What Causes Elevated Triglyceride Level?

Hello Dr Koregol, My husband is a very healthy active 70 yr old. He has normal blood pressure levels, and normal EKG readings as of last week. His blood work came back with high trigliseride levels. (800) Last year the same thing happened, but after 2 weeks of a strict diet, he was told his level were perfect . We questioned this at the time but were assured all was fine. Honestly his diet is high in fiber, low in fat and exercise daily. Now this year his trigliserides are high again. We typically eat pizza once a week, should he give this food up entirely? Thank you for your help.