Antioxidants and the
Nutritional Quality of Organic Vegetable
Consumer
awareness of the relationship between foods and health, together with environmental
concerns, has led to an increased demand for organically produced foods.
In
general the public perceives organic foods as being healthier and safer than
those produced through conventional agricultural practices. However,
controversy remains regarding whether or not organic foods have a nutritional
and/or sensory advantage when compared to their conventionally produced
counterparts. Advocates for organic produce claim it contains fewer harmful
chemicals, is better for the environment and may be more nutritious. There are
fundamental differences in organic and conventional production practices, but
limited information is available detailing how various practices influence the
nutritional quality, especially in terms of health-related antioxidants of
other food crops.
Fruits
and vegetables are a focal point of this controversy, since these foods are a significant
source of phenolic antioxidants, as phenolic acids and flavonoids, in the diet.
Epidemiological
studies consistently indicate an inverse correlation between the consumption of
fruits and vegetables and the risk of human cancers, cardiovascular disease,
diabetes and age-related declines in cognition. These chronic diseases are linked
to the oxidation of critical cellular macromolecules (e.g. proteins, lipids and
DNA) by reactive oxygen species (ROS).8 Phenolic antioxidants are thought to
neutralize ROS before they cause damage and lead to diseases. Dietary
guidelines set by the USDA now
suggest
increased consumption of fruits and vegetables (5-11 serving a day). Additionally,
reports by WHO and the Food and Agriculture Organization (FAO) of the United
Nations emphasize the role of foods and nutrition in the prevention of noncommunicable
diseases and point to a role for plant-derived phytochemicals in the prevention
of heart disease and cancer.
It
is important to recognize that both conventional and organic agricultural practices
represent dynamic systems that can vary greatly depending upon region, soil quality,
prevalence of pests, crop, climate and farm philosophies. This makes comparisons
very difficult. Conventional agriculture evolved globally in response to the availability
of high-yield crop cultivars, chemical fertilizers and pesticides, and progressing
irrigation and mechanization. Organic farming has also evolved, yet must adhere
to National Organic Standards set by the USDA in 2000. Accordingly, organic crops
must not be genetically engineered, irradiated, or fertilized with sewage
sludge. Additionally, farmland used to grow organic crops is prohibited from
treatment with synthetic pesticides and herbicides for at least three years
prior to harvest. Diseaseresistant varieties are often used and plant nutrients
are supplied through crop rotation, cover crops and animal manure.
Fertilization
is an important aspect to consider when comparing organic and conventional
agriculture. Organic fertilization typically does not provide nitrogen in a form
that is as readily accessible to plants as conventional fertilizers. The
accessibility of nitrogen has the potential to influence the synthesis of
phenolic antioxidants and soluble solids. For example, several studies
demonstrate there is a decrease in the concentration of phenolic antioxidants
in plants with increasing nutrient availability.
There are various overlapping hypotheses that
attempt to explain this relationship including the carbon/nutrient balance
(CNB) hypothesis, growth-differentiation balance (GDB) hypothesis and protein
competition model (PCM).15-17 In general, these theories state that high nutrient
availability leads to an increase in plant growth and development, and a decreased
allocation of resources towards the production of expendable metabolites such as
the phenolic antioxidants. The term phenolic antioxidant refers to both simple
phenolic acids and flavonoids. They are products of secondary plant metabolism
and are ubiquitous natural components of plants. Secondary plant metabolites
are defined as those compounds that are not essential to the life of the plant
(e.g. DNA, RNA, chlorophyll, amino acids and starch) and include phytochemicals
such as caffeine, isoflavonoids and phenolic antioxidants.
Plants
produce secondary metabolites as a defense mechanism against photo-oxidation, herbivory
(insect and animal predation), and for protection against pathogen attack.
Additionally,
they are critical components in the health of the plant, and many are pigments
that help to attract pollinating insects. The composition of secondary plant metabolites
differs between plants and within plant tissues. Genotype (i.e. the cultivar or
variety) is the primary determinant of the composition of secondary plant
metabolites, although their expression is strongly influenced by environmental
pressures, climate and UV-light exposure.
Scientists
have recently begun to question whether the levels of phenolic antioxidants are
lower in foods grown using conventional agricultural practices, since these
practices utilize levels of pesticides and fertilizers that can result in a
disruption of the natural production of plant-defense related metabolites.
Differences between the content of phenolic metabolites in organically and
conventionally produced fruits and vegetables allow for the possibility that
organically grown produce may benefit human health more than corresponding
conventionally grown produce.
Reviews
of existing literature demonstrate inconsistent differences in the nutritional
quality of conventionally and organically produced vegetables with the exception
of potentially higher levels of certain minerals, ascorbic acid and less
nitrates in organic foods. However, these data are difficult to interpret,
since cultivar selection and growing conditions varied widely and different methods
of sampling and analysis were used in the investigations reviewed.
Additionally, the majority of these studies did not assess levels of phenolic
antioxidants, as their role in human health was not yet
appreciated.
However, it is generally agreed that the levels of secondary metabolites have the
potential to differ the most between these two agricultural practices, since
they are produced in response to stress.
For
example, in two studies conducted by Carbonaro et al., higher levels of total
phenolics were found
in
organic peaches and pears when compared with their conventional counterparts.
In a study of five vegetables common in the
Japanese diet, Ren et al. demonstrated that organically grown spinach contained
120 percent higher antioxidant activity while Welsh onion, Chinese cabbage and
qing-gen-cai contained 20-50 percent higher antioxidant activity compared to
their conventionally grown counterparts.25 In our own studies, we have found
consistently higher levels of total phenolics and ascorbic acid in organic strawberries,
marionberries and sweet corn. Conversely,
Hakkinen and Torronen report that organic cultivation had no consistent effect
on the levels of phenolic compounds in
strawberries.
In more recent, unpublished studies at
the University of California Davis, we have found higher levels of total
phenolics, soluble solids and ascorbic acid, as well as the flavonoid aglycone
quercetin in two organically produced tomato cultivars.
Interestingly,
the same differences were not seen in organic bell peppers grown concurrently
with the tomatoes. This demonstrates the important point that differences in agricultural
practices will not affect all plants and all secondary metabolites equally.
Research
is needed to determine whether differences in agricultural practices affect the
levels of phenolic antioxidants in soybeans.
Contemporary
literature illustrates an apparent trend toward higher levels of phenolic
antioxidants, ascorbic acid and soluble solids in organic foods. However, there
are still far too few studies completed to establish a consensus regarding the
health benefit of organic foods. Ultimately, more research examining
relationships between agricultural production and the synthesis of
phytochemicals in specific crops is needed. Future studies should emphasize the
potential for agricultural manipulations to alter levels of both beneficial and
potentially toxic phytochemicals in foods. The ability to manage and control levels of beneficial
phenolic antioxidants in plants through cultivation has the potential to
enhance the nutritive quality of foods.
