Normally, my blogs are built around a recently published
scientific paper of relevance to public health nutrition. This week’s blog is a
rant. It is a rant against the tsunami of second-rate science that dominates
the media and that confuses the consumer allowing all sorts of drivel to be
peddled as nutritional science. The truth is that the great majority of these
stories are based on nutritional epidemiology, an academic area that gathers
vast amounts of data on people and uses statistical models to sort the wheat
from the chaff. Their statistical models are bound by the “known knowns” but
they necessarily exclude the “known unknowns” and the “unknown unknowns” and
thank you, Donald Rumsfeld for this addition to our lexicon. For example, a
journalist writing in the Irish times cited a paper, which was built on such
epidemiological models relating sugar to cardiovascular disease and ended up by
quoting the publication asserting that as the % calories from sugar in the diet
rose, the risk of cardiovascular disease rose by some huge figure. The
journalist argued that the higher sugar intakes “led” to a rise in
cardiovascular disease. It didn’t because there was no intervention that would “lead”
anyone anywhere. The statistical model implied so and that implication was limited
by the constraints of the “known knowns” of the statistical wizardry. But there
was no experiment in which subjects had their diet manipulated and then
followed to look for changes in cardiovascular risk factors such as might be
measured in blood or detected by imaging or simple monitoring blood pressure.
No, this study, like all epidemiological studies does not prove cause
and effect. They are
merely interesting observations of association awaiting confirmation by the
science of experimental nutrition.
No matter what the nutritional challenge is, there are normally,
with some few exceptions, the means to experimentally test the observed
associations of epidemiology. As the Nobel Laureate in Immunology,
Sir Peter Medawar wrote: “If politics is
the art of the possible, then science is the art of the soluble.”
This rant is written in the heart of the Chianti region of Tuscany
where village shops abound in beautiful fresh fruit, vegetables and legumes,
fresh fish, lean meat, nuts, breads and of course beautiful red wine. This is the Mediterranean diet and unless
you’ve been a hermit here in Europe for the last 30 years, you will know that
the Mediterranean diet is the most healthful on the planet. Or so the epidemiologists told us. But it took
a consortium of Spanish scientists and their funders to put in place the
critical test of the hypothesis that the Mediterranean diet was all it was
cooked up to be. The Predimed[1]
study, as it is called, had the following design as outlined in the abstract: “In a multicenter trial in Spain, we
randomly assigned participants who were at high cardiovascular risk, but with
no cardiovascular disease at enrollment, to one of three diets: a Mediterranean
diet supplemented with extra-virgin olive oil, a Mediterranean diet supplemented
with mixed nuts, or a control diet (advice to reduce dietary fat). Participants
received quarterly individual and group educational sessions and, depending on
group assignment, free provision of extra-virgin olive oil, mixed nuts, or
small nonfood gifts. The primary end point was the rate of major cardiovascular
events (myocardial infarction, stroke, or death from cardiovascular causes). On
the basis of the results of an interim analysis, the trial was stopped after a
median follow-up of 4.8 years”.
The outcome was clear. Both the intervention diets reduced
cardiovascular disease by about 30%. Now we have experimental data to search,
analyse and interpret, knowing that the data is the outcome of a dietary
intervention study. Some may not like the outcome and some may criticise the
design or query its policy implications. But in as far as is humanly possible,
this consortium of Spanish scientists has adhered to the correct scientific
rigour and has not stopped at epidemiological observations.
The media simply do not get this vitally important difference
of an association between two factors and a proven effect of
the causative
factor (the Mediterranean diet) on the effect under study (cardiovascular
disease). Consider for example the major feature article, which recently
appeared in Time magazine by Brian Walsh. The work of the epidemiologist Ancel
Keys, who first presented evidence of a link between saturated fats and heart
disease, is rubbished in this article. But Walsh never once refers to the follow
up to that epidemiological observation. Both Keys and his colleague Pepe Grande
subsequently conducted multiple dietary intervention studies in psychiatric
hospitals in which the major part of protein, carbohydrate and fat were
provided as milks with the subjects allowed a small selection of vegetables.
The results were conclusive. As the % calories from saturated fats rose, blood
cholesterol levels also rose while when polyunsaturated fats were increased,
blood cholesterol levels fell. In fact the effects was so consistent that a set
of predictive equations were established which can still operate today. Decades
later, Martijn Katan at Wageningen University in the Netherlands, also
concluded several dietary intervention studies which showed a role of olive oil
type fats (monounsaturated) and an undesirable effect of trans fats. Once
again, a rich stream of experimental data could inform policy. Indeed, Katan
still operates a web-based tool for calculating how blood lipids change in
response to changes in the composition of dietary fats (http://www.katancalculator.nl).
None of these dietary intervention studies appears in Walsh’s
article. He relies solely on epidemiological studies. He argues that
nutritionists were wrong to advocate a low fat diet. We didn’t. Going back as
far as the first set of dietary guidelines issued in the mid 1970s by Senator
George McGovern, nutritionists asked for a modest reduction in fat intake (from
40% to 35 % of energy) but with major reductions in saturated fat intakes
(based on experimental evidence). To this day, that advice still persists
within national and international bodies that are charged with issuing dietary
advice. It was the media, the food industry and the “pop nutritionists” which
sold an ever-decreasing fat intake as the holy grail of diet. We in nutrition
knew back then that a low fat diet would raise blood triglycerides and re-shape
that structure of the so-called bad cholesterol LDL, into a higher quantity of
very small and dense LDL particles. We in nutrition also knew that studies on
fasting subjects, which epidemiologists insisted on using, bore no relation to
the manner in which the body metabolised fats after a meal, an area, which is
today’s new “hot topic” in nutrition.
Public health nutrition has benefited greatly from the
theories that nutritional epidemiology has generated. But public health
nutrition that eschews experimental evidence where it exists, will ruin the
public’s confidence in nutrition. So to those readers who influence policy in
whatever way, might I ask that henceforth, any story you come across, which
claims to link some nutritional pattern with some health outcome, should ask
the following question: “Is the study
based on a dietary intervention that allows cause and effect to be established?”
If it isn’t, tread carefully until the evidence emerges from the true
scientific tradition of experimentation.
