If you were to ask people to name a health problem related to obesity, odds are good they’d say “heart disease” or “diabetes.” And they’d be right; those two chronic diseases have a very strong relationship to excess weight. They are the safe bets.

But if you like long odds, put your money on arthritis. It’s not as commonly known, but obesity in some cases raises the risk of getting a certain type of arthritis; in all cases, obesity makes arthritis worse. One in 5 Americans has been diagnosed with arthritis, but according to the Centers for Disease Control and Prevention (CDC), that number jumps to more than 1 in 3 among obese people – and 2 out of 3 Americans are either overweight or obese.

Here’s a look at what fat does to arthritis, as well as some tips to put you on the road to losing weight.

Fat and Osteoarthritis

What It Is

Osteoarthritis, OA, is the most common type of arthritis, affecting approximately 27 million Americans. It is characterized by the breakdown of cartilage – the flexible but tough connective tissue that covers the ends of bones at joints. Age, injury, heredity and lifestyle factors all affect the risk of OA.

Why Obesity Matters

OA has a logical link to obesity: The more weight that’s on a joint, the more stressed the joint becomes, and the more likely it will wear down and be damaged.

“Weight plays an important role in joint stress, so when people are very overweight, it puts stress on their joints, especially their weight-bearing joints, like the knees and the hips,” says Eric Matteson, MD, chair of the rheumatology division at the Mayo Clinic in Rochester, Minn.

Every pound of excess weight exerts about 4 pounds of extra pressure on the knees. So a person who is 10 pounds overweight has 40 pounds of extra pressure on his knees; if a person is 100 pounds overweight, that is 400 pounds of extra pressure on his knees. “So if you think about all the steps you take in a day, you can see why it would lead to premature damage in weight-bearing joints,” says Dr. Matteson.

That’s why people who are overweight are at greater risk of developing arthritis in the first place. And once a person has arthritis, “the additional weight causes even more problems on already damaged joints,” says Dr. Matteson.

But it’s not just the extra weight on joints that’s causing damage. The fat itself is active tissue that creates and releases chemicals, many of which promote inflammation.