I just scheduled an appointment to donate blood next week. I'm instructed to eat beforehand, but to "avoid fatty foods for 24 hours before donating." According to the Red Cross:
Eat a healthy meal before your donation. Avoid fatty foods, such as hamburgers, fries or ice cream before donating. Tests for infections done on all donated blood can be affected by fats that appear in your blood for several hours after eating fatty foods.
"Avoid fatty foods for twenty-four hours before you donate. Increased fat in your blood stream might make it impossible to get accurate readings on some of the mandatory screening tests conducted on samples of your blood after you donate. If they can't run all the tests, they have to discard your donation. You don't want all your effort wasted!"
It's quite possible that all these people who are telling you it's fine are never told there was something wrong with their donation, especially if it wasn't a real medical issue, just an error with the screening tests. Blood gets discarded, doesn't help anyone, and you never know.
Seems like the potential for an awful waste inefficient use of time - may as well just stick to low fat paleo for a day beforehand.
There is nothing on that link saying you can't eat eggs, beef, or butter. They list beef and eggs under sources of iron. It says nothing about NO fat. It just gives advice on the few hours before the test, as it affects your blood prior to donation. You can chug down your butter right after the test if you really want to make up for one meal. I'm sure it's not a BIG deal, but for blood donation, they probably just want to play it "safe" and increase the likelihood that blood will be useable.
The post-blood donation calls (to get you to donate blood) are from people who have no idea of your medical history. They go by a giant list of previous donors or even those who put their email on a list and never donated. And like someone else said before, unless you have AIDS, they don't have the time to call each person up whose diet interfered with their testing.
I think the confusion is due to the misconception that fat increases fat and triglicerides in your blood. From the looks of it they just advise not eating junk food which should be no problem for you since I doubt Mcdonals and ice cream are on the menu.