The “wear and tear” model of arthritis never really sat well with me. It just doesn’t make much sense if you understand physiology. It’s common for healthcare professionals to make analogies of the body being like a car. This can make sense in some ways such as fuel and metabolism. However, the problem is that many people, including healthcare professionals, say things such as, “like a car, your parts just wear out”. That reminds me of the quote, “just because it’s logical, doesn’t mean it’s physiologic”. 

The idea is that if you are obese, you are going to put more mechanical pressure on your joints and wear them out faster. Certainly, there is a correlation between obesity and arthritis, but I don’t think pressure is the main driver. There is a law in physiology called Wolff’s law: Bone grows and remodels in response to the mechanical stresses placed upon it. That’s for all tissues too, not just bone, including cartilage. So you’d expect that pressure would grow cartilage, not erode it. We’ve seen studies that show that runners have more knee cartilage than sedentary people. So it doesn’t quite add up. 

It also doesn’t make sense that those with obesity have more arthritis in the hands, which are non-weight bearing. Also, it is well known that astronauts come back from space with cartilage erosion and osteopenia, or bone loss. They are in a weightless environment, how does this make sense? This goes back to Wolff’s law that can be rephrased to: Bone breaks down in response to the mechanical stresses not placed upon it.

So what is the driver? Researchers have discovered that body fat plays an important role in joint health, because of the hormones fat tissue produces. One of the most important of these hormones is called leptin.

Leptin is best known as the hormone that tells the brain when we have enough energy stored. In theory, higher levels of leptin should make us feel full and encourage us to use energy. In practice, many people develop what is called leptin resistance, where the brain no longer responds properly. This is one reason why obesity is often difficult to overcome.

What makes leptin even more interesting is that it is found inside the fluid that cushions our joints. Cartilage cells and other tissues in the joint can also produce it. Instead of protecting the joint, leptin often promotes inflammation and activates enzymes that break down cartilage. It can also influence bone, encouraging the growth of bony spurs known as osteophytes that are a hallmark of osteoarthritis. Higher leptin levels act almost like a chemical messenger that speeds up joint damage, even in joints that do not carry much weight, such as the hands.

I’m not suggesting this is the only driver, but certainly a key driver. We know of other contributors such as pathogenic gut bacteria. We find the same gut bacteria in arthritic joints and they inhibit the turnover of new cartilage. We also know there is a strong correlation between excess sugar in the diet and arthritis. 

The reason that I think this is an important point is because it changes the prescription. Patients with arthritis are commonly told to rest and protect the joints. Perhaps that’s the worst possible thing to do. 

References:

Dumond H, Presle N, Terlain B, et al. "Evidence for a key role of leptin in osteoarthritis." Arthritis Rheum. 2003;48(11):3118–3129.

Francisco V, Pérez T, Pino J, López V, Franco E, Blanco FJ, et al. "Biological role of adipokines in osteoarthritis: a systematic review." Int J Mol Sci. 2019;20(18):4650.

Gandhi R, Takahashi M, Smith H, Rizek R, Mahomed NN. "The synovial fluid adipokine network in osteoarthritis." Arthritis Rheum. 2011;63(3):719–727.

Ku JH, Lee CK, Joo BS, An BM, Choi SH, Wang TH, Cho HL. "Correlation of synovial fluid leptin concentrations with the severity of osteoarthritis." Clin Rheumatol. 2009;28(12):1431–1435.

Mosher TJ, Liu Y, Torok CM. "Functional cartilage MRI T2 mapping: evaluating the effect of physical activity on knee cartilage." Osteoarthritis and Cartilage. 2010;18(3):358–364.

Tirman PF, Feller J, Janzen DL, Peterfy CG, Bergman AG, Mosher TJ, Genant HK. "MRI of articular cartilage in the knee: correlation with physical activity." AJR Am J Roentgenol. 1997;169(1):111–118.