Is Sunlight Good or Bad for You? (The Real Answer Is Here!)

Rob Siegel, M.D. — Sun, 30 Mar 2008 18:25:19 -0700

Sunlight causes cancer (bad)! It also helps us make vitamin D (good). Should we avoid it? On Wednesday the Indoor Tanning Association ran a full-page ad in the New York Times with their opinion (pdf). Are they correct?

Patients complain to me that medical research seems flawed because health recommendations seem to change; news reports suggest that doctors flip-flop with our opinions. However, we doctors are able to make sense out of all this, and you can, too. To understand the controversy surrounding sunlight and many other medical issues, the most important thing to know is:

Understand the strength of the evidence behind a health claim. For example, we are essentially certain about these statements:

-Fluoride reduces risk of cavities
-Cigarette smoking increases risk of premature death

On the other hand, we are uncertain about these:

-Avoiding sunlight reduces risk of skin cancer
-Obtaining vitamin D from sunlight improves health

The cases of fluoride and cigarettes help explain today's sunlight controversy.

We became certain about the effects of fluoride and cigarettes for several reasons. It is easier to detect something that has a powerful effect on health than to detect something with a subtle effect. Fluoride is especially good at preventing tooth decay, and cigarettes are particularly effective at killing people. Because these effects are so powerful, it's hard for scientists to fail to notice them.

Additionally, fluoride and cigarettes received unusually thorough attention because nonscientists generated controversy about them. Young readers may not know about the success of the tobacco lobby in creating confusion about whether or not cigarettes are bad for you. Also, some people used to argue that fluoride was a communist plot designed to poison American children in order to help Soviets conquer the world. These agendas seem ridiculous today, but they seemed somewhat reasonable thirty years ago. Social controversy pushed scientists to continue studying these claims even after they had become reasonably certain, making them among the most thoroughly proven claims in all of medicine.

Sunbathing also generates controversy among nonscientists, but scientists have not provided much useful information on this topic. There are problems with the few studies that have found harm from sunlight. (Here is a summary of the currently available evidence concerning sunbathing.) Why is the evidence so weak? We don't even know the answer to that yet. So long as scientists don't have good answers, the suntan lotion industry and the indoor tanning industry will continue to finance publicity endeavors such as SunLightScam.com, the site listed on the Times advertisement.

That site prominently declares that vitamin D from sunlight provides health benefits, but what's their evidence? They do not cite any studies. The only reference they provide is a link to a Newsweek health blog. And you, dear reader, understand that information in national magazine health blogs can be unreliable.

The bottom line: Sunbathing may be good for you, or it may be bad for you. We just don't know. The strength of the evidence is too weak to make decent conclusions at this time. Former Defense Secretary Donald Rumsfeld, while talking about something else, summarized the epistemology of health recommendations:

As we know,  
There are known knowns.  
There are things
we know we know.  
We also know  
There are known unknowns.  
That is to say  
We know there are some things  
We do not know.  
But there are also unknown unknowns,  
The ones we don't know  
We don't know.

-Feb. 12, 2002, Department of Defense news briefing

This blog is not intended to diagnose, treat, cure or prevent any condition or disease. Please consult with your own physician or health care specialist regarding the suggestions and recommendations made within.

The Claim: Caffeine Causes Birth Defects

Rob Siegel, M.D. — Fri, 21 Mar 2008 08:24:27 -0700

My pregnant friend has studiously avoided cats (toxoplasmosis), alcohol (fetal alcohol syndrome), tuna (mercury), soft cheese (listeriosis), and sushi (oy gevalt). Recently she sent me this, wondering if she should also give up her daily macchiato habit:

“Too much caffeine during pregnancy may increase the risk of miscarriage, a new study says.”
New York Times, January 21, 2008, “Pregnancy Problems Tied to Caffeine”

This finding is surprising because no serious clinician has any idea how this could possibly work. Most miscarriages are caused by chromosomal abnormalities. Caffeine does many things, but nobody has ever shown that it harms chromosomes. It’s difficult to imagine how caffeine could possibly cause miscarriage. Is this harm real?

Anytime the press reports a possible peril to a developing fetus, it’s easy for the report to add an item to the list of pleasures that are prohibited during pregnancy. Here’s how:

Pretend that we’re concerned that watching television might harm a fetus. (It sounds plausible, right? As a society we believe that some things are likely to cause harm that we do not yet know, and these things include caffeine and television.) Let’s do a thought experiment that could make a medical researcher famous.

1) Give questionnaires to some women. Ask how much TV they watched during pregnancy.
2) Ask about lots of bad things that could possibly happen to a developing child: Obesity, prematurity, attention-deficit-hyperactivity disorder, and more.
3) Send the questionnaires to statisticians; instruct them to analyze the results in hundreds of ways.

If you analyze imperfect data in enough ways, you can always find some meaningless, coincidental correlation. Biostatisticians call this practice “data dredging.” This technique frequently yields misleading results, but some researchers do it anyway. This is how the caffeine/pregnancy study was done. Our health journalists could have ignored this exercise in data dredging, but, instead, they broadcasted it on TV.

Back to the thought experiment. Imagine that the statisticians dredge up a correlation between watching television in pregnancy and having a child with Down syndrome.

4) Publish the result.

Down syndrome, just like miscarriage, is caused by a chromosomal problem. It’s completely unknown how television—or caffeine—could possibly harm chromosomes. However, now the American Pregnancy Association states that women should minimize caffeine intake, even though they acknowledge that studies on this topic have been “inconclusive.” How do they explain their recommendation? “It is still better to play it safe when it comes to inconclusive studies.”

Our television study was also inconclusive. I suppose it’s always better to “play it safe,” after every study result, no matter how flimsy. No more coffee (miscarriage) or TV (Down syndrome) for pregnant mothers!

Yes, this thought experiment is inane. However, it’s difficult to show how it is different from the caffeine study. There are other, worse flaws with the methods of the caffeine study, making it difficult to conclude anything from it. Good science may someday show that tiny amounts of caffeine in pregnancy cause harm. However, seeing how little we know, it may turn out that caffeine is actually beneficial in pregnancy. We just don’t know. This study didn’t tell us any more about caffeine than our thought experiment told us about television.

This is not trivial. Thousands of women will suffer headaches from caffeine withdrawal as a result of this study. In exchange for their headaches, these women receive protection from a condition that caffeine probably cannot cause.

Let them drink coffee.

*The proper study design to answer this question is a randomized, placebo-controlled, double blind clinical trial. This sort of data dredging study—the one reported in the Times—is, as described above, largely useless for answering a clinical question.

This blog is not intended to diagnose, treat, cure or prevent any condition or disease. Please consult with your own physician or health care specialist regarding the suggestions and recommendations made within.

Psychology Today Blogs - Let Them Eat Cake

Is Sunlight Good or Bad for You? (The Real Answer Is Here!)

The Claim: Caffeine Causes Birth Defects