss supports a less rigid definition of "ideal weight" for seniors.