There have been some studies indicating that those who are overweight are at lower risk of rheumatoid arthritis, which is not the same as osteoarthritis, and of dementia. So let’s break these down a bit more.
Rheumatoid arthritis (RA) is a condition where your own immune system attacks your joints causing severe pain and eventually deformity and sometimes death. Interestingly a 2016 study of ~450 people in the Journal Rheumatology showed obesity was associated with a lower risk of getting RA in men only but not women.
However, in a recent study of over 1000 women with rheumatoid arthritis they saw the opposite. The more overweight the more arthritis. And the younger you are overweight and the longer you are overweight the more likely the risk. Also, another study showed that people with RA who are obese have more active disease, more inflammation, more pain and worse overall health, and a big reduction in the response to effective medications.
So overall -that’s a no for RA, being overweight is not better.
In regards to dementia risk, a study in 2015 suggested that the more overweight you are the lower your risk of dementia. There were a few issues with this study, because dementia is a disease of old age, and if you are overweight you are less likely to get to old age and hence less likely to get dementia. However, even for those who did make it to old age, overweight and obese people were about 30% less likely to develop dementia.
However, this study only went for 9 years. Another more recent study in 2017 of 1.3 million people from Europe, the United States, and Asia that went for much longer found that a higher BMI was associated with increased dementia risk when weight was measured >20 years before dementia diagnosis.
Several long-term studies have demonstrated that obesity in mid-life is associated with a 70-100% increase in the risk of later developing dementia and Alzheimer’s disease. But it is not just being overweight, it is what that represents, generally, people are less active and have poor diets and this also increases our risk of dementia.
So overall the consensus is being overweight increases your risk of dementia.