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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).
Even at its best, science is a painstaking, deliberate process, which doesn’t fit very well into the cut-and-dry, newer-is-always better world of the mass media. And it’s the media reports on health that are responsible for much of the frustration the public feels toward the public health community. With their emphasis on short, “newsworthy” pieces, the media often only report the results of single studies, and many stories are chosen simply because the results run contrary to current health recommendations. Because such reports provide little information about how the new results fit in with other evidence on the topic, the public is left to assume that, once again, the scientists screwed up and are now backtracking.
Fortunately, in many cases it only takes a few incisive questions to get at the heart of a research-related news story and see how important the results are for you personally. One of the most crucial things to keep in mind is the issue we’ve already discussed above: How does a given study fit into the entire body of evidence on a topic? Whenever reading or watching a news story on health, keep these questions in mind:
- Are they simply reporting the results of a single study? If so, where does it fit in with other studies on the topic?
Only very rarely would a single study be influential enough for people to change their behaviors based on the results.
- How large is the study?
Large studies often provide more reliable results than small studies.
- Was the study done in animals or humans?
Mice, rats, and monkeys are not people. To best understand how food (or some other factor) affects human health, it must almost always be studied in humans.
- Did the study look at real disease endpoints, like heart disease or osteoporosis?
Chronic diseases, like heart disease and osteoporosis, often take many decades to develop. To get around waiting that long, researchers will sometimes look at markers for these diseases, like narrowing of the arteries or bone density. These markers, though, don’t always develop into the disease.
- How was diet assessed?
Some methods of dietary assessment are better than others. Good studies will have evidence that the methods have validity.
With these tips and a better understanding of the world of health research, you can look at health information with a more discerning eye. While this won’t be a guarantee against frustration, it should help you embrace health recommendations—and the healthy lifestyle they promote—with more confidence.
The aim of the Harvard T.H. Chan of Public Health Nutrition Source is to provide timely information on diet and nutrition for clinicians, allied health professionals, and the public. The contents of this Web site are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Web site. The information does not mention brand names, nor does it endorse any particular products.