You're listening to SCIENCE FRIDAY on NPR NEWS. I'm Ira Flatow.

Up next, the latest in fat. Now, we're talking good fat here. Now those of you who have tended to babies may have heard the term brown fat. That's the name of a type of fat that keeps infants and babies warm. And rather than storing calories, brown fat actually burns calories and produces heat. And scientists have long thought that brown fat disappeared by the time these babies became adults. Turns out, they were wrong.

This week, researchers are reporting that they have found active brown fat in adult humans, and that fat might play a role in obesity and regulating glucose levels. Joining me now to talk more about this good fat is my guest, C. Ronald Kahn. He is the Iacocca professor of medicine at Harvard Medical School and the Joslin Diabetes Center in Boston. Welcome back to SCIENCE FRIDAY, Dr. Kahn.

Doctor C. RONALD KAHN (Iacocca Professor of Medicine, Harvard Medical School): Thank you, Ira.

FLATOW: How come we never know about this stuff?

Dr. KAHN: Well, I think that people had thought that brown fat, at least in adult humans, was only going to be present in some extraordinary circumstance. For example, it was known that people who had a disease of the adrenal glands, a tumor called pheochromocytoma, would have more brown fat. And people also knew that people working in the northern part of Finland and outside in the cold had some brown fat. But people thought this was the extraordinary, unusual example and that normal people didn't have it. But obviously, our article and the other two that were in New England Journal this week have shown that that's not the case.

FLATOW: Is the fat really brown colored?

Dr. KAHN: It is, indeed, really brown colored. You know, most of the fat in our body is white or slightly off white and yellowish because it's full of lipid, it's full triglyceride, because that's where we store the energy from the food we eat. When we don't have access to food, we can burn up our fat.

But brown fat doesn't have very much a stored lipid, not much triglyceride. Instead, it's filled up with mitochondria. Mitochondria are those little parts of the cell that are important for burning energy. So this fat burns energy rather than stores energy.

FLATOW: And is that why we have it, so we make the energy from the fat?

Dr. KAHN: Well, the reason that these little babies have it and also some rodents have it is that it's used to create warmth. So, as it burns up energy, it actually creates warmth. As we get bigger, it's less important in keeping warm. We have other ways to keep warm, like we shiver and we have sort of some insulation by the white fat in our bodies. So I'm not sure that in adults, its major reason is for warmth. But I do think it plays a big role in keeping this energy balance between storage and burning.

FLATOW: So now that we've discovered that we have the brown fat, does that mean that when we get really cold, we can lose weight?

Dr. KAHN: Well, that's a great question. And, of course, we don't know for sure the answer. What we do know is that people who are exposed to the cold for even two hours, or when we did our study, people in the winter versus the summer have definitely both more brown fat and more brown fat if it's an active state. So I think one of the questions that we really need to look at is: Could we take advantage of this as a way to kind of naturally stimulate this energy burning?

FLATOW: Aren't you going to see people now outside, no jackets on, after hearing this?

Dr. KAHN: Well, maybe we will.

(Soundbite of laughter)

Dr. KAHN: I think that there may also be, you know, some genetic differences in how we respond to this cold treatment. But I think, of course, the thing about obesity is that it's - there are so many factors that contribute - you know, not only how much we might burn through brown fat, but, of course, how much we eat and how much we burn through exercise.

FLATOW: Is there a way to tell how much an individual, you yourself, has - have brown fat?

Dr. KAHN: The techniques that were used in the studies in these three papers are really pretty involved techniques. They would only be used in a really major clinical setting. The techniques are called PET-CT, and it's really the combination of two techniques. The one technique, CT, is computerized tomography, and that helps us tell the difference between fat and muscle and bone and other kinds of tissues. And the other technique, which is called PET -stands for positron emission tomography - really involves giving a kind of radioactive glucose tracer to see what tissues are most metabolically active.


Dr. KAHN: And when we do this, we say, well, this fat is highly active, it's likely brown fat. And sure enough, when we biopsied it, and when the other groups biopsied it, it was brown fat.

FLATOW: Let's go to the questions. Tony in St. Louis. Hi, Tony.

TONY (Caller): Good day. A question about - are there differences in things like omega 3s versus omega 6s, as there seems to be in other creatures like tilapia, which farm-raised have greater omega 6 and wild-raised have greater omega 3s?

FLATOW: You mean, in our own - in our brown fat, you mean? Stored there? Good question. Dr. Kahn?

Dr. KAHN: Well, we don't know exactly how dietary fats will modify either the amount or the activity of brown fat. We do know that some of the radiologists who are trying to interpret these PET/CT scans for cancer purposes find that the presence of brown fat kind of a distraction, it misleads the diagnosis. And so they actually do give people sometimes a high fat diet, which actually seems to turn down the activity of the brown fat.

FLATOW: I would imagine then if brown fat causes you to lose calorie, then you would find more of it in thinner people. Is that correct?

Dr. KAHN: That is correct. And that's exactly what we found and actually what one of the other studies found, that is, the people who have lower BMIs tend to have the more functional, active brown fat.

FLATOW: Mm-hmm. And so, what does - and, of course, people are going to say now, hey, how - what can I do to stimulate my body to make more brown fat?

Dr. KAHN: Right. And I think that in addition to cold, which we've already talked about, I think that one of the things we now need to look at more carefully is are there other ways we can do this? We do know that certain kinds of stress hormones can increase the activity of brown fat. But, of course, sometimes when you have stress hormones, you don't feel so good because your body is responding to the stress.

We, ourselves - and I know others as well - are looking at are there maybe certain kinds of growth factors that might stimulate brown fat. And we previously published another research paper showing that there's a bone factor called BMP, which is used to stimulate different kinds of tissue growth. And some of these may stimulate brown fat, as well.

FLATOW: Where would you find them?

Dr. KAHN: These growth factors?


Dr. KAHN: Right now, they are - well they're, first of all, they're naturally in the body. But then the question is, for therapeutic purposes, whether you would actually need to give more or give it in a special way to maybe stimulate the brown fat.

FLATOW: Mm-hmm. Now we always talk about obesity, fat, obesity as related to glucose, as related to diabetes. Is there a connection here?

Dr. KAHN: Oh, definitely. First of all, there's the connection between obesity itself and type 2 diabetes, which as you know, is very strong, that people who are overweight and obese have a much higher risk of diabetes because their body becomes resistant to insulin. And this is what we call type 2 diabetes, or what used to be called the adult onset form of diabetes.

The connection with brown fat is also there. We show in our study, for example, that people who have more brown fat - even if they're not diabetic - tend to have lower glucoses within the normal range than people who have less. So even within the normal range, this could be a modulator.

FLATOW: Have we found any place on the genome that makes brown fat that we would think…

Dr. KAHN: Well, there are a number of genes that are involved in this program to get brown fat. Actually, one of my colleagues here at Harvard, Bruce Spiegelman, has identified one of the major genes. It's a gene that goes by the name PRDM16. And a few months ago when we were looking at this morphogenetic protein, he was looking at BMP-16. And he found that this is one gene that is somewhat of a master - or partial master regulator for getting tissues to become brown fat. So…

FLATOW: Interesting. Question from Second Life, from Scott23hawker(ph) says: Is brown fat as good as muscle at burning energy? That's a good question.

Dr. KAHN: Well, actually, in some ways, brown fat is better at burning energy. It's been estimated that if it's maximally stimulated with hormones that stimulate it, like the stress hormones, then as little as 50 grams of brown fat, that would be about two ounces of brown fat, could, in a thin person, burn up about 20 percent of their daily caloric intake.

FLATOW: Do people taking certain kinds of drugs have a less amount of brown fat?

Dr. KAHN: Yes. In our study, we found that one of the drugs that affects at least - we don't know if it affects the amount or the activity, but it definitely decreases the amount of detectable active brown fat is a class of drugs called beta blockers.

These drugs are commonly used for the treatment of hypertension and heart disease, and they're very valuable drugs. So I wouldn't suggest that anybody stop the drug…

FLATOW: And they're widely used.

Dr. KAHN: And they're very widely used. But, indeed, it does reduce the amount of active brown fat because it's blocking the effect of some of these stress hormones like epinephrine norepinephrine on the fat. And it's actually been shown in clinical studies, larger clinical studies, that people who are on these drugs often do actually gain a kilogram or two - that is about two to four pounds.


Dr. KAHN: These are very important drugs, and I would not recommend anybody stop them on this basis. But they do affect its activity for sure.

FLATOW: Let's go to the phones. Ray in Reno. Hi, Ray.

RAY (Caller): Hi. Good show. Thanks. I need to know any relationship between high density cholesterol and brown fat. And is there any way you can increase that? I know it's genetic somehow, but is there any way, because I happen to have low-density cholesterol, very low, but also, at the same time, a very low high-density cholesterol. And I need to raise that ratio.

Dr. KAHN: Yeah. So the kinds of cholesterol that you're talking about circulating in the blood - the LDL, low-density cholesterol, and the HDL, high-density cholesterol, those are mainly made in the liver. And we don't think that brown fat would have any direct way to control the levels of LDL, which is the bad cholesterol, or the levels of HDL, which is generally regarded as the good cholesterol.

But, clearly, people who are overweight, of course, are often involved in what's called the metabolic syndrome, where, in fact, they have lower HDL cholesterol.

FLATOW: One last question: Where do you go from here, now that you know this stuff about brown fat?

Dr. KAHN: Well, we're going in a couple of different areas to follow up on this. One area is to see if there are other ways to detect brown fat that might be simpler, a little less invasive, less expensive so that you wouldn't need to use radioactive isotopes. The second is, of course, is to find out what we can do to stimulate either the amount or the activity of brown fat and see if that really will be an important adjunct to either prevention or treatment of obesity.

We don't think you would need to stimulate it very much. We hope that even just a little stimulation might help change the balance and keep more of us thinner or help those who are overweight, maybe keep those pounds off.

FLATOW: Dr. Kahn, thank you for taking time to be with us today.

Dr. KAHN: Thank for having me, Ira.

FLATOW: Good luck to you. Dr. C. Ronald Kahn is the Iacocca professor of medicine at Harvard Med School and the Joslin Diabetes Center in Boston.

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