SummaryWe have described for the first time the potential harmful effects of green tea on the metabolism and body composition of untreated juvenile experimental type 1 diabetic rats. The treatment containing 19.38% of epigallocatechin-3-gallate, its main catechin, increased blood glucose and water intake. It also increased oxygen consumption, enhanced energy expenditure and led to a lipid oxidation tendency in diabetic animals, which worsened the development of body fat in a way significantly more aggravated than diabetes alone. Taken together, our findings indicate that green tea treatment, when provided to juvenile diabetics, increases glycaemia, changes the body composition by reducing fat content and increases oxygen consumption, besides affecting energy expenditure. Therefore, the nutritional status of the juvenile type 1 diabetic rat is aggravated.