Inclusion Criteria:
* Be able to give written informed consent.
* Be between 18 and 59 years of age.
* Be in generally good health as determined by the investigator (excluding Major Depressive Disorder diagnosis).
* Community dwelling with a current diagnosis of Major Depressive Disorder, and current depression episode/symptoms as determined via Beck's depression inventory-II (score 13-31).
Exclusion Criteria:
* Change of pharmacological therapy less than 2 weeks prior to beginning of study (including beginning pharmacological treatment).
* Have a significant acute or chronic coexisting illness \[cardiovascular, gastrointestinal (GI) \[including functional GI disorders, inflammatory bowel disease, coeliac disease\], immunological, psychiatric \[to include formal/clinical diagnosis or as determined via participant self-report i.e., bipolar spectrum disorder, schizophrenia, or psychosis, but not anxiety disorder\], neurodevelopmental or neurodegenerative disorders, metabolic disorders \[to include type I or II diabetes\], or any condition which contraindicates, in the investigators judgement, entry to the study (including conditions which may prevent an individual from safely participating in low-to-moderate exercise intensities (57-76% heart rate max \[HRmax\], rating of perceived exertion \[RPE\]:9-13 (13))-i.e., cardiorespiratory disease\[s\]).
* Have a malignant disease or any concomitant end-stage organ disease.
* Having a condition or taking a medication that the investigator believes would interfere with the objectives of the study, pose a safety risk, or confound the interpretation of the study results; to include anti- inflammatory drugs, corticosteroids, laxatives, enemas, proton-pump inhibitors, antibiotics, or probiotics (within 1 month of starting study), anti-coagulants, thrombocyte-aggregation blocking medication(s) and over-the counter non-steroidal analgesics. Participants should have a wash-out period of four-weeks of the above-mentioned medication to be eligible for participation.
* Individuals who are considered to be poor attendees, in the opinion of the investigator, or unlikely for any reason to be able to comply with the trial.
* Participants must not be currently receiving treatment involving experimental drugs. If the participant has been in a recent experimental trial, these must have been completed not less than 30 days prior to this study.
* Are meeting the national physical activity guidelines (16) (i.e., at least 30 minutes a day of moderate intensity activity, five days a week (or 150 minutes a week), specifically in relation to structured aerobic exercise, as assessed via the international physical activity questionnaire (IPAQ). NB: if guidelines are met from occupational or incidental physical activity, individuals would not meet exclusion criteria.
* Current perimenopause, menopause, or post-menopause, in the case of females.
* Females who are pregnant, planning a pregnancy within duration of the study intervention period, or currently lactating.
* Participants who are not fluent in English or English is not first language.
* Are colour blind.
* Have dyslexia or dyscalculia.
* Are a current habitual daily smoker.
* Regular, illegal drug use.
* Alcohol abuse disorder.
* Acute suicidality or suicide attempt in the past 6 months.
* Current eating disorder (anorexia nervosa, bulimia nervosa, binge eating disorder).
