In the last year or so, we have seen a number of scientific
papers reviewing sets of older scientific publications linking saturated fats
to heart disease and calling for a reversal of prevailing wisdom that a change
in the composition of dietary fats (less saturates and more unsaturates) would
help reduce the public health burden of heart disease. The spin-doctors from
the food industry have whipped up the hype and last summer the front cover of Time
magazine headlined: “Eat butter. The scientists labeled fat the enemy. Why they
were wrong”.
Before I take at look at the detailed issues, I would make
two important points. The first is that no matter how unpalatable a critical
paper might be to the custodians of some aspect of conventional scientific
wisdom, there is an absolute need for continued critical analysis of existing
theories. As I have often argued, dissent is the oxygen of science. The second
point I would make is that the advent of any one paper is never in itself
sufficient to unravel prevailing wisdom. Philosophers of science have the
luxury of arguing that the just one black swan demolishes the “All swans are
white” theory. Well its not so black and white in an area as complex as public
health nutrition.
Last year, a paper was published in the Annals of Internal
Medicine[1],
which looked at previously published studies that sought to establish links
between different types of dietary fats and heart disease. In response to this
publication, the journal received many letters most of which were critical of
the study. I won’t re-hash their arguments but will offer my own. Back over
half a decade ago, Professor Ancel Keys conducted a study across 7 countries in
which he showed (a) that measures of the composition of dietary fat could help
predict blood cholesterol levels and (b) that the national statistics in these
countries showed a link between rising blood cholesterol and higher rates of
heart disease. His work is now often referred to as being flawed and certainly,
this study, the first of its kind, wasn’t exactly top drawer in today’s
standards no more than the falling Newtonian apple can rank with today’s Hadron
Collider in the study of gravity. It is not the 7 countries study that mark out
the great contribution of Keys to modern nutrition but to the subsequent
experiments he conducted on humans to study how precise control over the intake
of saturated, monounsaturated and polyunsaturated fatty acids would influence
blood cholesterol levels. In this he was joined by two other Doyens of that
great nutrition era, Paco Grande and Scott Grundy. They developed equations,
which would accurately predict the change in blood cholesterol that would occur
with changes in the composition of dietary fats. These were real experiments
with human subjects, not some fancy statistical model from the world of
epidemiology. What that world delivered, and delivered in spades, was
epidemiological data showing that rising population levels of blood cholesterol
were associated with rising levels of coronary heart disease. So here we had dietary data showing an
unequivocal link between the compositions of dietary fat and plasma cholesterol
and powerful evidence linking elevated plasma cholesterol levels with heart
disease. So, it made sense to recommend a reduction in the intake of saturates
and their partial replacement with unsaturated fats to minimize population
levels of blood cholesterol BUT NOT TO BE THE MAIN DRIVER IN ANY MOVE TO REDUCE
OVERALL HEART DISEASE. Why not? Because
heart disease was also partially caused by smoking, by high blood pressure, by
the advent of the menopause in women, by blood clotting potential, by bodyweight,
by fitness, by family history and by many other factors, some of which were
easy to measure on a population basis and some of which were difficult to
measure and thus were rarely entered into complex statistical models (platelet
aggregation, post-prandial lipemia, cardio-respiratory fitness, family history
of heart disease, common known gene variants etc).
So the biggest error in looking at dietary fat and heart
disease is that nobody ever said that there was a direct link between heart
disease and the composition of dietary fat. All that was ever said and in two
very clear stages was (a) that dietary fat for certain, influences blood cholesterol
and (b), blood cholesterol along with a wide range of other factors contribute
to heart disease. A more recent paper[2]
makes the same mistake of attempting to link dietary fat intakes to the
incidence of heart disease when the human intervention studies with a focus on
fats had blood cholesterol as the end point.
To hammer home this point, consider the generally accepted
public health nutrition success that is attributed to the reduction if not
elimination of hydrogenated trans fats from the human diets. It was a triumph
for NYC Mayor Michael Bloomberg in engineering a citywide movement for their
elimination. What constituted the risk
associated with trans fats? It wasn’t mortality from heart disease. It was a
series of human experiments conducted by the Dutch researchers Martijn
Katan and Ronald Mensink in showing an unequivocal effect of trans fats on
adverse levels of blood cholesterol. The epidemiologists contributed both the
only actual human studies were on blood cholesterol and trans fats intake[3].
Very recently, the industry spin-doctors debunking the
dietary fats heart disease putative link (the true link is dietary fats and
blood cholesterol) have acquired a new string to their bow, the 2015 US Dietary
Guidelines. The 2015 guidelines make the point that “available evidence shows no appreciable
relationship between consumption of dietary cholesterol and serum [blood]
cholesterol”. This is simply the conservative policy catching up with the
science since it has been know for decades that the main source of cholesterol
in our blood is manufactured by our guts and livers and the dietary
contribution from ingesting cholesterol is minimal. The report goes on to argue
that: “The DGAC encourages the
consumption of healthy dietary patterns that are low in saturated fat, added
sugars, and sodium. The goals for the general population are: less than 2,300
mg dietary sodium per day (or age-appropriate Dietary Reference Intake amount),
less than 10 percent of total calories from saturated fat per day, and a
maximum of 10 percent of total calories from added sugars per day”. For
many, this is taken to be a major turn around for dietary advice on restriction
of sugar intake but I’m afraid its no more than the US catching up with the
rest of the world who have held the 10%target for added sugars over several
decades now.
Despite the minor adjustments and catch up of the US to
dietary guidelines and notwithstanding the odd paper suggesting otherwise, its
business as usual for nutritional advisory committees globally. The press will report
on dramatic somersaults and loop-the loops and the public will continue to be
confused. In effect, nothing has changed.
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