Organic Produce – More Nutritious than Conventionally Grown Produce?

At last year’s Mini-College, a presentation on organic food production caused a minor amount of controversy when the speaker mentioned that organically produced foods are healthier than conventionally produced foods. Since the OSU Master Gardener Program strives to rely upon and present unbiased, research-based information, some Master Gardeners questioned the research underlying this statement. Thus, I thought I would blog on what is known (from peer-reviewed scientific studies) about the nutritional content of organic versus conventional produce.

First, let me define organic versus conventional farming methods. Although the USDA lists a lengthy definition and regulatory text for organic production systems, I think that it will be more useful to distill down the definition for this blog. Organic farmers work to build and maintain the organic matter in their soils and the natural pest control agents in and near their fields (i.e. predators and parasitoids). Organic farming systems tend to be more diverse (in both their crops and associated organisms) than do conventional systems. Cultural, physical and biological controls of pests are emphasized over chemical controls. External inputs of pesticides and fertilizers are reduced. When pesticides and fertilizers are used, organic forms of these products are used. Synthetic forms of these products are not allowed in organic production systems.

Conventional farming systems, by comparison, do not focus on soil management, biodiversity within their fields and cultural or physical controls of pests. Biological control of pests is often disrupted or difficult due to the use of pesticides (which may be organically derived or synthetically manufactured).

Organic farm production often occurs at a smaller scale, and requires more labor, than does a conventional farm. This is why organic produce can be more expensive than conventional produce.

In fact, a 1998 study of the cost difference between organic and conventional produce in Tucson, AZ found that red delicious apples were 44% more expensive than conventionally grown applies. Similar results were found for broccoli (+76%), carrots (+78%), leaf lettuce (+92%), and tomatoes (+62%). [Gary D. Thompson and Julia Kidwell. 1998. Explaining the Choice of Organic Produce: Cosmetic Defects, Prices, and Consumer Preferences American Journal of Agricultural Economics, Vol. 80: 277-287.]

Unfortunately, I could not find more recent numbers in the peer-reviewed literature. OSU Master Gardeners – would you care to help me out with a quick and easy research project? If so, contact me via this blog.

What about the nutritional content of organic versus conventionally-produced foods? A 2003 study by Asami and colleagues looked at the total phenolic content of marionberries, strawberries and corn that were produced by organic, ‘sustainable’ and conventional methods. In this study, organic fields were managed to adhere to USDA organic production standards. Sustainable fields were managed to ‘meet the needs of consumers without compromising the ability of future generations to meet their own needs’. Information is presented on the fertilization regime of the sustainable fields (synthetic fertilizers were used), and on the pesticides used (none in one field and herbicides in a second field) but additional information on sustainable management practices were not presented in this paper.

The authors looked at total phenolic content of these plants, because polyphenols, in particular, are known to have anti-oxidant properties. For marionberries and corn, higher levels of total phenolics were found in organically and sustainably produced, relative to marionberries and corn from conventionally managed fields. Sustainably produced strawberries had higher total phenolics, relative to conventionally produced strawberries (the authors did not include organically grown strawberries in their study) – but only when strawberries were frozen, and not when they were freeze-dried or air-dried. [Asami, D. K., Hong, Y-J., Barrett, D. M. & Mitchell, A. E. (2003). Comparison of the total phenolic and ascorbic acid content of freeze-dried and air-dried marionberry, strawberry and corn grown using conventional, organic and sustainable agricultural practices. J. Agric. Food Chem. 51(5):1237-1241.]

Two recent review papers found similar results (Zhao et al. 2006, Benbrook 2009). Nutrient density and secondary plant metabolites (many of which are known to have anti-oxidant properties) tended to be higher in organically-produced foods than in conventionally grown foods.

Why might organically grown foods have more nutrients and anti-oxidants that conventionally grown foods?

First, organically-grown foods are often grown in soils with abundant and balanced micro- and macro-nutrients. This is because organic food production emphasises the accumulation and retention of organic material in the soil. This may translate into produce that contains a more balanced array of vitamins and minerals.

Second, insects may feed upon organically-grown crops more often than they damage conventionally grown crops. This may be due to differences in pesticide use in organically versus conventionally-managed fields. When insects feed upon a plant, they can induce or trigger the plant’s chemical defenses against herbivory. These chemical defenses can include polyphenols – which are known to have antioxidant properties.

What does this mean for the home gardener?

Managing your soil to encourage and maintain a high level of organic matter, and minimizing or removing pesticid use in your vegetable garden can yield multiple benefits. First, these management practices are generally more environmentally friendly than are the repeated and unncessary use of synthetic fertilizers and pesticides in the home garden. Second, these management practices may result in you and your family enjoying fruits and vegetables that contain more vitamins, minerals and antioxidents than those that come from vegetable gardens that are managed without regard to soil health or the non-target effects of pesticides.