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Dear Science: I gave blood recently after drinking (many cups of) coffee and can't help but wonder — will any of the recipients of my caffeinated blood feel a caffeine jolt??? Thank you, I love reading the science column!

Here's what science has to say:

We hope you're not exceeding the maximum recommendation of five cups of coffee a day, but something about your choice of punctuation suggests otherwise. Just remember: A serving of coffee contains about 100 mg of caffeine. Consuming more than 500 or 600 mg a day can make you nervous, irritable, twitchy and nauseated, among other things.

The good news is that the recipient of your generous blood donation probably didn't notice a blood buzz.

"I know of no data that shows that it would have an impact on the recipient," said Jerome Gottschall, senior medical director of the BloodCenter of Wisconsin.

In fact, to our knowledge, no one has bothered studying this question at all: The researchers we talked to agreed that it was a fun one to talk about, but it just doesn't rank high on the list of concerns about the safety of the blood supply.

Caffeine, which works its magic by blocking the neuroreceptors for a chemical called adenosine, has a half life of around five hours. That means that while the amount in your system just about drops by half every five hours or so, it takes quite some time probably about two days  for it to disappear completely. If you give blood in the morning and drink a typical serving of morning joe beforehand, you probably have a good 100 or 200 mg of caffeine still kicking around when you go under the needle.

"Probably lots of donors have caffeine in the morning and donate sometime in the day, and this has gone on forever and ever," Gottschall said. "And for the most part that has no effect on the donor themselves."

Alan Mast, a senior investigator for the BloodCenter of Wisconsin and a spokesman for the American Society of Hematology, agreed.

"To be totally honest, usually when a patient is in the hospital getting a blood transfusion they have a lot of other things going on," Mast said. If they got a tiny perk from caffeine, they'd be unlikely to notice. They might not even be conscious until it had worn off.

Mast and Gottschall both think that even that tiny, easily ignored buzz would be rare. It's a question of dilution: Blood is made up of red blood cells and plasma, but most people who receive transfusions get what we call packed red blood cells. Those super bright red bags of blood you see hooked to IVs in hospitals aren't just wholesale blood they're full of donated blood that's had the plasma removed, then been mixed with other red blood cells.

It's the plasma that would carry the caffeine, Mast explained, and there would only be around 20 milliliters of that buzzy blood product diluting into the five liters of blood typically found in the human body.

"That's a very small cup of coffee," he said.

If someone received a plasma donation from a donor who drank coffee at just the right time, he said, it could potentially cause some kind of noticeable bump in the recipient's caffeine levels. But because it wouldn't hurt them and likely wouldn't make them feel any perkier, given their current state it's unlikely anyone would notice or care. Furthermore, because no one has studied this question, it isn't clear how the process of preparing donated blood for use after separation and testing, red blood cells are refrigerated for up to 42 days and plasma can be frozen for a year might degrade caffeine molecules.

That doesn't mean you should chug coffee before giving blood.

"It can be a very mild diuretic at times, so that’s probably not a good thing," Gottschall said. Caffeine makes you pee, which means less liquid in your body. Donating blood means losing even more liquid. Being dehydrated means you might be the guy who passes out on donation day, and no one wants to be that guy.

So caffeine falls firmly in the "who cares" category, as long as you stay well-hydrated. We asked whether any foods a donor might eat could prove problematic, and the answer is a resounding "probably not" (though you should avoid very fatty foods, which can interfere with some of the testing done on blood products, and bulk up on iron-heavy foods for a couple weeks before to avoid being too anemic to give).

"Occasionally patients have an allergic reaction, hives or something, and we don’t know exactly all the time what causes that," Gottscall said. "Could that be related to a food intake they have an allergy to? We’re not sure, but that’s not really considered when people donate."

But there are drugs that hematologists take more seriously than caffeine: Medications that cause birth defects, like certain prostate or acne drugs, are forbidden. There hasn't been much research on how likely it would be for blood tainted with these drugs to hurt a pregnant woman's fetus again, the dilution of the substance would probably make any exposure negligible but better safe than sorry.

Both researchers were cagey when we asked about more recreational drugs. Screening questions and track mark checks are meant to cut drug users from the donation pool, but blood isn't tested for things like cocaine or heroin it would just be too expensive to do so. Dilution should keep any errant illegal substances from posing a real threat, but it goes without saying that donating blood while high is a bad thing to do for many reasons.

And try to avoid aspirin-like painkillers before your donation. These drugs inhibit the function of platelets, the tiny particles that make blood clot. That's obviously not ideal during a risky surgery.

"You'll be giving the patient a product that doesn't work right," Mast said.

You'll be waived through aspirin-and-all if you're just giving whole blood, but you'll be prohibited from donating via aspheresis, a.k.a that super cool looking machine that takes your blood, spins it up and puts some of it back into your body. Clinics use this method to get more of the specific blood products they need on any given day. It's good practice to make sure you're capable of providing usable platelets when you go in to give blood, because someone in your region might need them.

Because Denmark and Sweden have nationalized health care and link records between blood donation, medical history and recipient outcomes, scientists now have intriguing data on other blood donor concerns, like cancer and Alzheimer's.

"People are starting to ask whether, if a person donates blood and is diagnosed with cancer a year later, they might have already been sick and given cancer to the recipient," Mast said. But Denmark and Sweden's follow-ups have shown that this doesn't seem to be a problem. Similarly, those who gave blood all their lives before developing Alzheimer's did not appear to increase the risk for the syndrome in their recipients.

"The obvious things that we test blood for are HIV and Hepatitis B and C," Mast said. "Nothing is ever 100 percent, but our testing accuracy is nearing that." So the likelihood of even these diseases contaminating the blood supply are very, very low.

At the end of the day, what you put into your body before donating has more of an impact on your own health than that of the lucky recipient of your caffeinated or fatty blood. Even rules about fevers and cold symptoms are meant to protect donors, not recipients.

"If a donor is so sick that they have bacteria in their blood, that could be passed on," Gottschall said. But someone with septicemia is unlikely to get out of bed and mosey down to a blood drive.

"It's really about the fact that if you have a fever, you know, you're sweating, you're not feeling well, you're dehydrated," he said. "You want the donor to be feeling healthy on the day of the draw, no question."