'Biggest Loser' Wedding -- NO BOOZE ALLOWED

"Biggest Loser" contestant Joe Messina's recent nuptials violated the one cardinal rule of weddings everywhere -- GET EVERYONE WASTED -- because booze was NOT allowed at the party.

TMZ broke the story ... the former Season 13 contestant tied the knot recently in Michigan -- and 12 other current and former contestants attended -- but the wedding party lacked a key ingredient .... alcohol.

Sources tell us ... the groom and his "Loser" guests are serious as a heart attack when it comes to their new healthy lifestyle -- so they axed all forms of alcohol from the menu (alcoholic drinks contain a lot of calories). Thankfully, we're told the crowd -- which was pretty religious -- didn't drink much anyway.

But Joe's reign of terror didn't end there -- we're told he also subbed a real wedding cake for low-fat pumpkin cake pops ... and served only food from the "Biggest Loser Cookbook" ... such as: 

-- Grilled Chicken with Mango, Thai Quinoa Salad
-- Tossed Salad
-- Fresh Fruit Salad
-- Asian Chopped Salad
-- Salad Salad


And just when you thought it couldn't get any worse ... we're told the guests actually WORKED OUT before the ceremony.

No one was injured.