The Allomantic Metals use Certain Ratios, With Iron, You can Only Burn Pure Iron, So When you Eat, the Iron in your food wouldn't be Burnable because it's compounded with Carbon, Fiber, and all the Food Stuff. And When the Iron Enters Your BloodStream, It's no Longer Burnable, the Metal Has to be in the Stomach (I think, Can anyone Confirm that?) Kelsier mentions that you have to burn it all before you go to sleep, or it will enter your Digestive Systems, and then it's not Burnable.
Although Kel does mention that Vin got Trace Metals from the Water, that's not really Compounded, but Mixed, Aqueous Solutions. Which is Why they always drink the Metal, rather than Sprinkling it over Steak or bread.