Iodine deficiency remains a global problem impairing health and development in affected populations. Although there has been remarkable global progress against iodine deficiency, mild and moderate iodine deficiency remain common globally, including European countries. Besides salt, milk and dairy products are important iodine sources in many industrialized countries, with varying contributions depending on the milk iodine concentration and the amount of milk and dairy consumed.

Iodine absorption in humans depends on the iodine species and possibly on the iodine status of the person. Very little data is available on iodine absorption or bioavailability from different dietary sources including milk. Inorganic iodide is thought to be absorbed almost completely (>90%). In contrast, only about two-thirds of some forms of organically-bound iodine are absorbed. The absorption of iodine from milk has not been quantified in humans. In this balance study, the investigators want to quantify the absorption of iodine in cow's milk in male and female adults and compare with the bioavailability from an iodine water solution (potassium iodide). The results of this study will inform on the bioavailability rate of iodine from cow's milk. Knowing the actual iodine bioavailability from milk is critical because milk and dairy products are major iodine sources in many industrialized countries.

The primary objective of this randomized, cross-over design study is to assess iodine bioavailability (measured using excretion in urine) from whole cow's milk delivering an iodine level of about 600 µg/L and compare them with a control iodine solution. The investigators will test three drinks within one subject: 1) a milk with an intrinsic iodine concentration of about 600 µg/L; 2) a milk with a naturally low iodine concentration and an added amount of potassium iodide (extrinsic iodine in milk matrix) to reach a level of about 600 µg/L (adapted to the intrinsic concentration in 1)); and 3) control iodine solution (extrinsic iodine in water matrix) with the same iodine concentration.

Inclusion Criteria:

1. Aged between 18 and 45 years
2. BMI 19-25 kg/m2
3. Current use of iodized salt at home
4. Signed informed consent

Exclusion Criteria:

1. Inadequate iodine status (defined as UIC <70 µg/L or >300 µg/L and assessed during screening from 5 urine spot samples)
2. Exposure to iodine-containing X-ray/ computed tomography contrast agent
3. Use of iodine-containing disinfectants (betadine)
4. History of thyroid disease (according to the participants own statement)
5. Any metabolic, gastrointestinal or chronic disease such as diabetes, hepatitis, hypertension, or cancer (according to the participants own statement)
6. Chronic use of medications (except for contraceptives)
7. Use of iodine containing supplements within 1 month prior to study start
8. Pregnancy (according to the participants own statement but confirmed by a pregnancy test with the first urine spot sample from screening)
9. Breast feeding
10. Vegan diet
11. Drug abuse
12. Extensive alcohol intake, defined as more than 3 (men) or 2 (women) standard drinks per day (i.e. 3dl beer, 1dl wine, 3-4cl liquor), with less than 2 days per week without alcohol consumption

Two of the three test drinks will consist of milk delivering extrinsic iodine or intrinsic iodine and will appear the same. These two milks will be labelled with "A" and "B" and masking will be applied to the participant only (single-blinded). The code will be held by the Study coordinator. However, for the water iodine-solution, blinding will not be possible.