Close to 40% of elderly Americans are obese or have osteoarthritis of the knee, or both, which, on average, eliminates 3.5 years of a positive quality of life for seniors, according to new research.

To determine the impact obesity and osteoarthritis has on the quality of life for people aged 50 to 84, researchers from Harvard Medical School calculated a mathematical model to evaluate years of good health lost. They found that about 33% are obese and do not have osteoarthritis—and can lose almost 2.5 healthy, pain-free life-years. People with osteoarthritis who are not obese (3.3%) are expected to lose 1.9 years of good health. About 3.3% of seniors who are obese and have knee osteoarthritis are projected to experience 3.5 years less of pain-free, good health.

Researcher Elena Rosina says telling obese patients to lose weight in order to avoid the disease isn’t very effective, “But if you say, ‘There is a likelihood your lifespan will be 3.5 healthy life-years less than if you were not obese,’ it’s easier to understand,” Losina said.  The study was published in the Feb. 15 issue of the Annals of Internal Medicine.