"If you read the newspaper or watch the news, it's impossible to ignore the dozens of reports about studies linking diet to health. But lately, some headlines leave people wondering if scientists are capable of making up their minds. Scientific research is a back and forth process and contradictions are bound to occur. With so many studies investigating the same topic in different ways, results won't always be the same. Another problem: News reports don't always capture the complexity of the research to help you determine if a study is important to you. Many of the nutrition studies you hear about are epidemiological -- they're studies that examine human populations to find links, or associations, between disease and certain foods, nutrients, or supplements. There are different types of study designs, each with its advantages and limitations. The following is a "quick and dirty" guide to help you decipher the next study that's reported in the news."
[Full article here and below... useful reading for the vegan or vegetarian, or anyone trying to figure out the validity of what they read in the mass media...]
FROM:
http://www.theglobeandmail.com/servlet/story/LAC.20060503.HBECK03/TPStory/specialScienceandHealth/columnists
05/03/06
A quick and dirty guide to nutrition studies
LESLIE BECK
If you read the newspaper or watch the news, it's impossible to ignore the dozens of reports about studies linking diet to health. But lately, some headlines leave people wondering if scientists are capable of making up their minds. From vitamin E (the heart healthy supplement that increased heart failure) to dietary fibre (the substance that did -- then didn't -- and then did help prevent colon cancer), dietary flip-flops can make even the savviest consumer's head spin.
Conflicting news stories are frustrating, especially if you're trying to make healthy lifestyle changes. Why go to the trouble of changing your diet when today's health food or supplement might be next month's bad news?
Scientific research is a back and forth process and contradictions are bound to occur. With so many studies investigating the same topic in different ways, results won't always be the same. Another problem: News reports don't always capture the complexity of the research to help you determine if a study is important to you. Space in the paper and time on the air is limited so it's not always possible to provide all the supplemental information required to fully understand the research and its implications.
Many of the nutrition studies you hear about are epidemiological -- they're studies that examine human populations to find links, or associations, between disease and certain foods, nutrients, or supplements. There are different types of study designs, each with its advantages and limitations. The following is a "quick and dirty" guide to help you decipher the next study that's reported in the news.
Case-control studies
These studies are retrospective -- they look back in time. Researchers contact people who have been diagnosed with a disease (cases) and people who have not (controls). Both groups are asked about the foods they ate during the years before the first group was diagnosed. The objective is to see if the people who are sick were more likely to consume certain foods, or nutrients, than people who are not sick.
These studies can be conducted relatively quickly (researchers don't have to follow people for years until some become sick), but their findings have potential limitations. People with illnesses often report diets differently than healthy people. As well, it's not always easy to recall what you ate 10 or 20 years ago. Both can lead to inaccuracies in the information gathered.
Prospective studies
These studies, such as the Nurses' Health Study and the Harvard Professional's Follow-Up Study, look forward. Researchers follow a large group of healthy people (the cohort) over a long period -- perhaps 10 years or more -- and gather diet, lifestyle, weight and medical information at regular intervals. After a specified time, researchers see who develops a certain disease and determine if those who became ill ate more or less of a certain food or nutrient than the people who remained healthy.
Prospective or cohort studies generally provide more reliable results than case-control (retrospective) studies because they don't rely on information from the past. People are asked what they are eating now, rather than what they ate years ago. Since these studies last so long, researchers can survey people's diets at regular intervals and look for diet-disease links at different times.
One potential limitation -- often people's diets don't vary enough to find a link between a disease and a nutrient.
For example, if eating 20 grams of fat a day is thought to protect from breast cancer, a study that followed North American women might find no link at all because very few women eat so little fat.
Clinical trials
Like prospective studies, these randomized trials follow a group of people over a period of time. The difference is that researchers intervene to see how a specific diet, food or nutrient affects a certain health outcome. Half the people in the study are randomly assigned to receive the prescribed diet or nutrient believed to prevent the disease (the intervention) and the others continue with their usual diet or take the look-alike treatment (the placebo). Researchers then wait several years to see if one of the two groups has a higher rate of the disease.
Randomized trials are considered the most reliable form of scientific evidence because their design eliminates biases that can compromise results. Researchers are also able to decide exactly how much of a certain nutrient each group consumes so there's enough of a difference to detect an association. However, clinical trials sometimes fail because the nutrient dose was wrong, the study wasn't long enough, or the wrong ingredient in a food was being studied.
Scientists don't expect final answers from individual studies and neither should we. Rather, they consider the balance of evidence from epidemiological studies, and animal and test-tube studies.
When deciding whether study findings are relevant to you, ask the following questions the next time you read a research-based nutrition story:
Does the story report the results of one single study? If so, how does it fit with other studies on the same topic?
How large is the study? Studies that enroll a larger number of people are more reliable than small studies.
Can the results be generalized to you? A study conducted in men might not achieve the same results in a group of women or children. How a food affects the health of mice or rats might have little to do with the impact it has on people.
Did the study look at real disease outcomes (e.g. heart attack, colon cancer, hip fracture) or some marker of disease (e.g. inflammatory proteins in the blood, colorectal polyps, bone density). Markers don't always develop into disease; it's best to put more weight on real-disease studies.
Think of dietary flip-flops as discussions among scientists. Rarely does one get the last word; conflicting opinions reflect the fact that science is an evolving process. Don't change your diet on the basis of one single study -- especially if it means giving up an entire food group."
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