Health benefits of organic food - experts respond

Monday, 14 July 2014, 4:34 pm
Press Release: Science Media Centre

Health benefits of organic food - experts respond

In contrast to earlier research, a new study has found that organic crops have measurable health benefits -- namely that they are higher in antioxidants and lower in toxic metals.

This new UK study compiled and analysed results from 343 earlier studies. It claims that organic crops and crop-based foods are up to 60 per cent higher in a number of key antioxidants than conventionally-grown crops.

The study also shows significantly lower levels of toxic heavy metals in organic crops including cadmium, which was found to be almost 50 per cent lower in organic crops than conventionally-grown ones.

Registered journalists can access the research and press material in the SMC Resource Library.

A 'Before the Headlines' analysis compiled by UK statistical experts is also available below.

NB: The original embargo on this research was broken over the weekend overseas, so the study and supporting materials are now for immediate release.

The SMC gathered the following expert commentary. Feel free to use these quotes in your reporting. For assistance reaching these or other experts for follow up, contact the SMC.

Peter Cressey, Environmental Health Scientist at ESR comments:

"This manuscript reports a meta-analysis study to determine if there are food compositional differences between plant foods produced under organic agriculture and those resulting from conventional agriculture. The study used a rigorous search regime and appropriate statistical analysis. Analyses were carried out on both weighted and unweighted data sets. It appears reasonable to place greater credence on the results of the weighted analysis.

"The study reports three major findings that are considered favourable to organic agriculture. Organic produce was found to contain statistically:

* higher concentrations of antioxidant polyphenolic, flavonoid and anthocyanin compounds;

* lower concentrations of cadmium; and

* lower prevalence and concentrations of pesticide residues.

"While not all previous meta-analysis studies have identified these differences, they are differences that are not unexpected."

Higher concentrations of antioxidant polyphenolic, flavonoid and anthocyanin compounds

"While these compounds are believed to have beneficial human health effects, these benefits have not been confirmed. It is probably more accurate to say that they are candidate compounds for the beneficial effects seen from a diet with high consumption of fruits and vegetables. This point is acknowledged by the study authors in their discussion.

"These compounds are also known inducible plant defence compounds, which may be produced by plants in response to biotic or abiotic stress. Organic agriculture proscribes the use of synthetic pesticides and it is plausible that organic crops may be under greater pest and disease pressure than their conventional counterparts.

"It should further be noted that the meta-analysis found a greater concentration of dry matter in organic produce, although this was only statistically significant in the unweighted analysis. Higher dry matter can result from the produce being less 'plump' or more shrivelled and will result in concentration of compounds in the edible portion."

Lower cadmium

"Cadmium is known to be elevated in phosphate fertilisers from some sources. These fertilisers are not used in organic agriculture and it is entirely plausible that organic produce may contain lower cadmium concentrations than the conventional equivalents."

Lower prevalence and concentrations of pesticide residues

"Synthetic pesticides are not permitted in organic agriculture and it would be very surprising of the prevalence of pesticides were not lower in these products. It is noteworthy that, across the studies considered, pesticide residues were detected in about 11% of organic samples. The authors of the study note this and state that it may be due to cross-contamination, persistence, or accidental or fraudulent use of pesticides.

"In a study carried out by our group, pesticide residues were detected in 22% of samples. Of the residues found on organic produce about 10% may have been due to persistence [in the environment]. Concentrations found were generally higher than would be expected from cross contamination."

Our colleagues at the Australia and UK SMCs collected the following expert commentary.

Dr Ian Musgrave is a Senior Lecturer in the Faculty of Medicine, School of Medicine Sciences, within the Discipline of Pharmacology at the University of Adelaide

"This is yet another study that shows that there is little difference in nutritional content between organically grown food and conventionally grown food. There have been many attempts to measure the nutritional differences between conventional and organic food, with largely inconsistent results. This is not surprising, as the nutritional value of foods is very variable, influenced strongly by local regional factors, variations in growing seasons and rainfall, ripeness of food when harvested and time of harvest. Even different cultivars of the same crop may vary significantly in composition due to the factors above. To try and avoid these limitations, several studies have looked at large numbers of these single studies (a meta-analysis), and generally concluded that there is no meaningful difference between conventional and organic food.

"The paper by Baranski and colleagues in the British Journal of Nutrition follows in the footsteps of these large comparison studies, looking at the largest group of studies to date (343). Their findings are largely similar to previous meta-analyses, only a handful of nutrients are statistically different between organic crops and conventional crops, and only one is of plausible biological significance.

"While some organic crops (typically fruits) had statistically higher levels of antioxidants than conventional, the level variations were small and unlikely to significantly affect health. While consumption of antioxidant containing fruit and vegetables have been associated with better health outcomes, the antioxidants themselves do not appear to have any role in this effect, and the extra amounts of antioxidants present in organic food are too low to be meaningful. Organically grown fruits tended to be higher in carotenoids, but fruit consumption is unlikely to boost carotenoid levels to those associated with higher risk of death. The levels of the toxic metal Cadmium were lower in organic foods than conventional foods. Although most peoples intake of cadmium in conventional foods is below that associated with any health risk, this is one area where organic foods may have a larger margin of safety. Overall, organic foods generally have no biologically meaningful health benefits (with the possible exception of cadmium), given their much higher price, people need to carefully consider any decision to consume organic foods."

Catherine Collins, Principal Dietitian at St George's Hospital NHS Trust, said:

"Is organic food better for you than non-organic? That's a question hotly debated for decades, and this large review of the evidence will be widely supported by the pro-organic lobby as evidence of a bigger 'Health Halo' with the organic cousins of the fruit and vegetable family compared to their non-organic equivalent.

"One of the key problems facing researchers in this field over the years is how to compare, well, apples and pears. Do you buy them at the supermarket and measure the difference in plant substances found in each type of produce at 'point-of-sale? (so-called 'basket studies'). Or do you choose two farms close to each other, one to grow organic and one to grow non-organic produce, and compare the two as picked? This removes the natural variation in soil richness as an influence of plant content. Or should you rely on formal experiments with sample crops? All these methods are valid but each generate very different answers depending on soil type, use of permitted plant additives to each type of produce, storage methods and time from farm to fork.

"That's why in this large review the results show such a wide range of values for different plant substances. For example, the antioxidant chlorogenic acid was sometimes lower and sometimes higher in organic foods when compared to non-organic varieties. So what to do? As a dietitian I'd suggest forget the big 'O' title, and just enjoy your morning coffee or a juicy peach as a between meal snack as a rich source of this particular antioxidant, without the worry of how it was grown.

"The key plant substances that appeared higher in organic fruits and vegetables were some of the plant antioxidants. Polyphenol antioxidants such as flavanones and flavonols were higher in organic produce than non-organic versions, as were antioxidant anthocyanins. When you compare the price and availability of the organic version of foods rich in these antioxidants, paying double for organic didn't provide you with double the antioxidant benefits - but it does reduce the amount of money left to spend on the rest of your diet.

"It's also worth remembering that all of the massive national, European and international studies showing the significant health benefits of eating at least 5 portions of fruit and vegetables daily have never made a distinction between organic and non-organic varieties. When it comes to health insurance all fruits and vegetables count. Bottom line? Just eat more."

Prof Richard Mithen, Leader of the Food and Health Programme at the Institute of Food Research (IFR), said:

"This paper provides some evidence that under organic agronomic regimes, there may be an increase in the concentration of certain phenolic compounds in some fruits and vegetables. The paper also reports a decrease in protein, nitrates and fibre in the organically grown crops, which may be undesirable, and which are maybe unsurprisingly not referred to by the authors in their advocacy of organically grown produce.

"The increase in phenolic compounds may not be entirely unexpected due to the greater pest and pathogen damage that crops under organic systems are exposed to which can results in the induction of these toxic defence compounds. Moreover, one can enhance the levels of phenolic compounds in crops through many different 'organic' or 'conventional' agronomic systems, or indeed through plant breeding and genetic modification, but this does not necessarily mean that the modes of production and the increases in these compounds are beneficial to health or the environment.

"Of greater significance is that there is no evidence provided that the relatively modest differences in the levels of some of these compounds would have any consequences (good or bad) on public health. The references to 'antioxidants' and 'antioxidant activity', and various 'antioxidant' assays would suggest a poor knowledge of the current understanding within the nutrition community of how fruit and vegetables may maintain and improve health.

"The additional cost of organic vegetables to the consumer and the likely reduced consumption would easily offset any marginal increase in nutritional properties, even if they did occur, which I doubt. To improve public health we need to encourage people to eat more fruit and vegetables, regardless of how they are produced."

Prof Tom Sanders, Head of Diabetes and Nutritional Sciences Division, School of Medicine, King's College London, said:

"This article is misleading because it refers to antioxidants in plants as if they were a class of essential nutrients, which they are not.

"The compounds referred to are mainly plant phenolics and are produced in higher quantities when plants are environmentally stressed. Plant phenolics have both toxic as well as potential beneficial effects. Some vitamins have anti-oxidant properties such as vitamin E, vitamin C and beta-carotene but the differences between organic and conventional produce are trivial.

"The article misleadingly suggest health benefits result from a high consumption of antioxidants particularly cancer protection. While the World Cancer Research Foundation in its systematic reviews concluded there is a relationship between fruit and vegetable consumption and a lower risk of cancer, they did state that there was insufficient evidence to make any claim for antioxidants and plant phenolics.

"Polyphenols also have an adverse effect on metal absorption and are considered as anti-nutrients. For example their antioxidant properties inhibit iron and zinc absorption.

"The article also claims that the lower levels of nitrate and nitrite in organic vegetables would be beneficial to health. However, this is opposite to more recent research, including some carried out at the University of Newcastle, which shows that nitrate in vegetables in fact lowers blood pressure because it is converted to the vasodilator nitric oxide (1).

"The article shows differences in cadmium concentrations in cereals but not in fruit and vegetables. Cadmium levels are dependent on the soil in which the plant is grown and haVE nothing to do with organic certification. There are naturally occurring areas (1,2) in the UK where cadmium levels of are high (e.g. Shipham in Somerset) and home-grown/organic food grown in these areas would therefore be high in cadmium. Cadmium can be high in soils derived from spoil from former lead, zinc and tin mines. Generally, shellfish are regarded as far more important source of cadmium in the diet, especially if fished from areas where sediment is naturally high in Cadmium (ie. South Coast) or from smelting.

"In terms of macronutrients (i.e. protein, carbohydrate, fat) ¬ the organic products contained less protein. Other nutrient differences were trivial and well inside the normal range of variation that occurs with different varieties, soil types and variations in weather. "This study provides no evidence to change my views that there are no meaningful nutritional differences between conventional produced and organic crops."

Refs:

1. Lidder S, Webb AJ. Vascular effects of dietary nitrate (as found in greenleafy vegetables and beetroot) via the nitrate-nitrite-nitric oxide pathway. Br JClin Pharmacol. 2013 Mar;75(3):677-96

2. Davies BE, Bailinger RC. Heavy metals in soils in north Somerset, England,with special reference to contamination from base metal mining in the Mendips Environ Geochem Health. 1990 Dec;12(4):291-300

3. Morgan H, Smart GA, Sherlock JC. The Shipham report. An investigation into cadmium contamination and its implications for human health. Intakes of metal. Sci Total Environ. 1988 Aug 15;75(1):71-100

Dr Alan Dangour, Reader in Food and Nutrition for Global Health at the London School of Hygiene & Tropical Medicine, said:

"Systematic reviews are powerful tools that are increasingly being used to help to resolve areas of controversy in science. By bringing together all of the available evidence, critically assessing its quality, and synthesising the findings in a standardised and pre-specified manner, systematic reviews substantially reduce bias and enhance the reliability of the 'answer'.

"The authors of this new systematic review that primarily aims to identify differences in nutrient content between organically and conventionally produced foods have brought together a large number of studies published over a 20 year period. The quality of the available data varies greatly and it is therefore very surprising that, in their analysis, the authors decided to include all the data that they found, irrespective of their quality. In fact the study authors themselves note that there are significant concerns with the consistency and reliability of some of their findings. Mixing good quality data with bad quality data in this way is highly problematic and significantly weakens confidence in the findings of the current analysis. It is a shame that greater care was not taken in trying to ensure that the analyses were based only on reliable and scientifically robust data from satisfactory quality studies.

"Furthermore, the public health significance of the reported findings have been worryingly overstated. There is no good evidence to suggest that slightly greater antioxidant or polyphenolic intake in the human diet has important public health benefits, and there is no robust evidence to support the claim that consumption of greater amounts of these compounds reduces the risk of chronic diseases such as cardiovascular disease and cancer in human populations.

"All natural products vary in their composition for a wide variety of reasons. This paper provides no convincing evidence to refute our earlier finding1, fully supported by a recent US-led systematic review2, that there are no important differences in nutrient content between organically and conventionally produced foods."

Refs:

(1) Dangour AD, Dodhia SK, Hayter A et al. (2009) Nutritional quality of organic foods: a systematic review. American Journal of Clinical Medicine, 90, 680-685.

(2) Smith-Spangler C, Brandeau ML, Hunter GE et al. (2012) Are organic foods safer or healthier than conventional alternatives? A systematic review. Annals of Internal Medicine, 157, 348-366.

Declared interests: Dr Dangour was project lead and lead author of the systematic reviews on the nutritional quality (2009) and nutrition-related health benefits (2010) for the Food Standards Agency. He is also an expert reviewer for the Department of Health and Public Health England.

/BEFORE THE HEADLINES ANALYSIS provided by the SMC UK

Title, Date of Publication & Journal

Higher antioxidant concentrations and less cadmium and pesticide residues in organically grown crops: a systematic literature review and meta-analyses, 15 July 2014, British Journal of Nutrition

Claim supported by evidence?

This study provides limited evidence that organically grown crops contain, on average, higher concentrations of some antioxidants, lower concentrations of cadmium, and are less likely to contain pesticide residues than conventionally grown crops; however, there could be other reasons for these apparent differences. The study found no evidence for differences in concentrations of a number of other antioxidants. It does not prove that organic crops are higher in antioxidants. There was also evidence that organic crops contain, on average, lower concentrations of fibre and protein and higher concentrations of carbohydrates, but these differences are small.

Summary

This was a review and meta-analysis of a large number of existing studies (all peer-reviewed). This allows evaluation of all the published evidence but does not include any unpublished data. • The results from the studies were combined to produce an overall average difference between organic and conventional crops with respect to levels of nutrients and contaminants. • For many of the outcomes considered there was evidence for moderate or strong publication bias. Publication bias can lead researchers to find effects that are not real; this is exacerbated when using a random effects meta-analysis (as here) - even more so if the meta-analysis is unweighted (also used here).

Study conclusions

This review includes a variety of types of study (field experiments, farm surveys, and basket studies, which compare organic and conventional products collected from shops) comparing organically-grown to conventionally-grown crops with respect to a number of different potentially beneficial or harmful constituents/contaminants. The main conclusion of the review is that there are higher levels of some antioxidants and a lower likelihood of pesticide residues in organic crops. This is supported by the evidence provided, although it is impossible to ascertain the likely impact of publication bias.

The authors have carried out weighted meta-analyses based on standardised mean differences, both of which are appropriate methods. However, they have also carried out unweighted meta-analyses, which are generally not considered appropriate because they give equal weight to all studies, regardless of their size (a study with a sample size of 10 would be considered on a par with a study with a sample size of 100, for example) or type (field trials are likely to be very different from basket surveys in terms of precision). In addition, they present unweighted mean percentage differences which would, for the same reasons, be considered inappropriate and potentially misleading. Unfortunately, these unweighted percentage differences are highlighted throughout. The study found evidence that the concentration of some antioxidants may be higher in organic foods. However, they also found no evidence for differences in concentrations of quite a number of other antioxidants. This is not highlighted in the conclusions.

The study supports the finding of a previous meta-analysis suggesting that organic crops are less likely to contain pesticide residues than conventional crops, but no claims are made about adverse effects on health.

Strengths/Limitations

Strengths This is a large study which considers all available published evidence and appropriate statistical methods have been used to combine the results from the different studies (although see previous comments about the unweighted analyses).

Limitations

Publication bias: The authors highlight the problem of publication bias but do not suitably discuss what impact this may have had on the results. Generally publication bias is likely to result in an overestimate of the effect size - or, in some cases, finding an effect that is not real. See glossary for more detail about this.

Heterogeneity (different studies found different things): There was a large amount of heterogeneity between the studies for most of the outcomes considered; in other words, differences between organic and conventional crops were inconsistent between studies. The authors discuss various factors which are likely to have contributed to this heterogeneity and highlight the importance of investigating these factors in future studies. Because of this heterogeneity, the average difference between organic and conventional crops for these outcomes may be fairly meaningless, since the actual true difference (as indicated in the paper) varies from country to country, from crop type to crop type and from species to species.

Multiple comparisons: The authors have carried out a large number of comparisons, only some of which are presented in the main part of the paper. This leads to an increased likelihood of obtaining "false-positives"- in other words, the more differences you look for, the more likely you are to find one just by chance. Some scientists advocate adjusting p-values to take account of this which the authors have not done in this case. At the very least, results that are borderline should be regarded with greater caution; the authors do not discuss this.

Additional comments

Some statements are particularly misleading:

The study does not prove that organic crops and crop-based foods are higher in a number of antioxidants. Statistical tests never allow you to prove something, only to provide degrees of evidence for or against it. In this case there is a strong possibility that the evidence is tainted by publication bias and, as such, should be treated with additional caution.

"Numerous studies have linked antioxidants to a reduced risk of chronic diseases, including cardiovascular and neurodegenerative diseases and certain cancers." This is incorrect. There is certainly evidence that a diet rich in fruit, vegetables and whole grains (which are a good source of antioxidants) is associated with lower rates of chronic diseases. However, although a few trials of antioxidant supplements have shown a benefit for specific conditions, most have produced negative results and some have even indicated that taking antioxidant supplements may be harmful. In addition, a recent meta-analysis provided evidence that taking antioxidant supplements may increase overall mortality.

Glossary

Meta-analysis: an analysis in which results from a number of different studies are combined to obtain an overall effect size; this combined result is (generally) a weighted average of the estimated effect in the different studies, such that larger, more precise studies are given greater weight than smaller, less precise studies.

Publication bias: this is bias resulting from the tendency for positive results (i.e. those finding a "statistically significant" result) to be more likely to be published than negative ones. Thus, all the unpublished negative findings are excluded from any meta-analysis. Publication bias cannot be addressed by reviewing larger numbers of papers, since the unpublished results will always be missing, but it is made worse if all possible studies are included in a meta-analysis rather than selectively including studies on the basis of their quality.

ends

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