The word pops up often in ads and stories about health, but misunderstanding what it really means can cause problems
EATING CHOCOLATE COULD REDUCE YOUR RISK OF HEART DISEASE BY A THIRD.
That was the lead of many news stories last week, after a study in the British Medical Journal reported that people who ate more chocolate had a 37 percent lower risk of heart disease and a 29 percent lower risk of stroke.
But what do those numbers mean? Do they apply to you? Are there other risks associated with eating chocolate, like weight gain? And if chocolate reduces your risk of heart disease by 37 percent, what was your risk to begin with? In other words - 37 percent of what?
Throughout the day, we’re bombarded with information about risk. Foods, medications, and lifestyle habits either reduce or increase our risk of a certain disease, say many studies.
That information can be valuable. But it is often misinterpreted by the media, consumers, and even doctors.
In the case of the chocolate study, the researchers never answered a key question: How likely is a person who doesn’t eat chocolate, or eats very little, to suffer from heart disease or stroke?
Without those numbers, consumers are in a situation that’s a little like having a 50 percent-off coupon without knowing the prices of anything in the store. You don’t want to use the coupon on a $1 item, saving only 50 cents, if you can use it on a $50 item and save $25 instead - but without those original prices, you’re flying blind.
The same is true for medical studies. In this story, we trace through a few hypothetical and real-life examples to show you how to better understand the concept of risk in medical studies aimed at consumers - starting with the chocolate study.
The findings by University of Cambridge researchers trumpeted in media outlets around the world as a victory for chocaholics, were actually the result of reviewing seven separate studies of the topic. Let’s take a look at just one of those studies, published in the European Heart Journal by German researchers.
The study found that the combined risk of heart attack and stroke for those who ate the most chocolate was 39 percent lower than those who ate the least. That’s the relative risk reduction - what’s the absolute risk reduction?
For those who ate the most chocolate, researchers documented 1.44 cases of heart attack or stroke per 1,000 people per year, or .144 percent - the modified risk. For those who ate the least, there were 2.72 cases per 1,000 people per year, or .272 percent - the starting risk.
Subtract those two and you get .128 percent. (That’s actually nearly 50 percent lower than .272 percent, but researchers made adjustments for other risk factors like blood pressure to get to the relative risk reduction number of 39 percent).
In other words, there are two ways to look at these results: Eating chocolate decreases your combined risk of heart disease and stroke by 39 percent, or it decreases your risk from about .272 percent per year to .144 percent (before adjusting for other risk factors). That’s a pretty small reduction.
STUDY ON PERCEPTION OF RISKS BY DOCTORS AND PATIENTS
A new drug for a viral disease has just come onto the market. It has severe side effects, but in clinical trials, it decreased the risk of dying from the disease by 33 percent, or one-third. Sounds impressive, right?
Now let’s look at the data a different way. Out of all patients who were given the new drug, 96 percent survived and 4 percent died. For those on the old drug, 94 percent survived and 6 percent died. So, the mortality rate decreased by one-third - from 6 percent to 4 percent.
Is the new drug better than the old one? And by how much? In a recent study published in the Journal of General Internal Medicine, doctors and patients were presented with these two scenarios. Both populations were more likely to rate the new drug as very effective when they were only presented with the data showing a decrease in mortality by one-third - the relative risk reduction.
If they also saw that death rates fell from 6 percent to 4 percent - the absolute risk reduction - patients and doctors were far less likely to consider the drug as effective as before.
STUDY OF NIASPAN IN ADVERTISING
For examples of where only relative risks are presented, just look at ads in medical journals, said Dr. Thomas Perneger, lead author of the study on risk perception: “The pharma companies know how to sell products, and it’s relative risk.
Take an advertisement for NIASPAN (generic name: niacin) in a copy of the New England Journal of Medicine from 2008. The ad claims the drug reduces the risk of a nonfatal heart attack by 27 percent over five years for patients with a history of heart attack and high cholesterol.
But how much is 27 percent? In very small print at the bottom of the page, the ad explains that 8.9 percent of patients given niacin had a nonfatal heart attack, compared to 12.2 percent of patients given placebos. In addition, the study the ad is based on was only done for men, meaning the results may not apply to women.
So, while it’s true that the risk of a nonfatal heart attack was reduced by 27 percent, the actual number of patients who had a nonfatal heart attack decreased by only 3.3 percentage points. It’s important to put this number in perspective with the side effects, which are often detailed in the fine print.
A clearer way to write the ad might be “For men with heart disease and high cholesterol, NIASPAN reduces the five-year chance of a nonfatal heart attack from 12.2 percent to 8.9 percent.’’ Note that the chance of a fatal heart attack is not reported.
STUDY OF ANTIDEPRESSANTS AND AUTISM
According to CNN.com’s coverage of a recent study by Kaiser Permanente researchers, “children whose mothers take Zoloft, Prozac, or similar antidepressants during pregnancy are twice as likely as other children to have a diagnosis of autism or a related disorder.’’
But what does “twice as likely’’ mean? How likely are “other children’’ to have a diagnosis of autism-spectrum disorder?
There’s no easy answer to that question. Studies have estimated the risk of autism in the general population as anywhere from 1 in 100 (1 percent) to 1 in 110 (.9 percent) to 1 in 200 (.05 percent). Within the population looked at by the Kaiser Permanente researchers, 6.7 percent of mothers taking SSRIs had a child with an autism-spectrum disorder, while 3.3 percent of mothers not taking SSRIs had a child with such a disorder. Hence the “twice as likely’’ label.
But “twice as likely’’ can only be applied to the population in the study - and that’s a pretty small population, of 1,800 mothers overall (only 300 of whom had children with autism).
A more precise way to report those results - although not as provocative - would be to say that women who had children with autism in this study population were twice as likely to have taken antidepressants in the year before pregnancy than the women who did not have children with autism.
NEXT TIME YOU HEAR ABOUT A STUDY:
If the study says that taking a certain medication increases your risk of having a heart attack, for instance, ask the following questions:
■ What’s the risk of having a heart attack for the people who took a placebo - the “control population’’?
■ What’s the risk of having a heart attack for those treated with the medication being studied?
■ How big is that difference? Is it significant?
■ What kind of people are being studied? Do their characteristics fit you? If all the study participants are heavy smokers, for instance, and you have never smoked, the study’s results likely do not apply to you.
■ What does the study outcome measure? For instance, did it follow people for five years to see if they had a heart attack after taking a certain medication? Or only for one year?
■ Are there any potential benefits to taking the medication that might outweigh the risks presented by the study?
Neena Satija can be reached at firstname.lastname@example.org
Correction: Because of a reporting error, a story in the g section today about the meaning of risk in medical studies misstated one study's finding about the reduction in risk from eating chocolate. There were 1.44 cases per 1,000 people per year of heart attack or stroke among those who ate the most chocolate, or .144 percent, and there were 2.72 cases per 1,000 people per year among those who ate the least, or .272 percent.