Inclusion Criteria:  Participant is willing and able to give informed consent for participation in the research Male or female Body mass index in the range of 18 to 33 Not currently taking any medications (except for contraception), including being antidepressant free for at least three months Have at least two previous episodes of depression, and have been recovered from the most recent episode of depression for six months Current PHQ-9 score < 10 (the cut off for DSM major depression)  Exclusion Criteria:  The participant may not enter the study if ANY of the following apply:  Any current Axis 1 DSM-5 psychiatric disorder Any previous episode of a severe mental illness, other than Depressive Disorder. Comorbid Anxiety disorders will be allowed, but not OCD (Obsessive Compulsive Disorder) or PTSD. A first degree relative diagnosed with Bipolar Affective Disorder Type 1 or Schizophrenia Body Mass Index outside the range of 18 to 33 inclusive Any significant current medical condition likely to interfere with conduct of the study or analysis of data Current use of psychoactive and / or medically significant medication as judged by a study medic, whether prescribed or bought over the counter (the contraceptive pill, the Depo-Provera injection or the progesterone implant will not result in exclusion) Ongoing psychopharmacological treatment for depression, including hypnotics (psychotherapy will be allowed as long as not newly-started in the last 6 weeks) High consumption of licit substances to an extent that would make complying with study protocol challenging (including alcohol, caffeine, nicotine) Past history of dependence to illicit substances, and any consumption of illicit substances in the three months prior to the study Currently pregnant or breast feeding Current, or a significant history of, gastro-intestinal disorder or irritable bowel syndrome Known lactase deficiency or any other problem absorbing lactose, galactose, or glucose Participation in a study that involves the use of a medication or novel vaccine within the last three months Participation in a study using the same tasks in the last two years Any physical (including visual and auditory) or language impairment that would make complying with the study protocol challenging