Influencing iodine levels in dairy

The wealth of literature reminds us that, in addition to iodised salt and seafood, milk and dairy products are the main sources of iodine in the human diet. The iodine level in milk is affected by numerous factors along the entire dairy food chain, from a feed-to-fork perspective. Of the factors, the main driver is the level of iodine in the feed administered to lactating animals, which has been demonstrated to be linearly associated with the final milk iodine concentration.

In humans, iodine maintains the functionality of the thyroid gland through the regulation of thyroid hormones; thus, its intake and availability are important for human health. Proper supplementation strategies in dairy species can guarantee animal and consumer health benefits. As a close relationship exists between the knowledge of iodine’s role and its proper dietary intake, efforts should be made to promote greater awareness in populations. Studies such as the current one aim to raise awareness.

Iodine concentration in animal feed

The type and amount of iodine supplementation in the daily ration is the major source of variation of milk iodine. Several studies have also demonstrated that milk iodine concentration is closely associated with iodine concentration in the feed; the correlation is linear and characterised by different dose-response effects according to the feed and the presence of supplementation. On the other hand, feed ingredients show variations in iodine levels. Among some of the analysed feed ingredients in the present study, hay, corn silage, and grass silage showed the lowest iodine concentration per kilogramme of dry matter.

The researchers emphasised that even if the diet is apparently balanced in terms of minerals and the intake of iodine seems adequate, the real amount of iodine absorbed may not be. “In fact, the amount of iodine absorbed is ‘overestimated’ in the presence of goitrogens,” they said. Goitrogens substances and their metabolites are organic compounds that interfere with the synthesis and secretion of thyroid hormones either by hampering iodine transport across the thyroid cell membrane or by inhibiting thyroperoxidase; they can inhibit the sodium iodide symporter, diminishing iodine uptake by the thyroid and mammary gland.

Goitrogenic substances such as glucosinolates (GLS), thiocyanates, and nitrates, have been identified and characterised in a variety of plant species, including ones in the cruciferous family (rape, canola, and kale), soybean, beet pulp, millet, linseed, white clover, and sweet potato. Studies show that within the same plant species, goitrogen concentration varies depending on the specific variety, and the technological processes that the crop is subjected to, for example, extrusion, may alter the antagonistic effect of goitrogens. The researchers suggested characterising the goitrogen content and goitrogenic potential of crops and strains included as feed ingredients and assessing the relationship between these dietary antagonists and final milk iodine concentration.

Influence of milking practices

It was clearly stated that the use of iodine-based products for teat cleaning is responsible for a notable artificial increase in the concentration of iodine in dairy cows. The application of iodised sanitisers either during the pre-milking phase or during post-dipping has carryover effects that result in increases in milk iodine through skin absorption. How the composition of sanitisers (e.g., iodised vs. non-iodised, the concentration of iodine) and their application (e.g., pre-dipping vs. post-dipping, immersion vs. spraying) influence the iodine concentration of milk is worth revisiting.

Genetic variation of milk iodine

Being mainly management dependent, the milk iodine concentration is lowly heritable. In a recent study published in the Journal of Dairy Science, researchers from the University of Padova attempted to estimate genetic parameters of predicted iodine concentration using milk data of Holstein cows; their results indicated that iodine concentration is lowly heritable (0.025 ± 0.005) and characterised by an extremely low coefficient of genetic variation (1.72%). The estimates suggested that directly improving iodine concentration in milk through selective breeding is far from being feasible and convenient. In the current study, researchers advised: “Further genetic investigations about milk iodine are advisable and should rely on more accurate phenotypes.”

Iodine concentration in dairy products

In general, from the literature review, the researchers concluded that milking practices and industrial processing can affect the iodine concentration in bovine milk. However, in the present study, the effect of milk technological treatments had a minor impact on milk iodine concentration.

It was remarked: “A simple comparison of differently treated commercial samples is not optimal for studying the influence of processing, because several other factors are likely to interfere with the iodine level. Therefore, it is advisable to evaluate the effect of processing on iodine concentration along the processing line. This would permit researchers to follow the same batch of milk along the different processing stages.”

Their results showed variations in iodine concentration in dairy products, including retail milk, different types of cheese, yoghurt, protein concentrates, and whey. “Although to date, iodine content has been well profiled in retail milk samples, addressing the effect of heat treatment (i.e., pasteurisation and UHT), fat content (i.e., whole, semi-skimmed, and skim milk), month and season of sampling, and farming system (i.e., conventional, organic); iodine concentration in retail milk is not subjected to standardisation and is thus extremely variable.”

Consumer awareness

In conclusion, the researchers emphasised that consumers need to be informed of the importance of dietary iodine and deficit implications, as the human recommended daily allowance could be reached through the dietary intake of milk and dairy products. They highlighted their study provides an important overview of factors that contribute to iodine concentration in milk and dairy products and is primarily intended to make the scientific community aware of the importance of this mineral in animal and human nutrition.

In addition, they stated: “Proper advertisement, attractive and informative labelling, and awareness campaigns are the best tools to spread knowledge on deficiency-related disorders. These are also useful ways to stimulate – or keep constant – the purchase frequency of dairy products.”

This article is based on the original article by G. Niero, G. Visentin, S. Censi, F. Righi, C. L. Manuelian, A. Formigoni, C. Mian, J. Bérard, M. Cassandro, M. Penasa, S. Moore, A. Costa, and M. De Marchi. 2023. Invited review: Iodine level in dairy products—A feed-to-fork overview. J. Dairy Sci. 106:2213–2229.