Locally advanced breast cancer is the most common cause of death in middle-aged women, with research focus shifted to preventative medicine. Polyunsaturated fatty acids (PUFA) is depleted in tumour initiation to sustain an inflammatory tumour microenvironment conducive to macrophage recruitment. Serotonin modulates macrophage activity and is a marker of poor 10-year survival in women with breast cancer. The relationship between serotonin and PUFA demands close examination for preventative treatment optimisation. We applied high sensitivity double quantum filtered MRS to accurately quantify PUFA between serotonin low and high breast tumours, and found PUFA depletion is associated with increased serotonin turnover.