The science of epidemiology is both a gold mine and a disaster for nutrition science. A quick look through the American Journal of Clinical Nutrition or simply scanning the news headlines and you find numerous new discoveries in nutrition health based on epidemiology studies. This research has uncovered many importance questions and led to multiple new lines of research. Unfortunately, epidemiology is too often misused and over-extrapolated to imply cause and effects based on simple statistical associations.
The problem with these studies is the health outcomes are related to dietary intakes of single nutrients or single foods and largely ignores the overall food patterns or lifestyles of the individuals at risk. The weakest part of the data are the food surveys that are supposedly validated, which simply means if you did the same survey 10 times you’d get the same answer 9 times, but it ignores the fact that the answer may be wrong.
A recent paper in Journal of Nutrition (JN 142:1652, 2012) highlights the problem. The investigators surveyed a large group of overweight individuals consuming higher protein diets for weight management and found a statistical association of higher intakes of “red meat and processed meats” with risk of colon cancer. However, after evaluating the overall food intake in greater detail, the investigators found that the higher risk of colon cancer occurred more specifically in individuals who were obese (ie. excess calories) and consumed limited dietary fiber, excess sugar, minimal fruit (ie. low vitamin C), high amounts of lettuce, spinach and beets (ie. high dietary nitrates), and high amounts of processed meats (ie. sausage and hotdogs). In the full context, it’s not surprising these individuals have increased health risks.
A second study in the Journal of Nutrition (JN 142:2112, 2012) sheds additional light on the importance of evaluating individual foods or nutrients in the context of the entire food pattern. These investigators evaluated the USDA NHANES dietary data related to cardiovascular risk factors (CVRF). They also used the food pattern model of the USDA Healthy Eating Index (HEI) which evaluates dietary outcomes against 10 food categories. They found that there was a strong inverse relationship of diet quality and CVRF. The better the overall diet quality the lower the health risks. So in an era when everyone wants to claim a new discovery about a magic or evil nutrient, we’re still finding that variety and moderation remain the most important goals for good nutrition.