GEOFF BENNETT: The World Health Organization has classified the artificial sweetener aspartame a -- quote - - "possibly carcinogenic to humans."

And that has led to some concerns and confusion about the risks.

Besides diet sodas, aspartame can also be found in a variety of foods, including breakfast cereals, chewing gum, and some desserts.

The WHO's cancer research group said it was categorizing the artificial sweetener as a possible carcinogen.

But the agency's food safety group said the evidence wasn't convincing.

There's quite a bit to sort through here.

And to help us clear up some of this, Allison Aubrey of NPR News joins us now.

Thanks for being with us.

ALLISON AUBREY, National Public Radio: Thanks for having me, Geoff.

GEOFF BENNETT: So, adding to the confusion, the FDA also says it doesn't believe that this artificial sweetener is carcinogenic.

So what are we make of all of this?

ALLISON AUBREY: I think you have to start by looking at, why did the World Health Organization decide to look at this?

The reason was that had there been several studies over the course of the last five or 10 years that found that people who consumed the most aspartame over time had a slightly elevated risk of certain cancers.

This was showing up in kind of population studies.

And the question was, well, why?

Could aspartame actually explain the risk?

So the scientists at the World Health Organization decided to review all of the evidence.

And what they determined is that, though these were good studies, they could not determine whether this finding of a slightly elevated risk of cancer was due to chance or bias, or just that the people who are getting cancer and were drinking -- consuming lots of aspartame might have other risk factors or other lifestyle habits that had put them at higher risk of cancer.

GEOFF BENNETT: The FDA says that aspartame is one of the most studied food additives in the human food supply.

It's been on the market for some 50 years.

How are there still unanswered questions about its safety?

ALLISON AUBREY: Right.

I think that's the confusing part to people, because the FDA says, look, it's safe.

It's well studied.

And, on one hand yes, it is very well studied.

There are lots and lots of short term studies.

I think that over time, if you think about cancer, it's a disease that develops over decades.

And so I think the question was, OK, this was introduced into the food supply in the 1980s.

That's when Diet Cokes first started using aspartame.

That's when Diet Coke was first blended with aspartame.

So the question was, was there some kind of long-term risk that people were missing?

And I think that's what the who was trying to assess and trying to look at.

But I come back to where the FDA concluded.

The FDA basically says, as a sweetener in the amounts that people consume it, the agency has reaffirmed its finding that aspartame is safe.

GEOFF BENNETT: The American Beverage Association, the lobbying group, the trade group that represents Coke and Pepsi, says that the WHO's finding is an expression of personal opinion, not the science, and they say their products are safe.

Setting that aside, or their recommendation shins from health organizations about how much people should consume?

ALLISON AUBREY: So the recommendation that comes from the agency within the WHO says that people could consume a maximum of 40 milligrams of aspartame per kilo of body weight.

So, what that translates to for a 130-pound person is about 12 Diet Cokes a day.

(LAUGHTER) GEOFF BENNETT: That's a lot of Diet Coke.

ALLISON AUBREY: Right?

That's a lot more than most people are consuming.

GEOFF BENNETT: So, bottom line, then people don't have to give up their Diet Cokes?

ALLISON AUBREY: I think bottom line here is that the agency is saying, you know what, we're asking for a bit of moderation here.

I think that if you ask yourself, why am I drinking Diet Coke?

Am I drinking diet soda because I want to help manage my weight, and I think that's helping me?

Perfectly reasonable answer there.

So a Diet Coke or two a day, I don't think that there's any evidence it's going to harm you.

But you also might say, hmm, given the uncertainties, I don't really enjoy Diet Coke, I don't really enjoy aspartame, I don't think it's helping me manage my weight, maybe it's a moment to reassess.

But, again, in moderation, the FDA says aspartame is safe.

GEOFF BENNETT: NPR health correspondent Allison Aubrey, thanks for coming in.

ALLISON AUBREY: Thanks for having me.