Objective:The objective of this study is to examine the independent associations of time spent in moderate-to-vigorous physical activity (MVPA) and sedentary (SED-time), with total and abdominal body fat (BF), and the bidirectionality of these associations in adults at high risk of type 2 diabetes.Design and subjects:We measured MVPA (min per day) and SED-time (h per day) by accelerometry, and indices of total (body weight, fat mass (FM), BF% and FM index) and abdominal BF (waist circumference (WC)) using standard procedures in 231 adults (41.3±6.4 years) with parental history of type 2 diabetes (ProActive UK) at baseline, 1-year and 7-year follow-up. Mixed effects models were used to quantify the independent associations (expressed as standardised β-coefficients (95% confidence interval (CI))) of MVPA and SED-time with fat indices, using data from all three time points. All models were adjusted for age, sex, intervention arm, monitor wear time, follow-up time, smoking status, socioeconomic status and MVPA/SED-time.Results:MVPA was inversely and independently associated with all indices of total BF (for example, 1 s.d. higher MVPA was associated with a reduction in FM, β=-0.09 (95% CI: -0.14, -0.04) s.d.) and abdominal BF (for example, WC: β=-0.07 (-0.12, -0.02)). Similarly, higher fat indices were independently associated with a reduction in MVPA (for example, WC: β=-0.25 (-0.36, -0.15); FM: β=-0.27 (-0.36, -0.18)). SED-time was positively and independently associated with most fat indices (for example, WC: β=0.03 (-0.04, 0.09); FM: β=0.10 (0.03, 0.17)). Higher values of all fat indices independently predicted longer SED-time (for example, WC: β=0.10 (0.02, 0.18), FM: β=0.15 (0.07, 0.22)).Conclusions:The associations of MVPA and SED-time with total and abdominal BF are bidirectional and independent among individuals at high risk for type 2 diabetes. The association between BF and MVPA is stronger than the reciprocal association, highlighting the importance of considering BF as a determinant of decreasing activity and a potential consequence. Promoting more MVPA and less SED-time may reduce total and abdominal BF.International Journal of Obesity advance online publication, 20 May 2014; doi:10.1038/ijo.2014.66.