Logically, we know that obesity, or excess weight, can worsen osteoarthritis. But what is the exact mechanism? Is it the extra pounding and pressure on the knee due to the excess weight? Let us see what research shows.
What is knee arthritis?
Arthritis is -itis of arthro-. ‘Arthro’ means pertaining to a joint and ‘-itis’ is a tag indicating inflammation. So arthritis is joint inflammation or swelling. Usually, arthritis happens because of damage to the cartilage that cushions and lubricates the movement of the bones that form the joint.
Types of arthritis
There are more than 100 types of arthritis. Many of them do not affect the knee. So we will restrict ourselves to knee arthritis. Amongst those, depending on how the knee cartilage is damaged, arthritis can be of four main types:
1. Osteoarthritis: This is caused by mechanical wear and tear of the joint cartilage.
2. Rheumatoid arthritis: This is caused by your body’s own immune system damaging it.
3. Gout: This is caused by crystals of uric acid getting deposited around the joint cartilage.
4. Trauma-related: This is caused by some accident or injury to the cartilage.
These four types suggest that obesity can possibly cause only the first type of arthritis. The other three causes have nothing to do with excessive weight. So we will focus on osteoarthritis of the knee in this article.
How common is knee osteoarthritis?
Ten percent of the USA population (all ages included in the calculation, though the condition is prevalent mostly amongst adults) suffers from osteoarthritis, as per Centres for Disease Control and Prevention (CDC), USA. If you consider people above 18 years of age, the prevalence is higher amongst obese people (31% versus 16%), and those with sedentary lifestyles (23% versus 18%).
Ten percent of the UK population suffers from osteoarthritis, as per National Health Service (NHS), UK.
Ten percent of the Indian population suffers from osteoarthritis. If you consider people above 40 years of age, the prevalence is higher amongst obese people (33% versus 28%), and those with sedentary lifestyles (37% versus 27%).
Lifetime risk for knee osteoarthritis
If these numbers sound confusing, consider that you have the following lifetime risk of developing osteoarthritis:
40%, if you are a man;
47%, of you are a woman.
Similarly, your lifetime chances of developing osteoarthritis are:
60%, if you are obese;
30%, if you are of normal weight or less.
The lifetime risk of osteoarthritis means the chance of your developing osteoarthritis some time in your entire life if you were born just now.
Most of the above risk arises as you age (a very small chance of developing osteoarthritis as a child or a young adult). So, if you are in your late 30’s, and you still do not have osteoarthritis, the above numbers are pretty much your risk of developing osteoarthritis in your ‘remaining’ lifetime.
If you are older, the ‘remaining’ lifetime risk is a bit less since you have lived some more years of your life without getting the disease. So, it is more than likely that you will get osteoarthritis if you are obese.
Back to the main topic of osteoarthritis versus obesity.
Mechanisms by which obesity affects osteoarthritis
If you are overweight, the excess weight puts additional pressure on the knee joints. Since osteoarthritis is the wear and tear of the knee joint, the damage can worsen. Or does it?
Forces on knee joint in daily activities
When you walk on a flat ground, the force on your knees is about 2–3 times your body weight. So an 80 kg person will put 200 kg of force on the knees. You might wonder why the weight does not get divided into half over the two knees. Well, that will happen when you are standing. But when you are walking, usually you put most of the weight on the leading, or landing, leg as you step forward.
If you climb the stairs, depending on the steepness, the pressure on the leading knee will increase to 3–4 times the weight. This is since the knee joint now has also to lift your body weight up.
Finally, when you squat down (which is the right way to pick up things; bending at the back is the wrong way) to pick up a pen you dropped on the floor, this pressure can go up to 4–5 times.
Here is the Harvard Medical School view on the subject. Read: Why weight matters when it comes to joint pain.
The following article has the views of some leading doctors about the situation in India. Read: Obesity and Arthritis are linked. Most developing countries must be facing similar situations.
We also know that reducing excess weight helps improve osteoarthritis conditions. Read: Weight loss for knee arthritis? Is more better?
So can we conclude that obesity leads to knee osteoarthritis due to the excessive pounding and pressures the knee joints get?
Is that how obesity contributes to knee osteoarthritis?
Contrary to what the experts have been saying above, in my view, no.
The evidence
Why do I say so? Look carefully at the statistics above from CDC and Indian researchers. I am re-quoting them:
In the USA, in people above 18 years of age, osteoarthritis is prevalent in obese people (31% versus 16% —31% of obese people have arthritis versus 16% of non-obese people have arthritis), and those with a sedentary lifestyle (23% versus 18%). In India, in people above 40 years of age, osteoarthritis is prevalent in obese people (33% versus 28%), and those with a sedentary lifestyle (37% versus 27%).
You will notice that a sedentary lifestyle is right up there, neck to neck with obesity, influencing your chances of having osteoarthritis. Since these are observed numbers, they indicate that obesity, or sedentary lifestyle, and osteoarthritis go hand in hand. But, that does not indicate that one causes another.
As a result, we can only hypothesize that a sedentary lifestyle, resulting from obesity, must be one mechanism. I have not read any confirmatory studies on this; so it is anybody’s guess. But this looks quite logical to me, though I am willing to be proven wrong.
What is your view? Do write in comments or if you have some data to point out, email me directly on madhur.kotharay@gmail.com.
In conclusion
Osteoarthritis was higher in sedentary people, compared to physically active people. And people, including doctors, think physical activity causes or worsens knee osteoarthritis.
Here is an article on this website about the most vilified physical activity for osteoarthritis, running. Read a comprehensive article on this website: Everything you want to know about running and knee osteoarthritis. It should tell you that running does not cause, nor worsen, knee osteoarthritis. In fact, it reduces the chances of getting osteoarthritis.
Here is the real culprit. Read on this website: Knee arthritis has doubled in the last 75 years, because of a sedentary lifestyle.
In other words, it is probably not the heavy pounding due to obesity that is causing knee arthritis, but rather the inactivity due to obesity that is the main cause of osteoarthritis.
Other mechanisms by which obesity affects osteoarthritis
I feel there are other mechanisms also by which obesity affects osteoarthritis. I will explore one such link in another article: inflammation. This was Harvard Medical School on that subject: Obesity and inflammation.
First published on: 14th October, 2016