Why is nutrition so hard to study?

In this feature, we discuss some of the reasons that nutrition research seems to be so indecisive, difficult, and downright confusing.In an ideal world, to understand the health impact of a given food — goji berries, for instance — an experiment would go something like this: Scientists recruit 10,000 participants (both males and females, from a range of nationalities and ethnicities) and house them in a laboratory for 10 years. In addition, participants do not always know the exact size of their portions, or the full list of ingredients in restaurant or take-out foods, for instance.Studies often ask questions about the long-term impact of a nutritional component on health. In reality, people’s diets can change substantially over the course of a decade.The issues associated with measuring nutrient intake are so ingrained that some authors have referred to self-reporting as a pseudoscience.These issues prompted a highly critical study, which appeared in the journal PLOS One, to pull apart data from the National Health and Nutrition Examination Survey (NHANES).The NHANES, which began in the 1960s, “is a program of studies designed to assess the health and nutritional status of adults and children in the United States.” Experts use the findings to guide public health policy in the U.S.The primary method of data collection for the NHANES are 24-hour dietary recall interviews. Another study in the journal PLOS Medicine looked at the impact of industry funding of research into soft drinks, juice, and milk.The authors conclude, “Industry funding of nutrition-related scientific articles may bias conclusions in favor of sponsors’ products, with potentially significant implications for public health.”To add to the confusion, media outlets tend to amplify these findings. So, when comparing diets that contain meat with those that do not, any health effects might not be due to the lack of meat but the addition of other foods.Every type of fruit and vegetable contains a vast array of compounds, and the type and amount of these can vary depending on where they grow, how people transport and store them, and how they process and cook them.There are so many variables to take into account that even when a study does find a statistically significant result, it is difficult to determine if it actually came from the food under investigation.Of course, humans are just as diverse as the foods they consume. The scientists continuously measured 800 participants’ blood glucose levels and found “high variability in the response to identical meals.” The authors explain that this suggests “that universal dietary recommendations may have limited utility.”To explain this issue, it may be helpful to describe the findings of an imaginary (but not entirely fanciful) study: People who eat a great deal of spinach live for 5 years longer than people who eat no spinach.From that result, one might quickly conclude that spinach increases life span.