Antihistamines are generally H1 antagonists, not agonists. They block the action of the histamine receptor(s); they don't activate them. They tend to stimulate appetite as a side-effect. "Prescription H1 antihistamine use did not significantly increase the OR for elevated glucose, insulin, or lipid levels." [1]

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3221329/#R6