In the U.S. it is recommended that children consume whole cow's milk (3.5% fat) from ages 1 to 2 years to support rapid early growth and brain development, and then at age 2 years transition to low-fat (1%) or non-fat milk to reduce saturated fat and calorie intake. To date, few studies have examined the optimal milk type for children to prevent obesity. This randomized controlled trial will evaluate the effect of consumption of whole versus 1% milk on child adiposity.

The prevalence of child obesity in the U.S. has tripled since the 1970s and excess weight gain - even in young children - is a precursor to adult obesity and associated co-morbidities. In the U.S. it is recommended that children consume whole cow's milk (3.5% fat) from ages 1 to 2 years to support rapid early growth and brain development, and then at age 2 years transition to low-fat (1%) or non-fat milk to reduce saturated fat and calorie intake. However, surprisingly few rigorous trials to support recommendations on optimal milk type have been conducted and existing observational studies paradoxically suggest that lower fat milk consumption is associated with increased adiposity in children. The effects of the types of fat found in milk on cardiometabolic disease risk have also been questioned. This randomized controlled trial will evaluate the effect of consumption of whole versus 1% milk on child adiposity and other health and developmental outcomes beginning after toddlers have successfully transitioned from breastmilk and/or formula to cow's milk at 2 years of age. Investigators will recruit 625 parents of toddlers and randomly assign 625 toddlers to either whole or 1% milk groups for 1 year (with estimated final sample size of 500). Our primary aim is to determine how milk fat type (whole versus 1%) consumed from age 2 to 3 years affects change in adiposity as measured by waist-to-height ratio (primary outcome), body mass index, tri-ponderal mass index, and waist circumference. Secondary aims are to evaluate how milk type consumed from age 2 to 3 years affects changes in milk, total and saturated fat, added sugars, and total energy intake and overall diet quality, as well as blood lipids and vitamin D status, and neurocognitive development. Results from the Milk-TOT Study can help pediatric health care providers give evidence-based dietary recommendations to improve child weight and health, and can inform the types of milk provided to participants in the federal nutrition programs which collectively provide milk to over half of all young children in the U.S.

Inclusion Criteria:

Parents of children: 1) willing to be randomized to provide either only whole or 1% milk to their child for 1 year, 2) access to a smart phone and a tablet, laptop or computer with internet and email at home, 3) ability to speak and read English or Spanish for the purposes of receiving study communications and completing surveys and dietary assessments (to avoid needing to translate all study materials and have research staff fluent in other languages due to budget limits), 4) not planning to move outside of the San Francisco Bay Area or discontinue being child's primary caregiver for the next year (e.g., foster care, parent separation).

Exclusion Criteria:

Children: 1) <23 months or >48 months old at recruitment; 2) condition or medication that affects growth or daily feeding, or cardiometabolic health such as hypopituitarism, growth hormone deficiency, inborn error of metabolism, syndromic obesity, familial hypercholesterolemia, 3) weight-for-length or height below the 2nd percentile, 4) lactose intolerance, milk allergy or other dietary restrictions (e.g., vegan) that impact ability to consume dairy or otherwise limit dietary intake, 5) resides in more than one household (e.g., shared care by separated parents), and 6) not covered by public or private medical insurance (may reduce well-child doctor visits). If two eligible children are in a household, one will be randomly selected to participate.