Are organically grown foods safer and more healthful than conventionally grown foods?

A major literature review and meta-analysis of studies comparing organically grown crops with conventionally grown crops has appeared recently in this journal, and it has concluded that organically grown crops do tend to have a notably lower Cd content than conventionally grown crops – on average, about 48 % lower. This estimate takes into account eighty-seven different comparisons previously reported in the literature, and hence there is little room for doubt that organically grown crops do tend to have lower Cd content. The heavy metal Cd is emerging as a major cause of vascular disorders, various common cancers, kidney disease, osteoporosis and other health disorders, even in populations that do not have occupational exposure to this toxin. Although tobacco smoke is a major source of Cd exposure, Cd is also found in the diet; green vegetables, root vegetables, tubers, grains, organ meats and shellfish often contain reasonable amounts of this toxin. Once Cd gets into the body, there is no physiological mechanism for excreting it; hence, the halflife of Cd in the body has been estimated to be 10–30 years. The concentration of Cd in the urine (expressed as mg/mg of creatinine) is thought to represent a reasonably accurate measure of the total body burden of Cd. Although Fe deficiency increases the efficiency of dietary Cd absorption, metabolic determinants of disease risk are not known to influence Cd absorption; moreover, the foods that are the predominant dietary sources of Cd are not inherently harmful to health. Hence, at least in non-smokers, the many studies that have correlated increased urinary Cd concentrations with an increased risk of health disorders very likely imply that Cd does indeed play a causative role in these disorders, consistent with its adverse effects observed in rodent studies. Cd can induce oxidative stress throughout the body, and interference with some of the physiological roles of Zn (as in DNA repair) may also contribute to the pathogenic impact of Cd. As an example of the magnitude of the risk involved, the four case–control studies, to date, to assess the correlation of breast cancer risk with urinary Cd concentrations have concluded that Cd exposure may be responsible for 27–68 % of the breast cancer cases – higher levels in Japan, where agricultural soils and rice tend to be rich in Cd. With regard to vascular diseases, recent multivariate-adjusted analyses of the National Health and Nutrition Survey cohort have concluded that Cd exposure may be responsible for 28 % of the myocardial infarction cases and 17 % of the total CVD and cerebrovascular disease cases. Analogously, data from the prospective Strong Heart Study (focusing on Native Americans) suggest that Cd exposure may account for 16, 23 and 28 % of the coronary disease, stroke and heart failure cases, respectively. Case–control studies attempting to correlate dietary Cd intake with disease risks have usually yielded null results, which probably reflects the fact that the Cd content of a given food can vary widely depending on the soil of origin; hence, estimating dietary Cd intake with FFQ is of dubious merit and has served to create the impression that the epidemiology correlating Cd status with disease risk is equivocal. In fact, if one focuses on urinary Cd concentrations when surveying Cd epidemiology, the hazard of Cd stands out crystal-clear. High supplemental intakes of Zn, and the antioxidant activity of spirulina, may have potential for attenuating the adverse health impacts of Cd that is already in the body. Traditional chelation therapies fail to remove Cd, as most of it is located intracellularly, where chelating drugs cannot reach it. Supplementingmeals withZn andMgmay lessen the absorption of dietary Cd to some degree, and avoidance or correction of Fe deficiency is quite important in this regard; the fact that women tend to have higher Cd concentrations than men probably indicates that they are more prone to Fe deficiency before menopause. Cd exposure can also be lessened by avoiding tobacco smoke – or by not smoking. However, avoiding dietary Cd poses a special challenge, as a high proportion of dietary Cd derives from plant foods ordinarily considered healthful – vegetables, tubers and grains. Indeed, a Slovak study has found that blood Cd concentrations tend to be higher in vegans than in omnivores. Cd occurs naturally in most soils, and plants grown in these soils will incorporate this Cd to a greater or lesser extent; less Cd is taken up from alkaline soils than from acidic soils. The new review by Baranski et al. provides the first compelling evidence that organic farming techniques have a major favourable impact on crop Cd content; on average, the Cd content of organically grown foods is approximately half as high as that of the same foods grown conventionally. This probably reflects the fact that many phosphate fertilisers used in conventional agriculture are significantly contaminated with Cd. Studies have concluded that the more frequently such fertilisers are applied to soil, the higher the Cd content of the foods grown in that soil is. It should be borne in mind, however, that organically grown crops do not inherently have a low Cd content; foods grown in soil British Journal of Nutrition (2014), 112, 1589–1591 q The Authors 2014