Iron salts and Omeprazole drug interaction?

there is clearly a drug interaction between the oral iron supplements like ferrous sulfate, gluconate,fumarate and proton pump inhibitors. reasoning: any agent that increases the gastric pH will ultimately impair iron absorption bec iron salts need to go into solution in an acidic medium in order for proper absorption to take place. Having said that where can i find a primary reference that states the percent decrease in absorption of the 3 iron salts. THANKS!!!