-fluorodeoxyglucose positron emission tomography combined with computed tomography (FDG-PET/CT) is widely used as a standard method for evaluating human brown adipose tissue (BAT), a recognized therapeutic target of obesity. However, a longitudinal BAT study using FDG-PET/CT is lacking owing to limitations of the method. Near-infrared time-resolved spectroscopy () is a technique for evaluating human BAT density noninvasively. This study aimed to test whether could detect changes in BAT density during or after long-term intervention. First, using FDG-PET/CT, we confirmed a significant increase (, ) in BAT activity in the supraclavicular region after 6-week treatment with thermogenic capsaicin analogs, capsinoids. Next, 20 volunteers were administered either capsinoids or placebo daily for 8 weeks in a double-blind design, and BAT density was measured using every 2 weeks during the 8-week treatment period and an 8-week period after stopping treatment. Consistent with FDG-PET/CT results, successfully detected an increase in BAT density during the 8-week treatment (, ), and a decrease in the 8-week follow-up period (, ), only in the capsinoid-treated, but not the placebo, group. Thus, can be applied for quantitative assessment of BAT in longitudinal intervention studies in humans.