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Summary Visceral adipose tissue is harmful to metabolic health. Exercise training reduces visceral adipose tissue mass, but the underlying mechanisms are not known. Interleukin-6 (IL-6) stimulates lipolysis and is released from skeletal muscle during exercise. We hypothesized that exercise-induced reductions in visceral adipose tissue mass are mediated by IL-6. In this randomized placebo-controlled trial, we assigned abdominally obese adults to tocilizumab (IL-6 receptor antibody) or placebo during a 12-week intervention with either bicycle exercise or no exercise. While exercise reduced visceral adipose tissue mass, this effect of exercise was abolished in the presence of IL-6 blockade. Changes in body weight and total adipose tissue mass showed similar tendencies, whereas lean body mass did not differ between groups. Also, IL-6 blockade increased cholesterol levels, an effect not reversed by exercise. Thus, IL-6 is required for exercise to reduce visceral adipose tissue mass and emphasizes a potentially important metabolic consequence of IL-6 blockade. Graphical Abstract Highlights • Exercise reduces visceral adipose tissue mass • Loss of visceral adipose tissue mass following exercise is dependent on IL-6 • IL-6 receptor blockade increases total cholesterol and is not influenced by exercise • Improvements in cardiorespiratory fitness following exercise are not IL-6 dependent Wedell-Neergaard et al. show that in abdominally obese people, exercise-mediated loss of visceral adipose tissue mass requires IL-6 receptor signaling. Given that abdominal fat is metabolically harmful to health, this study raises a potentially important side effect of IL-6 receptor antibodies, such as tocilizumab, used to treat some forms of arthritis. [ABSTRACT FROM AUTHOR]