1 Abstract 2 The prevalence of child obesity in the U.S. has tripled since the 1970s and excess weight gain – even in young 3 children – is a precursor to adult obesity and associated co-morbidities. In the U.S. it is recommended that 4 children consume whole cow’s milk (3.5% fat) from ages 1 to 2 years to support rapid early growth and brain 5 development, and then at age 2 years transition to low-fat (1%) or non-fat milk to reduce saturated fat and 6 calorie intake. Surprisingly few rigorous trials support recommendations regarding the optimal amount of fat in 7 milk. Paradoxically, existing observational studies suggest that lower fat milk consumption is associated with 8 increased adiposity in children. The effects of the types of fat found in milk on cardiometabolic disease risk 9 have also been questioned. We propose a randomized controlled trial to evaluate the effect of consumption of 10 whole versus 1% milk on child adiposity and other health and developmental outcomes beginning after 11 toddlers have successfully transitioned from breastmilk and/or formula to cow’s milk at 2 years of age. For the 12 proposed trial, Milk Type in Toddlers (Milk-TOT) Study, we will recruit 625 parents of toddlers and randomly 13 assign them to either whole or 1% milk groups for 1 year. Our primary aim is to determine how milk fat type 14 (whole versus 1%) consumed from age 2 to 3 years affects change in adiposity as measured by waist-to-height 15 ratio (primary outcome), and body mass index z-score, tri-ponderal mass index, and waist circumference z- 16 score (secondary outcomes). Secondary aims are to evaluate how milk type consumed from age 2 to 3 years 17 affects changes in milk, total and saturated fat, added sugars, total energy intake and overall diet quality, as 18 well as blood lipids, glucoregulatory indices, vitamin D status, and neurocognitive development. An exploratory 19 aim will assess changes in gut microbiota. Results from the Milk-TOT Study will inform evidence-based 20 recommendations about the optimal milk fat that children need to optimize their growth, development, and 21 health. Such recommendations will influence healthcare provider advice about the type of milk that children 22 consume as well as inform federal nutrition programs that collectively provide milk to half of young children in 23 the U.S.