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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?
Share on PinterestNutrition
is a maze. In this article, we ask why.
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.
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.
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.
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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.
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:
“[I]t is widely agreed that the regular
consumption of tasty, ripe blueberries can be unconditionally recommended.”
For more on this topic,
read our article on the sugar industry and how it manipulated
scientific discourse in its favor.
To reiterate, if a study
secures industry funding, it does not mean that people should dismiss the findings
out of hand. However, it should provide food for thought.
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. If, for instance, a study funded
by a chocolate manufacturer concludes that chocolate extends life, media
outlets will faithfully reproduce the conclusions, often without a mention of
the funders or a discussion of the study’s limitations.
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.”
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.
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.
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