['Specific Health Examinations and Guidance (Tokutei kenko shinsa/Tokutei hoken shido) are provided for people over 40 years of age to reduce the incidence of metabolic syndrome (MetS). In the present study, we evaluated the importance of weight control in people below 40 years of age.', 'Male subjects (n = 877), aged 30 years, without MetS, were examined. Subjects were classified into 3 groups based on body mass index (BMI): non-obese (BMI < 22), pre-obese (22 ≤ BMI < 25), and obese (BMI ≥ 25). Cox proportional hazards regression analysis was performed for each group to calculate the hazard ratios (HRs) and 95% confidence intervals (CIs) for the incidence of MetS in individuals in their 40s on the basis of changes in their BMI, systolic blood pressure, diastolic blood pressure, fasting plasma glucose, high-density lipoprotein, and triglyceride levels between 30 and 35 years of age. In addition, subjects were classified into 3 sub-groups based on changes in BMI: stable-decrease (BMI change < 1), slight increase (1 ≤ BMI increase<2), and increase (2 ≤ BMI increase). HRs for the 3 BMI change sub-groups for MetS were calculated for non-obese and pre-obese subjects.', 'There was a significant association between changes in BMI and the incidence of MetS for non-obese individuals in their 40s (HR: 2.80, 95% CI: 1.61-4.88) and pre-obese subjects (HR: 2.00, 95% CI: 1.44-2.77). There were also significant associations between the stable/decrease and increase (HR: 9.39, 95% CI: 1.52-57.70) sub-groups and MetS in the non-obese group, as well as for the slight increase (HR: 2.30, 95% CI: 1.03-5.11) and increase (HR: 10.13, 95% CI: 4.30-23.80) sub-groups in the pre-obese group.'] Is [ Increased body mass index in young adults associated with metabolic syndrome ]?