The risk of knee osteoarthritis in non-industrial societies is greater than previously realised and should be investigated.
Introduction
Osteoarthritis is a common condition that makes a person’s joints stiff and painful, particularly in the morning. It is caused by thinning of the cartilage within the joints, which allows the bones to rub against each other. Joint swelling and pain are frequent symptoms.
Osteoarthritis – especially in the knees – is a major public health concern in developed and newly industrialised countries. The risk of developing osteoarthritis depends on age, sex and certain genes, but one of the most common factors is obesity. Obesity is defined as a body mass index of 30 or higher. When people are carrying this much extra fat, it causes low-level inflammation all over the body. This inflammation then weakens and damages the joints. In non-industrial societies with access to less high-energy food, it is thought that people are less likely to suffer from diseases associated with obesity.
What did the authors hope to find?
The authors wanted to find out whether osteoarthritis happens in groups of people living in traditional, less industrialised societies such as hunter-gatherers or people who farm for their own use, without having any extra produce to sell. They also wanted to see if this was true in places where lifestyles and diets are quickly changing and becoming more modern.
Who was studied?
The study included men over the age of 40 from two groups. The first group included 157 men from an indigenous (native) population in Mexico called the Tarahumara. These men were all farming small areas of land to feed their families. The second group included 565 urban white men from Framingham, Massachusetts in the US. Men could join the study regardless of any knee pain or other health conditions.
How was the study conducted?
Each man in the study had his height and weight taken, as well as the measurement around his middle. Each man’s body mass index (often shortened to BMI) was calculated, which defines people as underweight, normal weight, overweight, or obese. All men also had an X-ray taken of their knees, and the investigators looked at these to see if there was evidence of osteoarthritis. Everybody was also asked whether they had pain in their knees on most days.
Some men also had their leg length measured, and compared as a ratio with their overall height. This ratio is believed to indicate how well-nourished a person was as a baby.
The authors then compared the data between the two groups.
What were the main findings of the study?
On average, the Tarahumara had lower BMIs and less obesity than the Americans. However, Tarahumara men tended to have larger abdomens for a given body weight than the Americans. This suggests that they store relatively more fat around their middle, which can cause chronic low-level inflammation. The authors think this could be caused by low-energy food consumption as babies, and high-energy food consumption later in life.
Importantly, the Tarahumara men were likely to have osteoarthritis in their knees at a lower BMI than the Americans. This increase in osteoarthritis seemed to be linked to the Tarahumara men’s abdomen size more than it was for the Americans. The authors think this means that the abdominal fat carried by the Tarahumara
men is stimulating degeneration of the knees.
Overall, these findings suggest that the risk of knee osteoarthritis is greater than might be expected in men living in non-industrial populations moving to modern high-energy diets and lifestyles, and cannot be predicted by obesity alone. Osteoarthritis in the knees is an obesity-related disease which is more common in people who have an imbalance between the food energy available to them in childhood and adulthood. These findings challenge the idea that a large proportion of the global population is not at risk of osteoarthritis, and highlights the need for greater research and clinical attention in non-industrialised areas.
Are these findings new?
Yes, this is the first time that this subject has been looked at.
What are the limitations of the study?
The study has some limitations, which mean we have to be careful in how we interpret the information. The data collected was from a small number of Tarahumara men and no women. Also, the authors assume that abdominal fat stores are linked to how well-nourished a person is as a child, but this was not confirmed in the study since they had no direct information about childhood nutrition. Nevertheless, their finding that relative leg length was negatively associated with abdomen size among the Tarahumara men provides indirect support for their arguments. It was also not possible to say whether differences in knee osteoarthritis between the two
groups could have been affected by different environmental causes, or a genetic link.
What do the authors plan on doing with this information?
The authors are currently planning more studies in other non industrial populations in South America and South East Asia who are undergoing rapid lifestyle changes.
What does this mean for me?
If you have osteoarthritis, these findings may not be relevant to you, although they provide interesting background on the way osteoarthritis develops, and how it may affect different people.
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Date prepared: December 2019
Summary based on research article published on: 13 September 2019
From: Wallace, IJ. et al. Knee osteoarthritis risk in non-industrial societies undergoing an energy balance transition: evidence from the indigenous Tarahumara of Mexico. Ann Rheum Dis 2019;77:1693–1698. doi:10.1136/annrheumdis-2019-215886
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