Iodine in Milk

• The trace element iodine is important in both human health and as a germicidal component of many pre-dip products. However, too much iodine in the human diet can lead to health problems. This article discusses how pre-dipping practices impact milk iodine content.

Iodine levels in milk resulting from pre- and post- dipping with iodine containing products are a concern for public health workers. Milk consumers and regulatory agencies demand milk that is free of harmful adulterants. 

Because of its known efficacy, iodine is the most widely used germicide in teat dips and sprays. Extensive research on iodine levels in milk has been conducted for at least the past twenty years. The following article reviews the scientific literature. Iodine is present in teat dips as both the germicidal iodine (I2) and as the non-germicidal iodide (I-). Product labels would report only the germicidal iodine and would typically contain 0.3 to 0.5% iodide for every 1% iodine. Iodine found in milk is the non-germicidal iodide.

Health Concerns and Benefits

Iodine has been identified as an essential dietary element and plays an important role in the prevention of goiter. Persistent iodine deficiency can result in hypothyroidism. Since the early 1900’s, iodine has been added to food, typically in the form of iodized salt.  Iodine supplementation of food continues to be recommended by the World Health Organization (WHO) (http://www.who.int/inf-fs/en/fact121.html).  Recommended daily intake levels have been established by various health organizations as shown in Table 1. Additionally, safe levels have been established by several organizations including the American Medical Association (AMA) and the WHO. Bruhn (3) has shown that lactating women secrete milk with 60 to 281mu g/l iodine ( average = 142 mu g/l).

Table 1 Recommendeddietary iodine intake

Health risks resulting from high iodine intake are not well-correlated with specific iodine intake levels.  High iodine levels, however, have been associated with Graves’ disease in children (4). Extremely high levels of dietary iodine (10,000 to 200,000 micrograms per day) have been found to cause goiter by reducing hormone release from the thyroid gland (5).  A detailed review of dietary iodine sources and health impacts is available on the WHO web site (http://www.who.int/water_sanitation_health/GDWQ/Chemicals/Iodinesun.htm).

Dietary iodine is derived from a variety of sources that will vary globally depending on diet and the iodine level in each source.  In one US study reported by the FDA in 1978 (6), the major sources of dietary iodine were identified as dairy products; meat, fish and poultry; and grains and cereal products (Table 2).

Table 2 Dietary Sources of Iodine

Food Commodity	Percentage of Total Iodine Intake
Dairy Products	56.1
Meat, fish and poultry	10.8
Grain and Cereal products	16.1
Potatoes	0.4
Leafy vegetables	0.5
Legume vegetables	1.0
Root vegetables	0.0
Miscellaneous vegetables	0.2
Fruits	0.4
Oils, fats and shortening	0.7
Sugars and adjuncts	10.7
Beverages (including drinking water)	4.2

A study in Poland (7) from 1988 indicates that milk provides 20% of the dietary iodine intake.  This potentially reflects both differences in diet and differences in iodine levels in dietary components.

Milk Iodine Levels

A large number of studies have been conducted to determine the iodine level in milk and milk products. A survey of those studies is shown in Table 3. Milk iodine levels of 500 ppb (parts per billion or mu g/l) have been cited as acceptable or legal maximum levels. In all of these studies the mean milk iodine level is below 500 ppb.

A small percentage of the milk samples in two studies (8, 9), had milk iodine levels greater than 500 ppb. This generally occurred in individual producer samples and not in tank truck or dairy silo samples (9). 

Several of the studies report seasonal and regional differences in milk levels (2, 10, 11, 12). Processed milk products often have higher levels of iodine than liquid milk (2, 7, 8, 13) as a result of concentration of raw milk. Studies from France (12) and the UK (14) identify milk and dairy products as important sources of dietary iodine to prevent goiter. Two US studies (2, 13) show that milk iodine levels decreased significantly from the late 1970s to the early 1980s and continued to drop through 1990.

Table 3 Milk iodine levels

Reference	Year	Location	Mean mu g/l	Range mu g/l	No.of Samples	Comments
Kammerlchmer (1)	1955	Germany		9-400		Review of literature - milk and dairy producers
Pennington (2)	1990	US	230	160-340		Seasonal and geographical variation in retail milk. Data reported for 1977 - 1990.
Bruhn (3)	1983	US	142	21-281	16	Human milk
Bulinski (7)	1988	Poland	24-521 59-515	10 10		Milk and liquid milk products condensed milk, butter, cheese
Cocchieri (8)	1989	Italy		28-920        82-643 8-426 8-4226	247           108 52 9	Dairy Silos Producers HTST Pasteurized milk UHT 5% of samples > 500
Demott (10)	1991	US	350-419	222-762	57	Raw milk - dairy schools, dairy farms, took from trends
Preiss (11)	1997	Germany	115			28 Bavarian Dairies sampled over 1 year
Lamard (12)	1992	France			838	Dairies: seasonal and regional variation. Milk iodine levels as low as 25 mu g/l
Fischer (29)	1993	Canada	117-456	87-1304		Whole and partially skimmed retail milk
Ohno (30)	1989	Japan		41-316		Retail milk
Bruhn (31)	1985	US	499 (438*) 328 (316*)	30-3484 22-4048	378      1021	Outside California In California * Volume weighted average
Bruhn (32)	1978	US	336     384		116       63	Raw Milk Retail Milk

Teat Dipping and Milk Iodine Levels

A variety of studies have been conducted to determine the impact of iodine teat dipping on milk iodine levels (Table 4). Several studies (15, 16), concluded that teat dipping was not correlated with milk iodine levels. In these studies, it is likely that changes in iodine levels in feed have masked any significant impact of iodine from the teat dips. The effect of iodine post-milking teat dips range from no effect, to a milk iodine increase of 130 mu g/l. 

Table 5 shows a trend of increasing milk iodine levels with iodine content in post-milking teat dips. However, in cases where multiple studies were conducted with the same teat dip iodine level, identical results were not obtained. Differences in the results could be affected by: formulation differences, differences in analytical method, differences in housing conditions, pre-milking udder prep procedures, application by dipping versus spraying, milk production levels and changes in baseline iodine levels. The impact of milk production on the iodine contribution of teat dips is pointed out by Sheldrake (17). Based on his results a 0.5% iodine dip would contribute 42 mu g/l for a 15 l/day (33 lbs/day) cow and 125 mu g/l for a 5 l/day cow (11 lbs/day). Pre-milking udder preparation could also impact the reported milk iodine levels, although research results are inconsistent (17, 18). The maximum reported milk iodine increase from post-milking teat dipping reported is 100 - 146 mu g/l for teat dips with 0.75% to 1% iodine.van

 

Reference	Year	Location	Mean mu g/l	Range mu g/l	No. of Samples	Comments
Kammerlchmer (1)	1955	Germany		9-400		Review of literature - milk and dairy producers
Pennington (2)	1990	US	230	160-340		Seasonal and geographical variation in retail milk.  Data reported for 1977 - 1990.
Bruhn (3)	1983	US	142	21-281	16	Human milk
Bulinski (7)	1988	Poland	24-52 159-515		10 10	Milk and liquid milk products condensed milk, butter, cheese
Cocchieri (8)	1989	Italy		28-920	247	Dairy Silos
				82-643	108	Producers
				8-426	52	HTST Pasteurized milk
				8-4226	9	UHT
						5% of samples > 500
Demott (10)	1991	US	350-419	222-762	57	Raw milk - dairy schools, dairy farms, took from trends
Preiss (11)	1997	Germany	115			28 Bavarian Dairies sampled over 1 year
Lamard (12)	1992	France			838	Dairies: seasonal and regional variation.  Milk iodine levels as low as 25 mu g/l
Fischer (29)	1993	Canada	117-456	87-1304		Whole and partially skimmed retail milk
Ohno (30)	1989	Japan		41-316		Retail milk
Bruhn (31)	1985	US	499 (438*)	30-3484	378	Outside California
			328 (316*)	22-4048	1021	In California
						* Volume weighted average
Bruhn (32)	1978	US	336		116	Raw Milk
			384		63	Retail Milk

Teat Dipping and Milk Iodine Levels

A variety of studies have been conducted to determine the impact of iodine teat dipping on milk iodine levels (Table 4). Several studies (15, 16), concluded that teat dipping was not correlated with milk iodine levels. In these studies, it is likely that changes in iodine levels in feed have masked any significant impact of iodine from the teat dips. The effect of iodine post-milking teat dips range from no effect, to a milk iodine increase of 130 mu g/l. 

Table 5 shows a trend of increasing milk iodine levels with iodine content in post-milking teat dips. However, in cases where multiple studies were conducted with the same teat dip iodine level, identical results were not obtained. Differences in the results could be affected by: formulation differences, differences in analytical method, differences in housing conditions, pre-milking udder prep procedures, application by dipping versus spraying, milk production levels and changes in baseline iodine levels. The impact of milk production on the iodine contribution of teat dips is pointed out by Sheldrake (17). Based on his results a 0.5% iodine dip would contribute 42 mu g/l for a 15 l/day (33 lbs/day) cow and 125 mu g/l for a 5 l/day cow (11 lbs/day). Pre-milking udder preparation could also impact the reported milk iodine levels, although research results are inconsistent (17, 18). The maximum reported milk iodine increase from post-milking teat dipping reported is 100 - 146 mu g/l for teat dips with 0.75% to 1% iodine.

Table 4 Teat dipping and milk iodine levels       

Reference	Year	Location	Mean Iodine Level mu g/l	Treatment	Comments
van Ryssen (9)	1985	South Africa	257	Tanker 1.75% of samples > 1000	2000 ppm teat dip increased iodine in milk 11-60 mu g/l
			294	Farm Tank 5.5% of samples > 1000
Clocte (15)	1989	South Africa			No milk iodine increases from 1500 ppm iodine post dip
Schultz (16)	1980	USA	466	multi-farm survey	Range <250 to >1500:  No correlation between milk iodine levels and teat dipping.
			85-203	1% iodine post dip	University herd using 1% iodine post dip, iodine udder wash, and iodine rinse for teat cups.
Sheldrake (17)	1980	Australia	84	0.5% iodine post dip, with wiping	Significant impact of proper teat preparation is shown.
			41	0.1% iodine post dip, with wiping
Conrad (18)	1978	US	80-100	1% iodine post dip	For a 1% iodine post dip.  Data supports iodine absorption through skin.  Premilking udder prep (water wash versus no wash) had no effect on iodine levels.
Galton (19)	1989	US	12	no treatment	Change in iodine level from baseline for a 0.25% iodine dip
			29	pre-dip with drying
			250	pre-dip, no drying
			86	post dip
			111	pre-dip with drying, and post dip
Galton (20)	1986	US	35	post dip, 0.1% iodine	Change in milk iodine level from baseline
			47	pre-dip, dry, post dip 0.1% iodine
			76	post dip, 1.0% iodine
			110	pre-dip, dry, post dip 1% iodine
Blowey (21)	1992	UK	35		Average milk iodine increase for 3 herds using a 0.1% iodine pre-dip and post dip versus post dip only.
Hemling (22)	1997	US	220-321	0.05% iodine, pre-dip no wiping	Dry towel wiping after pre-dipping has no effect on iodine milk levels for low iodine dip.
			248-314	0.05% iodine, pre-dip with wiping
Sheybal (23)	1980	Germany	41-58	Foremilk	From iodine 131 labeled teat dip
			16-28	Main Milk
Aumont (24)	1987	France	33-54	0.5% iodine post dip	1-8 times higher in foremilk than bulk milk
Bruhn (27)	1987	US	173	All Farms	Results of samples from 1572 farms with differing practices regarding iodine teat dipping and back flush usage.
			148	no iodine teat dip or back flush
			167	iodine teat dip
			202	back flush only
			251	back flush  and teat dip
Cantor (28)	1976	US	79	1% Iodine, 3 ppm Free Iodine	The effects of iodine feed supplements overrode any contribution from iodophor teat dip.  Use of iodophor udder wash caused no effect on milk iodine levels.
			142	1% Iodine, >50 ppm Free Iodine
			57	0.25% Iodine, 0.3 ppm Free Iodine
			47	4% Sodium Hypochlorite (Baseline)
Terplan (33)	1975	Germany	130	0.75% teat dip	14 day test.      3 cows per solution.
			85	0.60% teat dip	14 day test.      3 cows per solution.
			77	0.50% teat dip	14 day test.      3 cows per solution.
			47-58	0.60% teat dip	Iodine level in extended study on 2 farms.
Reybroech (34)		Belgium			Recommends < 3000 ppm. Iodine as safe level for teat dip.  No data.

Table 5 Effect of iodine level in post-milking teat dips on milk iodine levels

Iodine Level	Milk Iodine Increase (mu g/l)	Reference
0.1%	35	Galton (20)
0.1%	41	Sheldrake (17)
0.15%	0	Cloecte (15)
0.2%	11-60	van Ryssen (9)
0.25%	86	Galton (19)
0.25%	57	Cantor (28)
0.5%	33-54	Aumont (24)
0.5%	77	Terplan (33)
0.5%	84	Sheldrake (37)
0.6%	85	Terplan (33)
0.75%	130	Terplan (33)
1%	76	Galton (20)
1%	80-100	Conrad (18)
1%	79	Cantor (28)
1%*	142	Cantor (28)

*Contained >50 ppm of free iodine.

The impact of pre-milking teat dips on milk iodine levels has also been investigated.  Galton(19) showed that at a 0.25% iodine pre-dip increased iodine levels 29 mu g/l. If the teat dip was not wiped from the teat prior to milking, the increase was 250 mu g/l. In a separate study (20),a 0.1% iodine pre-dip, followed by a dry towel wipe, increased milk iodine 12 mu g/l. Blowey (21) has shown that a 0.1% iodine pre-dip increased milk iodine levels an average of 35 mu g/l in three commercial herds.  Using a 1% iodine product with dry wiping increased milk iodine by 34 mu g/l. Hemling (22) observed no difference in iodine milk levels between drying and no drying for a 0.05% iodine pre-dip. In an ongoing pre-dip trial in Australia using a 0.1% iodine product, milk iodine levels are reported to be less than 300 mu g/l.

The mode of entry of iodine teat dips into milk has not been well studied.  Possible routes include: migration up the streak canal into the udder; washing from the teat surface during milking; or penetration through the skin into blood or directly into milk.  Research can be found to support each of these routes; no clear evidence exists to favor one route over the others (18, 23, 24).  Differences in product viscosity, product water solubility, and/or housing conditions for the test animals may contribute to differences between studies.

Feed Additives and Milk Iodine Levels

The impact of iodine feed additives on milk iodine levels is given in Table 6. As in humans, adequate iodine is a nutritional requirement for healthy cows.  Excessive iodine is secreted in milk, urine, tears and feces. The necessary dietary level for cows has been established as no more than 50 micrograms per kilogram of feed (14).  

Organic iodine EDDI (ethylenediamine dihydroiodide) at levels of 50 mg or more per head per day has been used as a preventative for foot rot. However, current US Rood and Drug Administration Regulations set the maximum limit of EDDI at 10 mg per cow daily for the US market. 

The direct relationship between feed additive levels and iodine milk levels has been documented in several studies. Swanson (25) and Franke (26)haveshown similar increases in milk iodine levels caused by addition of 4 ppm of EDDI to the feed (equivalent to 91 mg of EDDI in 50 lbs of feed dry matter). Swanson showed the milk iodine increase was similar for both potassium iodide and EDDI. Further, milk iodine levels responded rapidly to changes in dietary iodine. In the Swanson study, teat dipping with a 1% iodine teat dip did not increase milk iodine levels above the levels obtained for supplemental iodine at the 1 mg or 4 mg per kg of feed levels.

In a survey of raw milk from California dairy farms (13) before and after a voluntary reduction of dietary iodine supplements, a significant reduction in the milk iodine level was observed.

Table 6 Feed additives and milk iodine levels

Reference	Year	Location	Mean mu g/l	No. of Samples	Comments
Bruhn (13)	1983	US	474	473	Raw milk, before and after reduction in dietary iodine supplement.
			256	1389
Phillips (14)	1997	UK			Iodine added to dairy feed to counteract endemic goiter
Swanson (25)	1990	US	205     basal diet 0.8 mg I/kg 404    (467) basal +1 mg I/kg 477    (575) basal +2 mg I/kg 755    (869) basal +4 mg I/kg		Milk iodine increases with iodine content in feed.  Iodine supplied from potassium iodide or (EDDI).
Franke (26)	1983	US	166	213	Difference caused by EDDI added to feed at 4 ppm.
			745	481

Other Sources of Iodine in Milk

Use of an iodine containing back flush, pipeline detergent-sanitizers or an iodine udder wash could have an impact on milk iodine levels. Bruhn (27) has shown that an iodine back flush can increase milk iodine levels by about 20 to 50 mu g/l.  Misuse or improperly functioning equipment can have the biggest impact because of the substantial volume of water that could be left in the milking unit. Udder washing with an iodine solution has been shown in one study to have no effect (28)on milk iodine levels. This is probably because low concentrations are used (25 ppm) and low volumes of liquid are left on the teat.

Summary

Iodine is an essential dietary ingredient for human and animal health. The literature supports the conclusion that iodine deficiency remains a greater health risk than does iodine toxicity. Iodine deficiency is still a problem in many parts of the world, including developed countries such as Germany (38, 39) and the Netherlands (40). Measured urinary iodine concentrations have also dropped more than 50% in the U.S. between 1971-74 and 1988-94 (41).  Recent studies have associated iodine deficiency with cardiovascular diseases (42) and fetal brain development (43).

Safe upper levels of iodine intake have been set at 1000 to 2000 mu g/day.  Upper limits of milk iodine levels have been set at 500 mu g/l. An extensive array of studies from numerous countries indicates that this value is not exceeded in fluid milk at the milk plant level. 

Feed additives have the biggest potential impact on milk iodine levels, as compared with teat dips. Use levels of such additives have been regulated in many countries. 

Post-milking teat dipping can contribute to milk iodine levels up to a level of 100-130 mu g/l. Proper udder cleaning or wiping prior to milking has been shown to minimize the potential iodine contribution from post-milking teat dipping.  Pre-dipping followed by wiping prior to milking does not increase milk iodine levels more than about 35 mu g/l, even for a 1% iodine teat dip. 

Continued education on proper use of iodine pre- and post-milking teat dips, and iodine feed additives should help minimize the risk of elevated milk iodine levels. New developments in the dairy industry such as robotic milking, with automated teat dip application, or the use of iodine barrier dips may warrant further study.

References:

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36. World Health Organization

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Related Links:

Iodine Deficiency Disorders
World Health Organization
Detailed discussion of the effects of iodine deficiency in human nutrition and development.