This is my own anecdotal experience.  I am not a doctor and this is not
intended to be medical advice.

I always thought that there was a connection in-between the food that I ate
and my psoriatic arthritis.  The food that would aggravate my symptoms the
most was pizza - my psoriasis and arthritis would flare up dramatically the
next day.

At first I thought that the pizza sauce was the culprit; I had previously
read about some people being allergic to the nightshade vegetables so I cut
them out.  It did nothing for me.

I then started on Enbrel after failing methotrexate and it was like a
miracle for me.  At one point, about four years ago, I could hardly walk.
After being on Enbrel for four months, my joint inflammation was normal.

I was taking Enbrel twice weekly for about a year.  The inflammation in my
joints was no longer there, but I still had the psoriasis.

Six months ago, I tried the Paleolithic diet which consists of fresh meat,
fruit and vegetables.  Amazingly enough, my stubborn psoriasis patches
started to clear up.  These are patches that 3 years of Enbrel,
methotrextate, prednisone and topical steroids (not all at once) failed to
clear up.

It occurred to me that it might be the gluten in the pizza that caused the
reaction.  I experimented by adding foods back into my diet.  I discovered
that both gluten and red meat (beef and lamb) cause a reaction.  I also
noticed that my digestive system works better when I avoid these foods.

After cutting out these foods (avoiding gluten basically means that you have
to cook most meals yourself), I was able to reduce my input of Enbrel
dramatically.  I am currently using Enbrel only twice a month, usually to
combat accidental gluten ingestion.  I have almost no psoriasis, no joint
inflammation and my skin feels healthy.

My hypothesis is that I have a food allergy which promotes my immune system
to attack my intestine.  After my intestinal wall was weakened, food
particles were able to enter my blood stream which then triggered the join
inflammation.

Does anyone have any similar experiences or comments?