GWEN IFILL: But, first, as you have probably heard in a headline or two by now, it was a sobering day for meat lovers, especially in a country that ranks second in the world for eating the most meat.

William Brangham has the story and some perspective on what you need to know.

WILLIAM BRANGHAM: This new report released today by the World Health Organization found that often-beloved meats, like sausage, bacon, ham, and hot dogs, can cause certain types of cancer. Prior studies had established similar connections, but the WHO is the most prominent health organization to specifically say processed meats can cause cancer.

It looked at more than 800 studies around the world. It also found that eating freshly prepared red meat like steak or pork or lamb probably can cause cancer as well.

Here to help us sort through this is Jonathan Schoenfeld. He is a professor of radiation oncology at Harvard Medical School. He co-wrote a notable paper for "The American Journal of Clinical Nutrition" in 2012 about the links between diet and cancer.

Dr. Schoenfeld, thank you for doing this.

The WHO certainly put the scare into meat eaters all over the country today. How much of this — if I'm someone who has a couple of slices of bacon or a turkey sandwich a few times a week, how worried should I be?

DR. JONATHAN SCHOENFELD, Harvard Medical School: Sure.

So, I Think it's important to realize that what the World Health Organization was, like you said, as looked at hundreds of studies and put the evidence from those studies together to show that the consumption of processed or red meats was quite possibly associated with an increased risk of a variety of different types of cancer.

Now, what these studies did was look at the overall risk of cancer. And so it's hard to say that any one piece of bacon, for example, or piece of meat would increase your risk of cancer by a certain amount, but, in total, you know, these studies show that being one of the people who consume the most amounts of meats or processed foods could potentially increase your risk of these cancers.

WILLIAM BRANGHAM: So, let's break this down a little bit. When we say processed meats, what are we talking about?

DR. JONATHAN SCHOENFELD: So, it's actually a definition that encompasses a variety of different ways to add flavor or to help preserve the meats.

So the different studies that the World Health Organization looked at might have had a slightly different definition of what processed meats meant. So it could be things like salting, it could be things like adding chemicals to help preserve the food, or to help add flavor to these different foods.

WILLIAM BRANGHAM: Is it the process of processing itself that causes the problems or is it the cooking of processed meats?

DR. JONATHAN SCHOENFELD:You really couldn't say.

And the authors of this report admit as much. What they could look at is the total. The act of processing the meat or the consumption of the processed meat was associated with an increased risk of cancer, but the studies don't allow you to actually dissect that further and figure out what exactly it is about processed meats that may lead to the increased risk.

WILLIAM BRANGHAM: I know a lot of people in the attempt to be — to lead a healthier life and eat healthier foods have switched to things like chicken and turkey.

If those meats are processed, do those same risks come along with them?

DR. JONATHAN SCHOENFELD: It's something that's unknown.

There is certainly some rationale to think that that might be the case, but these studies specifically looked at the processing of red meat and whether or not that posed an increased risk of these types of cancers.

WILLIAM BRANGHAM: So, my understanding is that the WHO has now put processed meats in the same category as cigarettes. And we have seen some headlines today asking, basically, is eating a hot dog now like smoking a cigarette?

Help me understand that. What is the level of risk here? Or is that outlandish to say?

DR. JONATHAN SCHOENFELD: Yes, I think that's a very important point.

The World Health Organization classifies things into their risk categories based upon the level of evidence, and not upon the magnitude of risk. So while it may be in the same category as cigarette smoking, the magnitude of the risk associated with processed meats doesn't nearly compare to the magnitude of risk that's been found with cigarette smoking.

What that determination just means is that there are numerous studies that have found similar findings. The risk — the relative risk for smoking is much higher, and unlike the consumption of meats, the consumption of excess cigarettes or tobacco doesn't provide any nutritional benefits.

WILLIAM BRANGHAM: I see.

How comfortable are you with the strength of this science? I mean, the industry and some other scientists have said that the WHO might be overstating the case a little bit. How strong do you feel that this is really a concern we all ought to be worried about?

DR. JONATHAN SCHOENFELD: Yes, I think it's important to understand the strengths and limitation of this type of study and this type of recommendation.

I think we can have more confidence in the results because the methodology that is used is looking at not one study by itself, but the aggregate findings of many and, indeed, hundreds of studies. But it's important to realize that the magnitude of the effects and the absolute increase in risk with the excess consumption of processed meats, of red meat, is actually relatively small, especially with other things that we can modify in our lifestyle and diet, such as cigarette smoking.

So, you know, I think it's reasonable to be mindful of these findings, but I don't think that it should warrant an overall change in people's lifestyles at this point.

WILLIAM BRANGHAM: All right, Dr. Jonathan Schoenfeld of Harvard Medical School, thank you very much.

DR. JONATHAN SCHOENFELD: Thank you.