Weight, obesity, psoriasis and psoriatic arthritis

You hear a lot these days about body weight, psoriasis and psoriatic arthritis. But what is fact and what is conjecture? We try to sort it out for you below.

Is being overweight or obese one cause of, or trigger for, psoriasis or psoriatic arthritis?

Suspected by not certain. More researchers are coming to believe that obesity can make one more susceptible to psoriasis and psoriatic arthritis. And certainly, the evidence is pretty strong that people with psoriasis, on average, have a body mass index (BMI) higher than people who do not have psoriasis. Put differently, there is a higher rate of obesity among psoriasis patients than in the general population. (BMI is a measure of body fat based on height and weight, and you can measure yours easily with this National Institutes of Health BMI calculator.)

Psoriasis patients who reported being overweight or obese at age 18 had a higher rate of psoriatic arthritis later in life than patients who reported they were not overweight at age 18:

“[P]atients who report having been obese at age 18 years are 3 times more likely to develop PsA in the course of their psoriasis than patients with normal BMI at age 18 years.”

According to an earlier summary by the same researchers, “Among patients with a BMI at [age] 18 of less than 25, 26.5% went on to develop psoriatic arthritis, compared to 35.4% of those with a BMI between 25 and 30, and 40% of those with a BMI over 40.” Someone is considered overweight with a BMI of 25 or greater, and obese with a BMI of 30 or greater. (One shortcoming of the study is that weight at age 18 was based on participants’ memories – an average of 30 years after the fact – rather than medical records.)

The fact that two conditions often appear together – psoriasis and obesity – does not, by itself, prove that one causes the other. Some people theorize that certain hormones, like the much-hyped protein leptin, found in higher levels in obese people, could contribute to psoriasis susceptibility. Or perhaps there are common genes that predispose one both to obesity and psoriasis. Certainly, both conditions involve chronic inflammation. Further psoriasis research may help us learn the answer.

Does psoriasis or psoriatic arthritis contribute to making one overweight or obese?

Some researchers believe that psoriasis and/or psoriatic arthritis contribute to obesity. The pain and embarrassment of psoriasis, for example, might lead people to exercise less. In addition, the higher rate of depression in psoriasis patients than in those without psoriasis, and even higher depression rate among those with psoriatic arthritis than those with psoriasis but not arthritis, might also contribute to reduced exercise. But the actual research on this is less definitive than the anecdotal theories.

Of course, it could also be that obesity can increase susceptibility to psoriasis or psoriatic arthritis, and that psoriasis can also lead to increased body weight. It could be that each reinforces the other.

Does being overweight or obese make psoriasis worse? Does losing weight make psoriasis or psoriatic arthritis improve?

Psoriasis patients with a higher BMI tend to have more severe psoriasis, on average, than psoriasis patients with lesser BMI. In addition, people with severe psoriasis have a higher rate of psoriatic arthritis than patients with mild psoriasis. The reasons for these associations are not yet clear.  Similarly, there is scant evidence (to date) that weight reduction (or BMI reduction) directly improves psoriasis symptoms. There is evidence (mostly in research done on other types of arthritis) that a reduction in BMI can reduce the negative effects of arthritis.

But beyond that, there are obviously many good reasons for obese or overweight patients to work on reducing their BMI. As we learn that psoriasis is associated with heart disease, heart attack, stroke, diabetes and other diseases, many of which are also linked to obesity, it becomes more important than ever for psoriasis patients – who can’t yet cure their psoriasis – to address other risk factors for these other diseases than can be addressed, like smoking and excess weight.

So while weight reduction is notoriously difficult to achieve and maintain, it is worth sustained effort.