Infants The purpose of this study is to measure breastmilk's vitamin D sulfate nutritional value in infant's saliva and digesta (gut).

Breastfeeding Mothers The purpose of this study is to measure Vitamin D sulfates in freshly expressed breastmilk samples before and after 28 days of Vitamin D supplementation in lactating mothers.

Infants

Sign the consent form to confirm participation:

You will be asked to bring a soiled diaper from the morning of the study visit. You will meet with a study team member who will collect up to 6 swabs total:

Up to 3 cheek/mouth swabs to collect saliva Up to 3 stool/anus/diaper swabs

Lactating Mothers

Sign the consent form to confirm participation:

Visit 1 (Baseline) You will meet with a study team member complete some demographic information and gestational age of the baby and birthdate, next you will have a blood draw and express breast milk in a private location, the study team will collect up to 50 ml of freshly expressed breast milk for the research study, you will be asked to bring your personal breast pump and collection container to each research visit, or manually express milk, see options below. The study team member will schedule visit 2. The study will provide the vitamin D supplement that you will take daily for 28 days.

Visit 2 (post intervention) Approximately 28 days later you will meet with a study team member, have a blood draw and express breast milk in a private location, the study team will collect up to 50 ml of freshly expressed breast milk for the research study, you will be asked to bring your personal breast pump and collection container to each research visit, or manually express milk, see options below. The study team member will ask for the container the Vitamin D supplement was in.

You have 2 options:

Option 1:

Bring along your personal breast pump and collection container to each study visit.

The study team member will collect up to 50 ml of freshly expressed breast milk for the study.

Option 2:

You do not bring along your personal breast pump and containers to each study visit, instead you will be asked to:

Manually express your breast milk into a specimen collection cup at each study visit.

Lactating Mothers

Inclusion Criteria:

• Lactating mothers
• ≥2 weeks after delivery
• No intentions of weaning during the study

Exclusion Criteria:

• History of sarcoidosis
• History of renal disease
• Premature birth- <37 weeks
• Taking daily supplement with ≥600 IU vitamin D in the past 30 days

Breastfeeding Infants

Inclusion Criteria:

• Female and male infants
• Exclusively breastfed

Exclusion Criteria:

• No known gastrointestinal diseases that require medical treatment
• Currently prescribed antibiotics or have taken antibiotics <2 weeks from collection