Researchers investigated the benefits of inhaling fennel essential oil in a high-fat diet-induced obesity model. Subjects were divided into four groups: normal diet control, high-fat diet no intervention, high-fat diet plus inhalation of 0.3% fennel oil, and high-fat diet plus inhalation of 1.0% fennel oil. Inhalation of fennel essential oil—0.3% or 1.0% fennel oil for 30 minutes daily over the course of 12 weeks—resulted in decreased body weight, body fat, and visceral fat in subjectd fed a high-fat diet. Improvement in lipid metabolism (improved cholesterol profiles and reduced insulin) and reduced systolic blood pressure and heart rate was also observed after inhaling fennel oil. Inhaling the higher concentration (1.0%) of fennel essential oil had the greatest impact on body weight. Interestingly, a decrease in LDL cholesterol and triglycerides and an increase in HDL cholesterol was observed in the 0.3% fennel oil group, but the 1.0% group experienced increases in triglycerides, HDL and LDL cholesterol. Insulin levels decreased in both fennel oil groups, while cortisol only decreased in the 1.0% fennel group. A marked increase in testosterone in male subjects was noted in the 0.3% fennel group. Overall, this study suggests that inhaling fennel essential oil protects against metabolic dysfunction associated with obesity and could promote healthy body weight despite a high-fat diet.