(David S. Seres, MD is Associate Professor of Medicine in the
Institute of Human Nutrition. The opinions expressed here are
those of the author.)
By David Seres
Aug 11 Have you ever wondered why nutrition
experts so often change their minds about what constitutes a
healthy diet? In the last six months, a variety of experts and
nutrition organizations have issued at least as many major
dietary guidelines proclaiming the next set of instructions on
what to eat to prevent cancer, whether processed foods are
really food, whether the type of fat you eat has any effect on
developing heart disease, and how to control diabetes with diet.
And the next set of U.S. Department of Agriculture (USDA) and
Health and Human Services (HHS) dietary guidelines, due out next
year, are already creating a buzz.
These new guidelines have, like most dietary guidelines in
the past, done little to solidify our understanding of the cause
and effect relationship between diet and disease. Even worse,
they're likely to discredit nutritional science. Moreover,
guidelines like these tend to suggest, without proper research
as proof, that people have control over whether or not they
develop certain diseases, and it is therefore their fault if
they become ill.
A large part of the issue comes down to funding, and a lack
of resources to do the kinds of studies that would help answer
some of our most pressing questions. Our insistence on making
recommendations - without having the proper research to back
them up - has unintended, harmful effects.
Nutrition research, like all medical research, is extremely
difficult and expensive. But nutrition research is even more
complex than research in other areas of medicine. First, many
well-designed studies have failed simply because the subjects
couldn't adhere to the diet: asking subjects to eat a diet lower
in salt, for example, can initially make food seem tasteless.
Second, changes in diets have the potential to change dozens
of other variables at the same time. When trying to determine
whether a low-protein diet decreases cancer, for example, a
study may involve reductions in grilled or processed meat -
which may contain cancer-causing substances. To control for
these coincidental effects, and subtle differences between the
subjects, studies of dietary changes often require tens of
thousands of participants.
Third, the impact of diet is subtle and researchers may need
to watch a subject for decades in order to observe any changes.
Within nutrition, and medicine as a whole, researchers rely
on two broad types of studies: randomized and observational. In
a randomized study, we recruit a group of subjects with a
desired set of similarities, and randomly assign them to a
treatment, which in this case is a diet. Researchers then
monitor the subject to see how the different treatments have
affected him or her. Because the subjects are relatively
similar, and the treatments randomly assigned, researchers can
establish cause and effect.
Observational research takes a snapshot of a population, and
looks to see whether two things occur together with a high
frequency - like smiling and happiness. While less difficult and
less expensive - and therefore much more popular - this type of
research can only generate hypotheses about cause and effect. In
this example, does smiling cause happiness? Most of our dietary
guidance is based upon this kind of research.
Not only does observational research not establish cause and
effect, it's often misleading and wrong, and the recommendations
that come from the conclusions of these studies can lead to
harm. When two things occur together in an observational study,
their relationship is actually more likely due to a third, often
unknown factor than to what the researchers conclude.
For example, in observational studies there is an
association between eating diets high in antioxidant vitamins
and a lower rate of cancer. However, in proper randomized trials
a diet supplemented with antioxidant vitamins has been shown to
increase the risk of cancer. This result shows how far off the
conclusions of observational research can be, and may help
explain the inconsistencies in our dietary recommendations.
Because contradictory guidelines are so frequent, the public
has every right to question the credibility of dietary
recommendations. By failing to educate the public about the
weaknesses in our research, or even acknowledge those weaknesses
to ourselves, nutrition experts continue to erode what public
trust remains. I worry that as long as we continue to make our
recommendations sound definitive, we send the message that we do
not need more research or the funding to do it.
There's another problem that occurs because experts act with
unjustified confidence in their research. The conclusion one
could draw from these guidelines becomes: If you develop cancer,
or another "preventable" disease, it's your fault. I have
treated thousands of cancer patients, many of whom feel failure,
remorse, and/or regret about steps that they think they could
have taken to avoid getting sick. People with cancer and other
diseases shouldn't be made to feel as though they were the cause
when the correlation between nutrition and disease is so poorly
I don't believe that we should throw out all dietary
recommendations. In fact, I feel fairly comfortable with the
recommendations made in these recent guidelines. But I also know
that they are not definitive - and so, intuitively, does the
general public. When we experts change our minds so frequently,
we send the message that all dietary guidelines should be
dismissed as the latest fad.
Until and unless the funding for very large, effective,
long-term randomized studies of the effect of different diets on
preventing disease becomes available, nutrition experts must
educate themselves and the public about the strengths and
weaknesses of the data on which their opinions are based.