Iodine concentration in plant-based milk products available on the Norwegian market

Lisbeth Dahl

Institute of Marine Research (IMR), 5817 Bergen, Norway.
Corresponding author: e-mail: lisbeth.dahl@hi.no

Tonje Eiane Aarsland

Institute of Marine Research (IMR), 5817 Bergen, Norway.
Department of Clinical Science, Faculty of Medicine, University of Bergen, 5020 Bergen, Norway.

Synnøve Næss

Institute of Marine Research (IMR), 5817 Bergen, Norway.

Inger Aakre

Institute of Marine Research (IMR), 5817 Bergen, Norway.

Maria Wik Markhus

Institute of Marine Research (IMR), 5817 Bergen, Norway.

Abstract

The intake of cow milk has steadily been decreasing over the past decade, while plant-based milk alternatives have had an increasing interest over the last few years. This may be of concern since cow milk is an important source of iodine, especially among women of childbearing age. The aim of this study was to measure the iodine concentration in plant-based milk alternatives available on the Norwegian market.

The iodine concentration was determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) in 40 plant-based milk alternatives and ranged from below limit of quantification (< LOQ) to 24 µg/100 ml (median; 1 µg/100 ml). Five oat-based products (12.5%) were enriched with iodine and had an analysed iodine concentration ranging from 22 to 24 µg/100 ml. Although most of the plant-based milk alternatives contained negligible concentrations of iodine, the enriched oat-based milk alternatives contained higher concentrations of iodine than cow milk.

Individuals substituting cow milk with plant-based milk alternatives should be encouraged to choose alternatives enriched with iodine.

Introduction

Iodine is essential for the thyroid hormones which are important for growth and development, especially during gestation and early life [1]. Although iodine nutrition is improving globally, iodine deficiency is still prevalent in Europe, especially among women of childbearing age [2-4]. Dietary choices are important for the iodine intakes as there are only a few good natural sources of iodine in the diet. Plant-based eating patterns have had an increasing interest over the last decade [5]. This is of particular concern regarding iodine intake, as exclusion of specific food groups such as seafood and dairy products will result in inadequate iodine intake unless these individuals use iodine containing dietary supplements. One of the largest studies in Europe assessing iodine status in vegans and vegetarians showed that vegetarians and vegans in Norway are unable to reach the recommended iodine intake from food, and they are dependent on iodine supplements [6]. A recently published study from the UK showed that exclusive consumers of plant-based milk alternatives had significantly lower iodine intakes compared to cow milk consumers [7]. Plant-based milk alternatives are mostly based on soy, rice, oat, almond, or coconut, not containing iodine unless fortified. Therefore, a shift towards consumption of plant-based milk alternatives may impact iodine nutrition and subsequently lead to a risk of inadequate iodine intake. The aim of this study was to measure the iodine concentration in plant-based milk alternatives available on the Norwegian market.

Methods

A convenient sample of plant-based milk alternatives (n = 40) were purchased from several grocery stores in Bergen, Norway, in March 2020. Thirty-seven products were plant-based milk alternatives, two products were specified for use as ’coffee-milk’, and one product was specified for cooking. Samples were prepared by mixing each product with ultrapure water (>17 MΩ cm-1) and ultrapure tetra methyl ammonium hydroxide (TMAH), before extraction at 90°C ± 3°C for 3 hours, as described in detail in Nerhus et al 2018 [8]. Iodine was determined in one replicate per sample by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The accuracy of all results was verified using the certified reference material Skim milk powder (ERM-BD 150) from National Institute of Standards and Technology (NIST). The limit of quantification (LOQ) was 0.8 µg/100 ml. Products with iodine concentration below LOQ (n = 15) were set to the LOQ-value in the calculations of median iodine content.

Results

Table 1 gives an overview of the analysed iodine concentrations (µg/100 ml) according to type of product included in the present study. Median iodine concentration was 1 µg/100 ml, ranging from <LOQ to 24.0 µg/100 ml. Twelve different brands were analysed and the products were produced in eight different European countries. Two brands were produced in Norway. All products were pasteurized, long-life products and twelve of the products were organic.

Five (12.5%) oat plant-based milk products from two different brands were enriched with iodine, and the analysed iodine concentration ranged from 22 to 24 µg/100 ml, whereas the product label value was 16 µg/100 ml in two products and 22.5 µg/100 ml in three products.

The measured iodine concentration in the products not enriched with iodine (n = 35) ranged from <LOQ to 11 µg/100 ml.

Four products, with an analysed iodine concentration between 5 and 9 µg/100 ml, declared algae (Lithothamnium calcareum) in the ingredients list.

Discussion

In this study, the median iodine concentration of plant-based milk alternatives from the Norwegian market was found to be very low. According to legislation, the iodine content should not be declared on a drinking product if the content is below a ‘considerable amount’; defined as a minimum of 7.5% of the recommended intake (RI) [9]. For adults, this corresponds to 11.3 µg/100 ml (RI = 150 µg/day). Only five of 40 products had a considerable amount of iodine (>22 µg/100 ml). All of these five products were oat-based milk alternatives enriched with iodine, and the iodine analysis agreed with labelled iodine concentration. Four of the plant-based milk alternatives had a low, although not negligible, amount of iodine in the range of 5 to 9 µg/100 ml. These products had declared algae (Lithothamnium calcareum) in the ingredients list, which we presume explains why these products contained iodine. The fact that iodine was not on the product label in these algae products, is therefore in accordance with legislation. The product specified for cooking contained 11 µg/100 ml and also here iodine was not listed on the product label.

In a study from Australia, plant-based milk alternatives samples with tree nuts/seeds (i.e. almond, hazelnut or hemp etc.) (n=44), with legumes (i.e. soybean or pea) (n=27) and mixed products types (i.e. almond, hazelnut, coconut, oat and or rice etc.), were reported with an iodine concentration of 0 µg/100 ml [10]. Plant-based milk alternatives of coconut (n=10) and grains (i.e. rice, oat and or quinoa) (n=19) had 0.15 and 5.2 µg iodine /100 ml, respectively [10]. In a study from UK they determined iodine concentration in 47 samples of seven types of plant-based milk alternatives [11]. Median iodine concentration in products with no added iodine (n=44) was 0.7 µg/100 ml. Three products of soya, oat and rice were fortified with iodine and the iodine concentration was 28, 28.7 and 26.6 µg/100 ml, respectively. Our findings suggests that few of the plant-based products may serve as dietary iodine sources. This is in accordance with results reported from similar products available in Australia and UK [10,11]. As most of the products available on the Norwegian market are produced abroad, our results are most likely relevant for other European countries as well.

Table 1

Product type

Original Product name (brand)

Iodine concentration (µg/100 ml)

Oat

Gryr Havredrikk fyldig naturell (Sunniva)c

Gryr Havredrikk naturell (Sunniva)c

Oatly! Havredrikk kalsium (Oatly)b

Oatly! Havredryck chokolad (Oatly)b

Oat Drink natural (Coop)

Oatly! Økologisk havredrikk (Oatly)

«Mylk» oat (Berit Nordstrand)

Oat drink chocolate, ecological (Tylle)

23

22

22

22

1

1

< LOQ

< LOQ

Soy

So soya calcium (Sojade)

Soya calcium (Provamel)

Soy drink ecological with vanilla and calcium (Tylle)

Alpro soya chocolate (Alpro)

Soya organic-bio no sugar (Provamel)

Alpro soyaunsweetened (Alpro)

Alpro soya original eko organic (Alpro)

Soya chocolate (Provamel)

So soya natural (Sojade)

Soy drink vegan (Änglamark)

Alpro soya orginal (Alpro)

9

6

5

2

2

2

1

1

1

1

< LOQ

Coconut

Coconut milk ecological no sugar (EcoMil)

Alpro Coconut orginal (Alpro)

Coconut drink ecological (Tylle)

Koko unsweetened (Koko)

1

< LOQ

< LOQ

< LOQ

Almond

Almond drink ecological, origianal calcium (EcoMil)

Almond drink ecological nature no sugar (EcoMil)

«Mandaorla» almond original (EcoMil)

Almond drink vegan (Änglamark)

Roasted Almon original (Alpro)

5

1

1

< LOQ

< LOQ

Rice

Rice drink ecological (Änglamark)

Alpro rice orginal (Alpro)

Rice Dream organic (Rice Dream)

Rice Dream almond-hazelnut (Rice Dream)

Rice drink ecological (Tylle)

1

< LOQ

< LOQ

< LOQ

< LOQ

Hemp

Hemp drink ecological without sugar (EcoMil)

1

Quinoa

Quinoa nature ecological without sugar (EcoMil)

< LOQ

Almond and oat

Rice Dream almond-oat (Rice Dream)

“Mylk” oat and almond (Berit Nordstrand)

< LOQ

< LOQ

“Coffee milk”

Oatly ikaffe havredryck att skumma (Oatly)b

Alpro i kaffe soya (Alpro)

24

1

Cooking

Gryr coconut and rapeseed (Sunniva)

11

a Values (rounded to the nearest whole number) based on one replicate per sample, analysed with inductively coupled plasma mass spectrometry with a LOQ of 0.8 μg/100 ml ml. b Declared iodine content on product: 22.5 µg/100 ml. c Declared iodine content on product: 16 µg/100 ml. Abbreviation: LOQ, limit of quantification.

In view of the increasing awareness of the environmental impact of food, the consumption of plant-based alternatives may increase further. A recent media report claim that the sales of plant-based milk alternatives have increased by about 40 % over the two last years in Norway. In addition, the consumption of cow milk has decreased gradually for the last decades [12]. It is interesting that both analysed and declared iodine concentration in the oat-based milk alternatives are higher (22-24 µg/100 ml) than Norwegian produced cows’ milk (~15 µg/100 ml) [8]. It is worth noticing that the Norwegian produced cow milk is indirectly enriched with iodine, through the cow fodder [13]. Although most of the products analysed in the present study were low in iodine, the results show that a plant-based milk alternative could contribute with a higher amount of iodine in the diet, given that the consumer choose products enriched with iodine. A daily intake of one glass (i.e. 2.5 dl) of the iodine-enriched plant-based milk alternative in our study would contribute with 55-58 µg iodine, which corresponds to almost 40 % of the recommended daily intake in children (>10 yr), adolescents and adults. A similar intake of cow milk contributes with 38 µg iodine, or 25% of the recommended iodine intake.

Our study is limited by the use of only one product of each plant-based milk alternatives at one time-point and one replicate per sample. The results are therefore limited to the current marked. However, in view of the generally low iodine concentrations in plants it is unlikely that there is a large variation of iodine during the year and between different productions of the plant-based milk alternatives. Changes will primarily occur if more manufacturers start to fortify with iodine, and future monitoring of plant-based milk alternatives will be required.

Conclusions

This study is the first study providing analytical data of iodine concentrations in plant-based milk alternatives in samples from the Norwegian market. Most of the plant-based milk alternatives contained negligible concentrations of iodine, however the enriched oat-based milk alternatives contained higher concentrations of iodine than cows’ milk. As milk and dairy products are important sources of iodine in Norway, individuals substituting cow milk with plant-based milk alternatives should be encouraged to choose the alternatives fortified with iodine.

Conflict of Interest: The authors declare no conflict of interest.

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