BACKGROUND:Diet and life-styles are considered as the main factors that determine the high prevalence of obesity in Western societies. Although some countries have registered a decrease in fat intake, the percentage of overweight and obesity has increased. Therefore, it is thought that fat intake may not be the main factor that determines the current epidemic of obesity. The objective of this study was to determine the role of a sedentary life-style and eating between meals (snacking) as major determinants of a recent weight change (over last 5 years). METHOD:By using cross-sectionally baseline data of the SUN cohort, we adjusted non-conditional logistic regression models to estimate the odds ratio (OR) of gaining weight according to age, physical activity in leisure time, watching television, taking a nap, smoking, snacking and the intake of macronutrients. RESULTS:A statistically significant inverse association between leisure-time physical activity and the probability of gaining weight was found for men (OR = 0.93; CI 95%, 0.88-0.98) and a trend was also present among women. Snacking was positively associated with a higher probability of gaining weight among men (OR = 1.88; CI 95%, 1.40-2.53) and among women (OR = 1.38; CI 95%, 1.10-1.73). CONCLUSIONS:Our data suggest a direct association between snacking and weight gain in middle-aged people.