Is dairy good or bad for health? Is cholesterol evil? Does red meat kill or cure? Is the ketogenic diet a godsend or a health hazard? Can the vegan, vegetarian, pescatarian, or raw food diet extend disease-free life?
Nutrition is wrapped in multiple confusions. Why is it so hard to determine whether a food is good or bad for health?
In medical science, proving any theory is difficult. The science of nutrition is no different, but it also has some unique challenges. In this feature, we outline just some of these stumbling blocks.
Despite the many issues that nutrition scientists face, understanding which foods benefit or harm health is essential work.
Also, the public is growing increasingly interested in finding ways to boost health through diet. Obesity and diabetes are now highly prevalent, and both have nutritional risk factors. This has sharpened general interest further.
All areas of scientific research face the following issues to a greater or lesser degree, but because nutrition is so high on people’s agenda, the problems appear magnified.
A changing world
Although the water is muddy and difficult to traverse, there have been substantial victories in the field of nutrition research. For instance, scientists have determined that vitamin C prevents scurvy, that beriberi develops due to a thiamine deficiency, and that vitamin D deficiency causes rickets.
In all of these cases, there is a link between a particular compound and a specific condition. However, the picture is rarely so clear-cut. This is especially true when investigating conditions wherein multiple factors are at play, such as obesity, osteoporosis, diabetes, or heart disease.
Also, nutrition-related conditions have changed over time: The most common threats to health used to be deficiencies, whereas in Western countries today, overeating tends to be the primary concern.
Understanding the role of food in health and disease is essential and deserves attention. In this feature, we discuss some of the reasons that nutrition research seems to be so indecisive, difficult, and downright confusing.
The ‘perfect’ nutritional study
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. The scientists feed each person the exact same diet for the duration of their stay, with one difference: Half of the participants consume goji berries surreptitiously — perhaps blended into a mixed fruit smoothie.
Alcohol and tobacco are banned for the duration of the study.
The participants must also exercise for the same amount of time each day; if some people exercised more, they might become healthier, regardless of their goji berry intake. This would skew the data.
Neither the researchers nor the participants are aware of who is receiving the goji berry smoothie; if the participants knew they were receiving a “superfood,” they might benefit from the placebo effect. This so-called double-blinding is vital when running clinical trials.
During the decade-long study, the scientists monitor the participants’ health intensively. This might involve running regular blood tests and medical imaging.
Of course, the astronomical cost of this type of study is the very first stumbling block. Also, ethics and good sense say that this is beyond impossible.
In lieu of perfection
Nutritional research has to make some concessions, as the perfect study is unachievable. So, in “observational studies,” nutrition scientists look for links between what a person consumes and their current or future state of health.
Observational studies can be incredibly useful. Using this method, scientists proved that tobacco causes lung cancer and that exercise is good for us.
However, these studies are far from perfect.
One issue with observational studies is the researchers’ reliance on self-reported food intake. They ask participants to note down everything they eat for a set amount of time, or to remember what they ate in the past. This could refer to yesterday or months earlier.
However, human recall is far from perfect. Also, some people might purposely miss certain food items, such as their third candy bar of the day. 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. However, researchers tend to take dietary information at just one or two points in time. 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.
The role of industry
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. Researchers use this information to calculate energy intake.
The authors of the critical paper conclude that “the ability to estimate population trends in caloric intake and generate empirically supported public policy relevant to diet-health relationships from U.S. nutritional surveillance is extremely limited.”
In an opinion piece, lead author Edward Archer pulls no punches, explaining that their paper demonstrated “that about 40 years and many millions of dollars of U.S. nutritional surveillance data were fatally flawed. In […] nutrition epidemiology […], these results are commonplace.”
Here, we meet the double-edged sword of industry: The PLOS One paper declares that funding for the critical study “was provided by an unrestricted research grant from The Coca-Cola Company.”
Industry funding certainly does not invalidate the findings of studies, but it should prompt us to wonder what the funder might gain from such research. In this case, a company that produces sugary drinks might benefit from destabilizing people’s faith in the research that has deemed their products unhealthful.
Perhaps this example is a little unusual; more commonly, an industry with a vested interest will fund studies that demonstrate the benefits of a product.
As an example, the California Walnut Commission regularly fund research concluding that walnuts are good for us. Meanwhile, one study supported by the U.S. Highbush Blueberry Council proudly states in its abstract:
Diving into complexity
Another issue that seems to plague nutrition research is complexity. Sometimes, a study will focus on the impact of just one specific food item or compound on a health outcome. This is slightly easier to manage.
Often, however, studies will try to investigate the impact of a particular diet. The Mediterranean diet, for instance, has gained a lot of attention over recent years.
The difficulty here is that one person’s version of the Mediterranean diet could be very different from another person’s version. For instance, one person might have one small glass of red wine, 25 olives, and an artichoke each day, while someone else — who may be in the same experimental group of a study — might drink no wine and eat neither olives nor artichokes.
A related issue is that of replacement: If someone eats no meat, for instance, they are likely to replace meat with other sources of protein, such as beans or pulses. 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. Eating a single peanut might provide one person with beneficial nutrients, while that same peanut could be fatal for someone with an allergy.
One 2015 study makes the same point but in a more subtle way. 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.”
The scourge of confounding variables
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. However, before rushing out to stock up, it is worth considering the alternatives.
In this case, the extended life span might not be due to the spinach alone; someone who eats a lot of spinach might also eat a lot of other vegetables. Conversely, someone who eats no spinach might eat fewer vegetables overall.
Also, someone who regularly eats vegetables is possibly more likely to indulge in other healthful pastimes, such as exercise. Someone who never eats spinach might, perhaps, be less inclined to work out.
These are all assumptions, of course, but it is clear that other factors are linked to spinach consumption, and these might influence the results.
In most studies, researchers attempt to “control” for these variables. However, there is always a risk that some unmeasured factor is significantly altering the findings.
Confounding variables affect all areas of medical science, of course, but because diet and lifestyle are so intimately entwined, they are a particular hazard in nutritional research.
The take-home message
Nutrition is a minefield of confusion. Determining what is healthful and what is not can be a challenge. Although we can be fairly confident that fruits and vegetables are good for us while high fat, high salt, high sugar products are not, there are many gray areas.
One study that brings the confusion in nutritional science into stark relief involves a literature of research associated with 50 ingredients randomly selected from cookbooks. These included pepper, veal, lemon, carrot, lobster, rum, raisin, and mustard.
According to the paper, of these 50 ingredients, 40 were associated with either an increase or a decrease in the risk of cancer. In their conclusion, the study authors write that “[m]any single studies highlight implausibly large effects, even though evidence is weak.”
So, what can we do? Scientists should keep improving their study methods and adding to what we already know. Consumers, the public at large, and media outlets all need to be more critical.
Overall, there are no quick answers in the world of nutrition. However, because we all need to eat, interest is unlikely to disappear, and science will continue to forge ahead.