The UK Biobank is a large-scale epidemiological study designed to understand how genetic, lifestyle and environmental factors influence incident disease and commenced data collection in 2006 (1). Diet is a recognised key determinant of health, but some studies using UK Biobank data have reported only small or insignificant associations with health outcomes. This review sought to answer the following questions. What is the effectiveness of eating a higher quality diet versus low quality on the incidence and mortality from chronic disease in the UK biobank cohort? How does it vary according to the dietary indices being used to determine quality of diet? A protocol was published in PROSPERO (CRD420251153616). MEDLINE, EMBASE and CINAHL were systematically searched for English-language studies published between 2006 and 2025. Eligible studies included cohort and nested case-control designs using UK Biobank participants, a diet quality index or equivalent dietary measure, and chronic disease incidence or mortality as outcomes. Risk of bias was assessed by the JBI Critical Appraisal Checklist for Cohort Studies. From 1013 studies screened, 79 met inclusion criteria and 44 unique diet indices were identified. The main dietary indices were the Plant-based Diet Index and its healthful and unhealthful variations (n = 16), EAT-Lancet diet index (n = 12) and ultra-processed foods as a % of intake (g, serves or kJ) (n = 12). Other indices used were Healthy Diet Score, Dietary Approaches to Stop Hypertension (DASH), and Alternate Healthy Eating Index-2010 (AHEI-2010) each with six studies and there were 12 studies using a variety of nine Mediterranean diet indices. Most studied outcomes were all cause mortality (n = 14), cancers (n = 28), cardiovascular disease (n = 23), Type 2 diabetes (n = 14), and dementia (n = 11). Studies demonstrated a protective association with higher diet quality and all-cause mortality for all dietary indices studied if dietary data from the comprehensive 24 h Oxford WebQ was used but only two of the five that used the shorter food frequency questions collected at baseline found a significant effect and of note three of these five studies appear to have used the same bespoke index. For cardiovascular disease healthy eating index, alternative healthy eating index, heart protective diet score, planetary diet index, diet quality score, alternate Mediterranean score and higher intake of plant-sourced non-ultra-processed foods showed protective effects. For incident type 2 diabetes the range of healthful dietary indices all showed protective effects, and for cancer there were inconsistent results as for dementia with the named high diet quality indices. Overall, evidence from the UK Biobank found that adherence to healthier dietary patterns was generally associated with lower incidence of and mortality from non-communicable chronic diseases, however, findings for cancer need more nuanced investigation.