When was the last time your knees made a noise getting up off the floor? A pop, a crunch, a little grind on the stairs?
Mine started early. A doctor told me I had arthritis in my knees when I was 14. Three knee surgeries later, I have zero interest in a fourth.
My genes don’t help. I have Ehlers-Danlos syndrome, a condition that affects the collagen holding your tissues together. Collagen also forms the framework of your cartilage. So I’ve spent decades learning how knees work, mostly out of self-defense.
Your cartilage has no blood vessels
The cartilage covering the ends of your bones has no blood supply and no nerves. Almost every other tissue in your body gets food and oxygen delivered through tiny vessels. Your knee cartilage gets nothing delivered.
Instead, you feed it every time you move.
Your knee sits inside a capsule filled with synovial fluid, a slippery liquid that carries nutrients. When you put weight on your knee, you squeeze fluid out of the cartilage. When you lift the weight off, fresh fluid soaks back in and brings nutrients with it. This happens thousands of times a day.
Nutrients still seep in slowly when you sit. But sit still for eight hours and you lose the squeeze that keeps fresh fluid moving through.
And when your joint gets inflamed, the only cells living in your cartilage, called chondrocytes, change their priorities. Inflammatory signals like IL-1β and TNF-α push them to release enzymes that break cartilage down faster than they repair it. Your own repair crew starts doing demolition.
Your knees run on teenage collagen
In 2000, Dutch researchers measured how fast your body replaces the collagen in knee cartilage. They estimated a half-life of about 117 years. In other words, this structural protein turns over incredibly slowly.
A Danish team confirmed it a different way in 2016. They used carbon-14 left over from Cold War nuclear bomb tests to date the collagen in people’s knee cartilage. Once people stopped growing, their bodies made almost no new cartilage collagen. They got the same result in healthy knees and arthritic ones.
So the collagen framework you walk on today formed before you finished growing. The rest of your cartilage keeps turning over, but whatever happens to that collagen, you keep.
Sugar is one of the things that happens to it. When blood sugar is chronically high, glucose sticks to proteins and forms advanced glycation end products. Scientists call them AGEs, which is a pretty good name for them, since they do age you. Your body clears short-lived proteins before AGEs pile up on them. Collagen with a 117-year half-life sits there and collects them for decades.
The same Dutch group also found that when they raised AGE levels in human cartilage, the AGEs cross-linked the collagen fibers and made the tissue stiffer. Stiff cartilage handles impact worse.
So years of high blood sugar leave their mark on tissue you plan to use for the rest of your life.
Your knees change without telling you
Boston University researchers did MRIs on 710 adults over 50 in the Framingham Osteoarthritis Study. Every one of them had knee X-rays with zero signs of arthritis.
89% had at least one abnormality on MRI anyway. Cartilage damage, bone marrow lesions, meniscus tears, bone spurs. Plenty of these knees felt fine with no pain at all.
Some of these knees will never hurt. Either way, knees change. The best time to take care of them is while they still feel great.
Joggers had the healthiest knees
You’ve heard running wrecks your knees, but listen to this.
A 2017 meta-analysis pooled 17 studies and nearly 115,000 people. Here’s how often each group had hip or knee arthritis:
Recreational runners: 3.5%
People who didn’t run: 10.2%
Competitive and elite runners: 13.3%
The people jogging for fun had the healthiest joints. The couch crowd did worse. And the competitive runners did worst of all. I still don’t think running or chronic cardio is a good idea unless you really love it. There are better ways to strengthen your knees (coming up later).
The habits that protect your cartilage are basically free and take less time than you’d think.
Here’s what’s coming up:
The main way to take pressure off of your knees
The leg muscle linked to a higher risk of knee arthritis when it gets weak, and how to train it without beating up your joints
The walking trick that saves your knees
The supplement stack for your joints
