Results from nutrition  research seem to change almost daily, leaving most consumers confused: Do I really need that much fiber? Are eggs in or out? What about caffeine? Read some interesting examples on Ever-changing Data from Nutritional Research .
The media coverage, which leaps to publicize the latest study, contributes to the confusion. Unfortunately the news stories don't usually investigate the limitations of the research or explain the complexity of the findings. Below are two keys reasons why the results of nutrition research can be flawed or misunderstood.
Until recently, nutrition research emphasized the role of single nutrients acting as a magic bullet to miraculously prevent disease or, conversely, as the sole agent responsible for the development of disease.
During the past five years, however, research is uncovering the concept of food synergy; which is the additive influence of multiple nutrients or food patterns. A 2003 study published in the American Journal of Clinical Nutrition found that the beneficial effects of whole grains might be more profound than any single nutrient contained therein. A 2007 study published in Circulation found that it was the synergy of fish, fruits, and vegetables that led to a reduced risk of blood clots.
In other words, it is not the effect of one nutrient that leads to health, but a person's overall diet. In 2004, the Journal of the American Medical Association published the results from the HALE project showing that adherence to healthful lifestyle practices such as a Mediterranean Diet  pattern was associated with a nearly 70 percent reduction in chronic disease.
Data collection for nutrition studies is severely flawed because people simply don't report accurately. They tend to underestimate what they eat, but even more fundamentally, they don't remember what they ate to report it back. And if they eat any meals out, they don't know what all the ingredients in the food were (for example what oil was used). Finally, they (or the nutritionists running the study) don't have accurate data about the actual nutrient content of their food, which can vary depending on freshness and where it was grow or raised.
As Marion Nestle, a prominent nutritionist, says in her book Food Politics, "The most intellectually demanding challenge in the field of nutrition is to determine food intake."
Furthermore, in studies comparing one diet to another, it is not always possible to control what people eat. For example, in a study comparing a group eating a low-fat diet to those eating a higher-fat diet, those in the low-fat group cheated and ended up eating a higher fat diet than the researchers recorded, while those in control group (normal diet) lowered their fat intake voluntary (perhaps because the study made them aware of it). In the end, unbeknownst to the researchers, the two groups were not far apart in the amount of fat they were eating. Thus it was not surprising that the "low-fat diet" group didn't show any health benefits!
Another confounding variable in nutritional research is genetic difference among the people studied. These differences can impact how individuals digest and use nutrients in food. Sometimes, when a study of a nutrient shows a difference only in a small number of people, it could be due to individual metabolic differences. We simply can't isolate the metabolism of a food from the entire makeup and functioning of the person eating it.
Fortunately, while individual studies may give different data about a particular nutrient, the value of a healthy diet is not in doubt. Study after study shows that good food choices have a positive impact on health and poor diets have negative long-term effects.
A healthy diet gives your body the nutrients it needs to perform, maintain function, and fight disease. Americans whose dietary pattern fit the USDA's Healthy Eating Index have a lower incidence of major chronic diseases.
That said, the challenge is to identify the best foods for you at any point in your life. This website can help you address that.
Knoop, K.T.B. et al. (2004). Mediterranean Diet, Lifestyle Factors, and 10-Year Mortality in Elderly European Men and Women: The HALE Project. Journal of the American Medical Association, 2(9), 1433-1439.
Nestle, M. (2002). Food Politics. Berkeley, CA: University of California Press.
Ostrzenski, A. Ostrzenski, K. (2005). WHI Clinical Trial Revisit: Imprecise Scientific Methodology Disqualifies the Study's Outcomes. American Journal of Obstetrics and Gynecology, 193, 1599-1604.
Pollan, M. (2008). In Defense of Food. New York: Penguin Press.
Singh, P.N., Sabate, J. Fraser, G.E. (2003). Health benefits of fruit and vegetables are from additive and synergistic combinations of phytochemicals. American Journal of Clinical Nutrition, 78(3), 526S-532S.
Steffen, L.M., Folsom, A.R., Cuhman, M., Jacobs, D.R. & Rosamond, W.D. (2007). Greater fish, fruit, and vegetable intakes are related to lower incidence of venous thromboembolism: The Longitudinal Investigation of Thromboembolism Etiology. Circulation, 115(2), 188-195.