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Read more about these types of research studies:
Read an example of how different types of studies shaped health recommendations about fiber and colon cancer.
Over the past 100 years, great strides have been made in improving the health of people the world over. In the United States, for example, life expectancy has increased by about 25 years for men and 30 years for women, largely due to scientific advances that have helped prevent and treat many serious diseases.
The end results of science undeniably benefit us, but to people who don’t work in the field, the scientific process can also be frustrating. This is particularly true for anyone trying to make important decisions about his or her health. What should I eat? How much exercise should I get? How often should I see the doctor for health checks? We all expect that science—specifically, health research—will provide answers to these very important and personal questions.
Unfortunately, it’s often hard to get a straight answer. One day the “experts” say one thing. The next, they seem to say another. Then it all appears to switch back again. Such flip-flops can be maddening, especially when you’re making your best effort to live a healthy lifestyle. Why go to the trouble of making big changes when today’s highly recommended choice may be tomorrow’s bad example?
The classic case of such flip-flopping in nutrition is the butter-versus-margarine question. More than 30 years ago, as research began to suggest that saturated fat might be bad for the heart, recommendations were made that encouraged people to switch from butter, which is high in saturated fat, to low-saturated-fat margarine. However, further research showed that, in many cases, margarine contained a type of fat (known as trans fat) that’s even worse for the heart than saturated fat. This seeming about-face led many people to throw up their hands in frustration.
But the butter-versus-margarine question is also a well-known, if somewhat vexing, example of how research often works. Scientific research is a dynamic process that moves forward slowly. Recommendations are made based on the best science available at the time. However, with new research and new results, these recommendations may be revised.
On the other hand, such radical shifts in advice are rare today. There’s far more research on diet and health available now than there was 30 years ago, when the original recommendation to switch to margarine occurred. This means that contemporary diet recommendations tend to be better-grounded—based on the results of many studies—and involve much less guesswork than in the past.
But contradictions in research results still occur. They are an inevitable part of the scientific process. Researchers are constantly performing studies and reporting their results. And when so many different people study each topic in so many different ways, it’s natural that the results won’t always be the same. What is the key, though, and what drives health recommendations is the weight of evidence on a particular topic—what all the results as a whole point to.
The research process is like placing stones on an old-fashioned balance scale. When enough weight accumulates on one side, the scale tips in favor of a particular recommendation. And the more weight there is on one side, the stronger the recommendation is and the more evidence it would take to change it.
If, on one side of the scale, you have over 40 studies showing that moderate alcohol intake can lower the risk of heart disease and, on the other, one or two studies that contradict those results, the scale would hardly budge. The weight of evidence would still be greatly in favor of moderate alcohol intake protecting against heart disease. Indeed, the link between alcohol and heart disease is so strong that it’s known as an established relationship.
But not all topics are as clear-cut as this. Often, the weight of evidence is not as great. In some cases, only a handful of studies have addressed a particular question. In other cases, a large number of studies may support one side, but there may be some particularly significant studies supporting the other side as well—just enough to cast some doubt. In these instances, we’d say that there is a probable link between a behavior and a disease. The value of eating moderate amounts of nuts to protect against heart disease is an example of a probable link.
There are also possible links, where the weight of evidence is still less and, in effect, the scale only tips slightly to one side. Possible links often develop in new, emerging areas of study, where a few studies have found a relationship, but more studies need to be done to confirm the results. A high intake of trans fat and an increased risk of diabetes is an example of a possible relationship that needs to be confirmed.
To carry this analogy further, the scale’s likelihood of tipping reflects not only the number of stones placed on one scale, but also the size of those stones. Bigger stones will make the scale tip faster than smaller ones. Likewise, big, well-designed studies tend to play a more important role in establishing a relationship—and in shaping health recommendations—than smaller, less-well-designed studies.
Although the details can get complicated, large studies that follow human participants over time (randomized trials and cohort studies) tend to provide more reliable results than smaller studies that ask people about their past activities (case-control studies).
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