In the course of a recent lecture on the subject of obesity,
an issue arose in the discussion about an incremental approach to solutions for
obesity. Thus individual action plans might seem very modest but a suite of
such action plans could be successful in treating obesity. Allied to this
viewpoint about incremental solutions came the old chestnut that sometimes, in
epidemiology, we need to take action even if we don’t have definitive proof of
the efficacy of that action. This chestnut, hankers back to the founder of
modern day epidemiology, John Snow, who disconnected the tap from the water
pump in Broad Street London, thereby ending a cholera outbreak. It is argued
that Snow did not have definitive proof and this is the root of the view that
sometimes, epidemiology needs to take a leap of faith. However, as Snow himself
points out, of the 71 deaths he investigated, 61 were of people living in the
vicinity of Broad Street and thus users of the pump. Three deaths occurred in
children who lived nearer another pump but who went to school close to the
Broad Street pump. Five others “preferred” the water at Broad street over their
local pump. Only two deaths could not be linked to the guilty pump. All of this
happened prior to our understanding of pathogenic microorganisms so Snow
clearly couldn’t have used culture techniques to verify the organisms presence.
He could have asked persons living away from the pump in question to drink the
water in question but even in Snow’s time, the unwritten ethical position was:
“First do no harm”. And so, armed with a dot map and his 71 case histories, the
pump was disabled. This may have been a leap of faith in that definitive proof
of cause and effect wasn’t to hand, but the quality of Snow’s data was
excellent and compelling.
So in the field of human nutrition, how much of our public
health nutrition policy is based on fact and how much on faith. In the early
1950’s. Ancel Keys studies the relationship between dietary fats, plasma
cholesterol and the rate of heart disease in seven countries across the globe.
Keys and his colleague Paco Grande then completed a series of human
intervention studies in which patients in a psychiatric hospital were fen on
reconstituted milk with a wide variety of fats and oils. The net outcome was
definitive proof that the effects of dietary fat on plasma cholesterol could be
accurately predicted using equations derived in their studies. But was there
proof that lowering cholesterol would reduce the risk of heart disease. Endless
studies ensued and all showed that high levels of plasma cholesterol were , at
a population level, predictive of a higher risk of heart disease. Thus the
dietary lipid hypothesis was upheld and entered the policy arena of public
health nutrition.
A contrasting story is that of antioxidant micronutrients,
particularly vitamins C and E. In the early 1980s, there was a widespread
belief that plasma antioxidant levels played a major role in cardiovascular
disease, in cancer and in ageing. The data was dominated by associations
studies linking published levels of plasma antioxidant status in different
countries with national disease rates. The relationships were most impressive.
Animal studies also added to the theory and in vitro studies abounded showing
how anti-oxidant vitamins could protect fractions such as low-density
lipoprotein from oxidative damage which would otherwise render them very
atherogenic. As often happens in the field of health research, someone wanted
to cut to the chase and head for glory with a human intervention study. And so
the ATBC (alpha tocopherol {vitamin E} beta carotene) study was designed and
implemented. It failed to uphold the hypothesis and many reasons were put
forward as to why the study was “unsuccessful”. Based on knowledge of these
flaws, more intervention studies were rushed along and, all in all, the
antioxidant theory was abandoned.
We can look at some other “successes” and “failures” in
nutritional epidemiology. The protective role of folic acid in reducing the
risk of a neural tube defect birth was shown in a randomized controlled trial
leading to a major initiative in public health nutrition with the fortification
of flour with folic acid. Trans fatty acids were removed from the food chain
wherever possible on the basis of a strong human intervention study. In
contrast, notwithstanding the strong evidence from correlational studies in
humans of a link between fish oil fatty acids and cognitive decline, endless
intervention studies have failed to show a protective effect.
Thus we have an excellent track record in human nutrition in
translating observational studies that show an association between some aspect
of diet and some health attribute into dietary intervention studies and then
basing our policy interventions on food of those intervention studies.
Taxing sugar-sweetened beverages to reduce the incidence of
obesity requires data from human intervention studies that show a direct link
between weight gain and consumption of such beverages at rates that correspond
to reality before anything is done. There are intervention studies and
meta-analyses of such studies and the evidence is very weak if non-existent. To
suggest that we trust these data so much and that we are fully confident that
fiscal measures will be always positive and rarely negative is simply wrong and
is bad science. To argue that sometimes we need to take leaps of faith in
nutrition policy flies in the face of 6 decades of rigorous research on which
nutrition policy has been built. Facts and not faith should drive policy. Facts
are universal but faith is a subjective value.
