Bitter orange is otherwise known as Citrus Aurantium. One suggested effect is due to the active component of synephrine which can augment thermogenesis (67). There is little evidence however to support the claims that citrus aurantium is effective for weight loss, this is due to research studies not using citrus aurantium alone and instead including it with substances such as caffeine which is how changes in participants fat mass can be attributed (68).
67 – Preuss, H. G., DiFerdinando, D., Bagchi, M., & Bagchi, D. (2002). Citrus aurantium as a thermogenic, weight-reduction replacement for ephedra: an overview. Journal of medicine, 33(1-4), 247-264.
68 – Colker, C. M., Kaiman, D. S., Torina, G. C., Perlis, T., & Street, C. (1999). Effects of< i> Citrus aurantium</i> extract, caffeine, and St. John’s Wort on body fat loss, lipid levels, and mood states in overweight healthy adults. Current Therapeutic Research, 60(3), 145-153.