Early childhood (~3-7 years of age) is an important window for determining body composition trajectory and may be a critical period for the development of tissue partitioning patterns that influence obesity risk. As adiposity accelerates during this critical period, deposition/ preservation of fat stores may be sustained at the 'expense' of other tissues; i.e. energy homeostasis may be inherently biased toward fat gain. The type and amount of tissue mass accrued in early childhood has implications for metabolic profile, glucose/insulin homeostasis, hormone profile and resting energy expenditure.

The interplay between fat and bone deposition represents a physiologic trait enabling the body to choose between shuttling 'energy' towards accrual of a particular tissue. Plausibly the phenotype underlying obesity and diabetes risk may be determined by the differentiation of cell type (adipocyte, osteocyte, etc.) during this early stage of growth and development. In vitro studies demonstrate transdifferentiation under the influence of specific external stimuli, which can switch phenotypes toward other cell lineages. Further, rodent models have demonstrated that exposure to stimuli (exercise) early in life may prevent excess fat mass accrual in adulthood, even when the stimulus is later removed (animals are no longer exercising). Children's early experiences (engagement in physical activity vs. sedentary behavior) may 'environmentally induce' alterations in body composition and predispose individuals to obesity throughout life.

Aim 1. To examine the associations between body composition via DXA and objectively-measured physical activity/inactivity.

1. Hypothesis 1.1: There is a positive association between physical activity and bone mass.

2. Hypothesis 1.2: There is a positive association between sedentary behavior and total fat mass.

   Aim2. To examine the associations between adipose tissue distribution via MRI and objectively-measured physical activity/inactivity.

3. Hypothesis 2.1: There is an inverse association between physical activity and bone marrow adipose tissue.

4. Hypothesis 2.2: There is a positive association between sedentary behavior and bone marrow adipose tissue.

Inclusion Criteria:

• children aged 3 to 7 years
• healthy, not under the care of a doctor
• not taking medications known to alter body composition or metabolism