Vitamin D deficiency is common. It is caused by limited sun availability together with a low supply of vitamin D in the food system. There is a high prevalence of low vitamin D status around the world. In Ireland, our relatively northern latitude and prevailing weather mean that UVB availability for skin synthesis of vitamin D is limited in this country and our population relies on the dietary supply of vitamin D to prevent deficiency. Thus, the endemic Irish problem of vitamin D malnutrition is due to the lack of vitamin D in our food system. Dietary guidelines cannot address this issue because foods naturally rich in vitamin D are very few and infrequently consumed.

The OASIS study will test the hypothesis that a vitamin D-fortified bread as part of a healthy diet that includes vitamin D-fortified foods is effective in preventing low vitamin D status during winter, and safe for older adults to consume.

This randomised controlled dietary intervention study aims to recruit 200 older adults (>65 years) who will be randomised to receive vitamin D-fortified bread and advice to consume additional commercially available fortified foods (Treatment group) or identical unfortified bread and advice to consume additional commercially available fortified foods (Control group). The outcome of the study is to test whether increasing vitamin D intake through food (Treatment group) is sufficient to prevent wintertime vitamin D deficiency (measured using serum concentrations of 25-hydroxyvitamin D <30 nmol/L) in comparison with the Control group.

The target vitamin D intake for the treatment group in this study is >15-20 μg/day vitamin D. This will maintain serum concentrations of 25-hydroxyvitamin D (25(OH)D) (the biomarker of vitamin D status) > 30 nmol/L, the clinical deficiency threshold to prevent metabolic bone disease, in over 90% of participants. At this vitamin D intake level, about 80% of participants will have a 25(OH)D concentration > 50 nmol/L, the personal intake target in the US and EU.

Inclusion Criteria:

1. Provide written informed consent.

2. Be an adult ≥65 years.

3. Willing to consume bread provided and have storage capacity for frozen bread.

4. Be in good general health.

5. Be willing to follow the assigned diet for 10 weeks and attend the required appointments.

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Exclusion Criteria:

Regularly consume a single high dose vitamin D supplement (>10µg/d /400 IU equivalent).

2. Exposure to factors that may influence vitamin D status, such as winter sun holiday, tanning bed usage.

3. Are following a medically prescribed diet. 4. Have a diagnosis of an acute or chronic medical condition that in the opinion of the investigator would interfere with the outcomes of the study such as Coeliac disease, Inflammatory Bowel Disease, liver disease, renal disease, pancreatic disease or gastric disease.

5. Have a history of active cancer or a diagnosis of cancer within the past 5 years.

6. Are taking certain medications that can impact vitamin D status, such as glucocorticoids.

8. Have regular excessive alcohol intake (≥28 units per week). 9. Have a known food allergy. 10. Are participating in another research study with an intervention or other lifestyle programme that would interfere with the outcomes of the study.

11. Are unable to read, write, or understand English.

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Please contact m.kiely@ucc.ie for any queries related to data access