An advance copy of a study appeared today that will be published in the
September edition of the “American Journal of Clinical Nutrition.” The
published paper, “Nutritional quality of organic foods: a systematic
review,” was written by a team led by Alan Dangour, at the London
School of Hygiene and Tropical Medicine and funded by the United
Kingdom’s Food Standards Agency (FSA).
In
their written report, the London team downplayed positive findings in
favor of organic food. In several instances, their analysis showed that
organic foods tend to be more nutrient dense than conventional foods.
Plus, their study omitted measures of some important nutrients,
including total antioxidant capacity. It also lacked quality controls
contained in a competing study released in 2008 by The Organic Center
(TOC). Last, the FSA-funded team also used data from very old studies
assessing nutrient levels in plant varieties that are no longer on the
market.
The London team reported finding statistically
significant differences between organically and conventionally grown
crops in three of thirteen categories of nutrients. Significant
differences cited by the team included nitrogen, which was higher in
conventional crops, and phosphorus and tritratable acids, both of which
were higher in the organic crops. Elevated levels of nitrogen in food
are regarded by most scientists as a public health hazard because of
the potential for cancer-causing nitrosamine compounds to form in the
human GI tract. Hence, this finding of higher nitrogen in conventional
food favors organic crops, as do the other two differences.
Despite
the fact that these three categories of nutrients favored organic
foods, and none favored conventionally grown foods, the London-based
team concluded that there are no nutritional differences between
organically and conventionally grown crops.
A team of scientists
convened by The Organic Center (TOC) carried out a similar, but more
rigorous, review of the same literature. The TOC team analyzed
published research just on plant-based foods. Results differ
significantly from the more narrow FSA review and are reported in the
study“New Evidence Confirms the Nutritional Superiority of Plant-Based Organic Foods.”
The
TOC findings are similar for some of the nutrients analyzed by the FSA
team, but differ significantly for two critical classes of nutrients of
great importance in promoting human health – total polyphenols, and
total antioxidant content. The FSA team did not include total
antioxidant capacity among the nutrients studied, and it found no
differences in the phenolic content in 80 comparisons across 13 studies.
Unlike
the London study, The Organic Center review focused on nutrient
differences in “matched pairs” of crops grown on nearby farms, on the
same type of soil, with the same irrigation systems and harvest timing,
and grown from the same plant variety. It also rigorously screened
studies for the quality of the analytical methods used to measure
nutrient levels, and eliminated from further consideration a much
greater percentage of the published literature than the FSA team.
While
the FSA team found 80 comparisons of phenolic compounds, the TOC team
focused on the more precise measure of total phenolic acids, or total
polyphenols, and found just 25 scientifically valid “matched pairs.” By
mixing together in their statistical analysis the results of several
specific phenolic acids, the FSA team likely lost statistical
precision.
Instead, the TOC team focused on studies reporting
values for total phenolic acids, and also applied more rigorous
selection criteria to exclude poorer quality studies.
The TOC team found –
- Twenty-five
matched pairs of organic and conventional crops for which total
phenolic acid data was reported. The levels were higher in the organic
crops in 18 of these 25 cases, conventional crops were higher in 6. In
five of the matched pairs, phenolic acid levels were higher in organic
crops by 20% or more. On average across the 25 matched pairs, total
phenolics were 10% higher in the organic samples, compared to
conventional crops. - In seven of eight matched pairs
reporting total antioxidant capacity data, the levels were higher in
the organically grown crop. Of 15 matched pairs for the key antioxidant
quercetin, 13 reported higher values in the organic food. In the case
of kaempferol, another important antioxidant, the organic samples were
higher in six cases, while five were higher in the conventional crops.
In
the TOC study, there were an ample number of matched pairs to compare
the levels of 11 nutrients, including five of the nutrients in the FSA
review. For the five nutrients covered in each review, the TOC team was
in general agreement with the FSA findings for two (nitrogen and
phosphorus).
The London team did not assess differences in key
individual antioxidants, nor in total antioxidant activity, important
nutrients that have been measured in several more recent studies.
Across
all the valid matched pairs and the 11 nutrients included in the TOC
study, nutrient levels in organic food averaged 25% higher than in
conventional food. Given that some of the most significant differences
favoring organic foods were for key antioxidant nutrients that most
Americans do not get enough of on most days, the team concluded that
the consumption of organic fruits and vegetables, in particular,
offered significant health benefits, roughly equivalent to an
additional serving of a moderately nutrient dense fruit or vegetable on
an average day.
Why the Different Results?
A review of
the London-based team’s methodology and study design points clearly to
why the FSA and Organic Center studies reached some different
conclusions.
Inclusion of Older Studies
The FSA review
included studies over a 50-year period: January 1958 through February
2008. The TOC team included studies published since 1980. Most studies
published before 1980 were found flawed for purposes of comparing the
nutrient content of today’s conventional and organic crops.
Most
of the older studies used plant varieties no longer in use, and did not
measure or report total phenolics or antioxidant capacity (since these
nutrients were just being discovered). The older studies used
analytical methods that are now considered inferior, compared to modern
methods.
Further, since the 1950s, plant breeders and growers
have consistently increased the yields of food crops, leading, in some
cases, to a dilution of nutrients. In 2004, one of us (Donald R. Davis)
reported evidence for a general decline in some nutrient levels in 43
garden crops between 1950 and 1999 (Davis et al., “Changes in USDA Food
Composition Data for 43 Garden Crops, 1950 to 1999,” Journal of the
American College of Nutrition, Vol. 23(6): 669-682; asummary of the Davis paper is posted).
Similarly,
an Organic Center report by Brian Halweil describes in detail the
evidence linking higher yields and nutrient decline (“Still No Free Lunch: Nutrient levels in the U.S. food supply eroded by pursuit of high yields,”).
Thus, results in the FSA study are likely confounded by the team’s decision to include data from over three decades ago.
New Studies Support Greater Nutrient Density in Organic Foods
Since
February 2008, the cut-off date of the London study, some 15 new
studies have been published, most of which use superior design and
analytical methods based on criticisms of older studies. The Organic
Center is updating its earlier analysis with these additional studies.
These new studies generally reinforce the findings reported in the
March 2008 TOC report, particularly in the case of nitrogen (higher in
conventional crops, a disadvantage), and Vitamin C, total phenolics,
and total antioxidant capacity, which are typically higher in
organically grown foods.
The Center’s study finds that protein
content and beta-carotene, a precursor of Vitamin A, are typically
higher in conventionally grown foods, but since both are present at
ample or excessive levels in the diets of most Americans, these
differences do not confer a nutritional advantage nearly as important
as heightened levels of phenolics and antioxidants in organic foods.
Exclusion of Studies Analyzing Results on “Integrated” Farms
The
FSA team excluded studies comparing organic foods to “integrated” and
biodynamic production systems, stating that “integrated” systems are
not conventional. Most conventional U.S. fruit and vegetable producers
are now using advanced levels of Integrated Pest Management. Thus,
“integrated” systems are now a more accurate description of
“conventional” agriculture in the U.S., than a definition grounded in
monoculture, the calendar spraying of pesticides, and excessive
applications of chemical fertilizers. The London team did not report in
the published paper which “integrated” studies were dropped, but we
suspect some important U.S.-based studies may have been eliminated.
TOC Study Applied Much Stricter Screens for Scientific Validity
The
two teams agree that many published studies are methodologically
flawed, and hence should not be included in comparative studies. But
the FSA and TOC teams used very different rules to screen studies for
scientific quality and to select matched pairs for analyses.
The
FSA team cites five criteria: definition of the organic system;
specification of the plant variety (i.e., crop genetics); statement of
nutrients analyzed; description of laboratory method used; and, a
statement regarding statistical methods for assessing differences. The
London team states that they simply required some discussion of these
issues in published papers, but did not set or apply any qualitative
thresholds in judging scientific validity.
The Organic Center
team focused on the same factors (plus several others) and used stated,
objective criteria for assessing them. The TOC team reviewed the
statistical power and reliability of the analytical methods, a process
that eliminated dozens of results. Finally, the TOC team insisted upon
a close match of soils, plant genetics (variety), harvest method and
timing, and irrigation systems, all factors that can bias the results
of a comparison study.
Inclusion of Market-Basket Studies
The
FSA team included some market basket studies, for which there is no way
to know the specific circumstances of the farm locations, the plant
genetics, the soil type, or harvest method and timing. In the Organic
Center study, market basket results were judged as “invalid” based on
several quality-control screening criteria.
This review is also available as a pdf document below.