Friday July 31 2009
Researchers looked at thousands of studies on organic food
“The supposed health benefits of organic food are one of its great selling points… but can be damaging to good nutrition,” The Times has reported. The newspaper says that the evidence that organic is healthier than conventional produce has always been weak, and that certain organic lobbyists are ignoring the bigger picture by citing only selected studies which show that organic food has more nutrients.
The newspaper suggests that because organic food is expensive, people on a budget who choose it as a ‘healthy option’ might actually be harming their health by reducing the total amount of fruit and vegetables they eat.
To resolve the issue, the Food Standards Agency funded a systematic review. This well-conducted comprehensive review, identified over 50,000 articles but found that only 55 were of satisfactory quality. It found only 11 papers with direct relevance to human health, five of which involved testing cell cultures rather than people. Of the six human studies, four included fewer than 20 participants, giving them little statistical power. The taste, surface pesticide content or appearance of the food was also not researched.
Regardless of production method, fruit and vegetables are, of course, still good for you. People on a budget can still get their five-a-day from fresh, frozen or tinned fruit and vegetables.
Where did the story come from?
This research was conducted by Dr Alan D Dangour and colleagues from the Nutrition and Public Health Intervention Research Unit, and colleagues from other units in the London School of Hygiene and Tropical Medicine. The study was funded by the UK Food Standards Agency, which had no role in the methods of study design, data collection, analysis, interpretation, or in the writing of the final report.
The study was published in the American Journal of Clinical Nutrition, a peer-reviewed medical journal.
What kind of scientific study was this?
This was a systematic review examining the nutrient content of organic foodstuffs compared to the conventionally produced varieties.
In order to gather studies, the authors systematically searched recognised databases for studies published from 1958 until February 2008, and they contacted 40 subject experts, and went through the reference lists of the studies they found. They included studies that had abstracts in English and compared nutrient content between organic and conventional foodstuffs. They used two specialist reviewers to extract study characteristics, quality, and data.
The authors were interested in a range of nutrients (more than 450), and they categorised them into groups for comparison. These groups were nitrogen, vitamin C, phenolic compounds, magnesium, calcium, phosphate, potassium, zinc, total soluble solids, copper and titratable acidity. Titratable acidity is a measure of a fruit’s ripeness when harvested.
The authors assessed the quality of studies using five criteria that addressed key components of the design. The studies had to include:
- a clear definition of the organic production methods,
- specification of the ‘cultivar’ (variety) of crop or breed of livestock,
- a statement of which nutrient was analysed,
- a description of the laboratory methods used,
- a description of the statistical methods
To be considered satisfactory in quality, the study had to meet all five criteria.
What were the results of the study?
From a total of 52,471 articles, the researchers identified 162 studies (137 crops and 25 livestock products). Of these, 55 were of satisfactory quality.
When the authors looked at only those studies of a satisfactory quality, conventionally produced crops had a significantly higher nitrogen content, which is a measure of specific fertiliser use. Organic crops had a significantly higher content of phosphorus and higher titratable acidity (a measure of ripeness of the fruit at harvest). They found no evidence of a difference in the remaining eight crop nutrient categories from the 11 analysed.
When the authors analysed the limited database on livestock products available, they found no evidence of a difference in nutrient content between livestock products produced organically and conventionally.
What interpretations did the researchers draw from these results?
The researchers say that there is no evidence of a difference in nutrient quality between foodstuffs produced organically and conventionally.
They go on to explain that the small differences in nutrient content relate to differences in production methods or were biologically plausible, meaning that they relate to differences such as fertiliser use or the timing of harvest.
What does the NHS Knowledge Service make of this study?
This was a well-conducted review in which the authors tried hard to identify relevant studies, used two specialised reviewers, and carefully described their methods.
- The researchers say that their systematic approach agrees with some but not all the findings from previous reviews. For example, some previous reviews also found a higher content of phosphorous in organic foods. In contrast, this review did not support the conclusions of some other reviews which had shown that organic foods had a higher content of vitamin C and magnesium.
- The reviewers noted other minor limitations. Because the reviewers excluded ‘grey literature’ (conference abstracts and unpublished studies) and non-English language abstracts, it is possible that some relevant data was not included in the review. Also, the researchers knew of two studies that were published after their cut-off date and were therefore not included in their analysis.
- A large number of studies were excluded at the abstract stage, with 52,179 out a total of 52,471 studies deemed unsuitable. This suggests that the strategy used for identifying the studies in the database may have been too sensitive (i.e. it found a lot of irrelevant studies). The number of studies excluded later in the process was also high, suggesting that stringent criteria for inclusion and quality were used.
Overall, the study confirms speculation that the nutritional content of organic and conventional foods is much the same, except for content that is a feature of individual production methods. It should be noted that the study did not look at other differences that concern those who buy this food, such as taste, pesticide content, appearance or the environmental effect of agricultural practices.