The researchers, from Centro Technológico Agroalimentario de Lugo (CETAL) and Universidade de Santiago de Compostela, examined trace and toxic elements in samples of milk from 22 organic and 10 conventional dairy farms in Northwest Spain.
Tested samples of organic milk were found to contain lower concentrations of several essential trace elements, including copper, zinc, iodine and selenium.
“The results of our study indicate that the essential trace element content of organic milk in NW Spain was significantly lower compared to conventional milk,” said a report documenting the findings, Essential trace and toxic element concentrations in organic and conventional milk in NW Spain.
“This was especially evident for elements that are routinely supplemented at high concentrations in the conventional concentrate feed,” it added.
Unbalanced soil mineral content
The researchers attributed these lower trace element concentrations to the absence of certain mineral substances in the diets of organically-reared cows.
In a conventional production environment, milk cows are provided with mineral diet supplements, while organically-reared animals depend on the mineral content of soil they graze on.
According to the researchers, this may not be sufficient.
"Since the main source of minerals in organic dairy cattle is the own-farm feed, mineral deficiencies in milk can occur in areas with low and/or unbalanced soil mineral content, and these should be evaluated," said the report.
Iodine's "special mention"
While copper, zinc, and selenium are essential nutrients for infants - aiding normal growth and development - the researchers gave "special mention" to the lower concentration of iodine in the organic milk tested.
“Iodine is possibly the essential trace element that better exemplifies this situation,” they said in their report.
“Iodine is needed for the production of thyroid hormones which are vitally important for a correct metabolism, especially during pregnancy and infancy.”
To overcome this issue, the researchers suggested that the diets of organically-reared cows be supplemented with a natural source of iodine, such as seaweed.
Source: Food and Chemical Toxicology
Title: Essential trace and toxic element concentrations in organic and conventional milk in NW Spain
Authors: F Rey-Crespo, M Miranda, M Lopez-Alonso