Consuming fast food more than twice a week has a strongly positive association with weight gain and doubles the risk of insulin resistance.

Reference

Pereira, M.A., Kartashov, A.I., Ebbeling, C.B., Van Horn, L., Slattery, M.L., Jacobs, D.R. (2005) Fast-food Habits, Weight Gain, and Insulin Resistance (the CARDIA study): 15-year Prospective Analysis. Lancet. 365, pp.36-42.