August 11, 2011 | 312
If you’re like me, the word “organic” conjures up picturesque images of untouched green fields, majestic trees blossoming with fruit, happy free-ranging cows and chickens living together in harmony…ok maybe its just me. For many of us though, something visceral does happen when we decide to choose the organic apples as opposed to the conventionally grown ones.
Studies have found that consumers who choose organic foods over conventionally produced foods do so for several reasons including, but not limited to, environmental and health concerns over pesticide and fertilizer use in conventional practices, and perceived improved nutritional value of organic foods. Since consumers are willing to shell out more money for organic foods in part because they believe they are more nutritious, we should at least determine if this is actually the case.
Disclaimer note: I am solely reviewing evidence of nutritional content of organic foods…i.e. not touching on pesticide or fertilizer content, taste, appearance, etc. So yes, those organic strawberries may taste better, but are they nutritionally better for you? Read on my friends.
And the answer is…(drum roll please…)
Unclear. Ugghh, I know. I hate that answer too. I would love to be able to say unequivocally, that the money we spend on organic food is totally worth it and we will reap the benefits with longer, healthier, and happier lives. And while I can’t say this isn’t the case, I don’t think we can yet say it is. There are a ton of studies out there that have attempted to determine whether organically produced foods have more nutritional value than their conventionally grown counterparts. The vast majority of these studies have focused on fruits and vegetables and specifically have looked at micronutrient levels, these being your vitamins and minerals.
Probably the most consistently reported beneficial findings are those suggesting decreased nitrate content (makes sense, due to the lack of nitrogen-based fertilizers), increased vitamin C, and increased mineral content (including calcium, magnesium, iron, phosphorous, and sodium) in organically produced foods. These findings have been reported for strawberries, peaches, pears, apples, green leafy vegetables, potatoes, wheat, and the list goes on and on (reviewed in Bourn and Prescott 2002, and Williams 2002).
BUT, don’t go out and spend all your cash on organic strawberries just yet. For all these positive findings, there are just as many that report no difference between organic and conventionally grown produce. And this is where the real story begins…
In one word, variability. As scientists, we HATE this word. The reason for all these conflicting findings is in part due to the sheer amount of variability in the way these studies are designed and carried out. There is SO much variability that it is virtually impossible to compare findings.
Lucky for me, the authors of a handy little paper (Bourn and Prescott 2002) have reviewed the literature examining the differences in nutrient content between organic and conventional foods and broadly grouped these into four types of study design; 1) comparison of produce bought straight from the retailers, 2) comparison of crops treated with synthetic fertilizer vs. manure, and 3) comparison of produce obtained straight from the farm. The fourth category, in which studies compared the effects of organic and conventional foods fed to animals and humans, will not be addressed here as these studies do not look directly at nutrient content. (Note: go to Bourn and Prescott, 2002, for more details regarding the studies mentioned below).
So what have these various studies found, and more importantly, what is wrong with these studies? Let’s review.
1). Retail studies. A few of these studies have reported increased vitamin C and mineral content such as calcium, magnesium, iron, and sodium, in organic foods (mostly fruits and vegetables) as compared to conventional foods. However, while this study design may have its merits since the supermarket is where most consumers get their organic foods from, it also allows for a LOT of variability. Studies performed in this manner essentially ignore details such as growing conditions, harvest maturity, freshness, and even cultivar. The produce has likely been grown in different soil types, climates, been harvested at different times, been stored and shipped differently, etc. The authors of one study (and there are likely more) did not even verify that the foods with organic labels were in fact organically produced. For these reasons any differences found in terms of nutritional value cannot be taken too seriously as these differences may well be explained by the factors listed above.
2). Fertilizer studies. One of the most common study designs, because they are relatively cheap and easy to carry out, is to examine the effect of fertilizer type (nitrogen-based synthetic fertilizers vs. manure) on nutritional composition of produce. Some studies have reported that manure fertilization caused decreased nitrates, increased vitamin A, B12, and C in various crops. On the flip side, studies have also found manure fertilization to cause no difference in nitrate content and decreased vitamin C in various crops. Confused yet? Again, we have the problem of lack of control of crop location, climate, growing conditions, etc. Since these factors have been found to be an important factor in response to different fertilizer types, these findings cannot be attributed to fertilizer or manure alone.
3). Farm comparison. The nice thing about these types of studies are that effects of the whole farming system can be compared and therefore more variables can be controlled for, such as soil, cultivar, maturity, etc. But, just because these variables can be controlled for, doesn’t mean they always are. Findings from these types of studies are again, highly variable. For example studies have shown higher, lower, or equal vitamin C levels in several organic crop types as compared to conventional ones. In general, it seems that these types of studies have shown fewer significant differences regarding nutrient content of organic vs. conventional foods, than studies of other designs. This could be a “real” finding, meaning when you get right down to the level of the farm, there just is no difference, or it could be due to so many factors at play at this level that can affect nutritional content…remember, just because some of these factors can be controlled, does not mean they are.
Some recent studies have been performed at the “farm” level examining the differences in phytochemical content between organic and conventional foods (Raigon et al 2009, Stracke et al 2009). Phytochemicals are naturally occurring substances in plants that are thought to affect health, but are not considered essential nutrients. Examples of these (and foods they are commonly found in) are beta-carotene (carrots), lycopene (tomatoes), sulforaphane (broccoli), and resveratrol (red grapes). These natural components of plants are produced as a protective response to stress such as environmental conditions and insects or other pests.
It might be expected then that the differences in stresses produced by organic vs. conventional growing practices might cause differences in phytochemical levels in the products. Yet again we see variability in the findings of these studies, and again this seems to be due to several factors, including year-to-year variation, climate, location, etc. Since this is still a fairly young area of research we can hope for more quality studies on the horizon.
So in summary, it seems the same problems in variability of study design and uncontrolled factors are being encountered over and over again, and until these are resolved it will be difficult to determine whether organic foods have more nutritional value than conventional.
The bigger problem…
While the issues with the study designs outlined above are problematic, experimental control over absolutely everything is not going to be possible. At the very least though, the quality of the research and subsequent publications should be produced at the highest level attainable for a given study design (translation: make things as reproducible as possible!).
A systematic review (actually the only systematic review) of studies examining the nutritional quality of organic vs. conventionally produced foods was carried out in 2009 (Dangour et al 2009). As expected, the authors found a variety of study designs, but more importantly they found that the quality of this research was also extremely variable and very few were considered of satisfactory quality. In this review, quality was based on 5 criteria: 1) clear definition of organic production methods, 2) specification of cultivar or breed, 3) statement regarding which nutrient(s) were analyzed, 4) description of laboratory methods, and 5) statement of statistical methods. Studies were considered to be of satisfactory quality if they met all 5 criteria.
Now it seems to me that these standards for reaching satisfactory quality are extremely low. I have a hard time imagining how any study without these basic criteria would even see publication. Even more shocking, only 1/3rd of the studies examined (out of 162) were considered satisfactory in quality. That’s 55 satisfactory studies. Keep in mind these 55 studies would be all over the map in terms of study design, food product analyzed, nutrients examined, etc., so likely comparison between them would be limited.
Nevertheless, analysis of these 55 revealed no significant difference in vitamin C, magnesium, potassium, calcium, zinc, copper, and phenolic compounds. Conventional crops appeared to have higher nitrogen content (due to synthetic fertilizers) and organic crops tended to have higher phosphorous and were more acidic (which may have to do with ripeness at harvest). Of the 55 studies, only 9 of these were livestock product studies and very limited data were reported in these, therefore conclusions cannot be drawn. Based on this systematic review, it appears that organic foods do not offer superior nutrient content as compared to conventionally produced ones.
Sorry to say, but as of now, there is no clear-cut answer as to whether organic foods are nutritionally advanced as compared to conventionally grown foods. Several studies have reported an increase in vitamin, mineral, and phytochemical content of organic produce, and while I would LOVE to believe them, these findings must be taken with a grain of salt due to the major problems with study design variability and just poor science in some cases. Not only do we need more quality studies looking into this question, we need more studies examining the bioavailability of organic vs. conventional foods (which I’m sure would just open up a whole new can of variable-poor-study-design worms, but need to be done nevertheless).
So as a final point, no studies have really shown that organic foods are nutritionally worse than conventional, in fact the trend is for them to be no different or slightly better. If organic foods are actually nutritionally superior, I have a feeling it won’t be by much, meaning it may make no substantial difference in terms of your health. Again, I’m just talking nutrients here – whether fertilizers and pesticides used in conventional farming methods have health effects is an issue for another blogger to tackle. So for now, I say whether you choose the organic bag of apples or the conventionally grown ones really doesn’t make a big difference. As long as you’re choosing any bag of apples over a bag of Doritos, you’re on the right track.
Bourn D and Prescott J. A comparison of the nutritional value, sensory qualities, and food safety of organically and conventionally produced foods. Critical Reviews in Food Science and Nutrition, 2002; 42(1):1-34. DOI:10.1080/10408690290825439
Williams CM. Nutritional quality of organic food: shades of grey or shades of green? Proceedings of the Nutrition Society, 2002;61:19-24. DOI:10.1079/PNS2001126
Dangour AD, Dodhia SK, Hayter A, Allen E, Lock K, and Uauy R. Nutritional quality of organic foods: a systematic review. American Journal of Clinical Nutrition, 2009;90:680-685. DOI:10.3945/ ajcn.2009.28041
Raigon MD, Rodriguez-Burruezo A, and Prohens J. Effects of organic and conventional cultivation methods on composition of eggplant fruits. Journal of Agriculture and Food Chemistry, 2010;58:6833-6840. DOI:10.1021/jf904438n
Stracke BA, Rufer CE, Weibel FP, Bub A and Watzl B. Three-year comparison of the polyphenol contents and antioxidant capacities in organically and conventionally produced apples (Malus domestica bork. Cultivar ‘golden delicious’). Journal of Agriculture and Food Chemistry, 2009;57:4598-4605. DOI:10.1021/jf803961f
Photo credit: jdickert on Flickr.