This study is examining the relationship between infant nutrition, gut health, and development. The fecal microbiota changes and develops, in large part due to the food that infants eat. These changes are important for many aspects of development. This study is designed to examine how the fecal microbiota changes when exclusively breastfed infants are first introduced to solid food, and how changes of the fecal microbiota are related to other aspects of development.

The purpose of this study is to determine: 1) how the gut bacteria of exclusively breastfed infants changes in response to ingesting solid foods; 2) how infant cognition develops in response to ingesting solid foods; and 3) the relationship between infant gut bacteria and infant cognition during the first year of life.

This study is designed to determine how specific complex carbohydrates in commonly used first foods encourage the growth of different bacteria in the infant gut. The two foods used in this study are commercially-available sweet potato (Plum Organics) and pear (Earth's Best). These two foods have been chosen because they differ substantially from each other in their carbohydrate composition. For example, sweet potato is mostly made up of starch which is digestible and pear is made up of other types of sugars found in fruits and vegetables that are not digestible and may have "prebiotic" effects (food for good bacteria in the gut). Thus, the use of these two foods could provide a good contrast for comparing how gut bacteria respond to different carbohydrate compositions during complementary feeding.

Inclusion Criteria:

• Women, age 21 to 45 years who have delivered a healthy single infant by vaginal delivery and their infants, age 4 to 7.5 months;
• Infants who are developmentally ready for solids;
• Generally healthy women and infants;
• Mothers who plan to exclusively (without solids or infant formula) breastfeed (at the breast or feed breast milk by bottle) their infants for at least 5 months of age and plan to continue to breastfeed with solids and/or infant formula until 12 months of age;
• Mothers who are willing to either use their own breast pump, or hand-express, or use a manual pump provided by the study to collect milk samples;
• Mothers who are willing to refrain from feeding their infants infant formula, non-study solid foods; probiotic or iron supplements (confounding variables of the intestinal microbiome) before the end of the feeding intervention period;
• Term infants born >37 weeks gestation;
• Mother-infant pairs who live within a 20-mile radius from University of California, Davis campus in Davis, California (includes Woodland, Vacaville, Dixon and surrounding areas) or within a 20-mile radius of the University of California, Davis Medical Center (UCDMC) (2221 Stockton Blvd, Sacramento, CA 95817).

Exclusion Criteria:

• Infants with any GI tract abnormalities;
• Infants born by cesarean section;
• Family history of immunodeficiency syndrome(s);
• Multiple infants born to one mother at the same time (no twins, triplets, etc.);
• Infants born with medical complications such as: respiratory distress syndrome, birth defects, and infection;
• Mothers diagnosed with any metabolic or endocrine, liver, kidney disease, any autoimmune disease, cirrhosis, hepatitis C, HIV, AIDS, cancer, obesity (pre-pregnancy BMI >34.9), polycystic ovary syndrome (PCOS), celiac disease, Crohn's disease, heart disease, hyper- or hypothyroidism, hyper- or hypotension (including pre-eclampsia), type 1 or type 2 diabetes.
• Mothers who smoked cigarettes less than one month before becoming pregnant, during pregnancy, and currently or mothers who plan to initiate smoking during the study duration;
• Infants who have taken antibiotics within the past 4 weeks;
• Infants who have taken iron supplements within the past 4 weeks;
• Infants who have consumed infant formula in the past 4 weeks;
• Infants who have consumed infant formula more than 10 days between birth and 4 weeks prior to screening;
• Infants who have consumed any solids;
• Mothers who plan to feed infants solids before 5 months of age;
• Mothers who plan to administer any probiotics to infants throughout the feeding intervention period (first 18 days of the study);
• Infants who have consumed probiotics containing Bifidobacterium within the past 4 weeks or other probiotics within the past 7 days;
• Mothers who live in more than one location (should only live in one house to ensure samples are correctly collected and stored);
• Infants who have hypotonia,
• Infants who have been diagnosed with any medical or nutritional condition that would require iron supplementation.
• Infants who on average pass less than one stool per week.