Hypoglycaemic potential of beetroot and blackcurrant juice : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Health Sciences, Massey University, Albany, Aotearoa New Zealand

Abstract

The global rise in type 2 diabetes mellitus (T2DM) and its precursor, prediabetes, have created an urgent need for accessible and effective interventions targeting early glycaemic and metabolic dysregulation. Metabolic syndrome (MetS), comprising central adiposity, hypertension, dyslipidaemia, and impaired glucose tolerance, significantly increases T2DM and cardiovascular disease (CVD) risk. Functional foods rich in dietary polyphenols and nitrates may offer a low-risk, scalable approach to improving glucose regulation and vascular health, yet data on their efficacy in at-risk populations remain limited. This thesis investigated the acute and short-term metabolic effects of beetroot (BR), rich in dietary nitrate and polyphenols, and blackcurrant (BC), rich in anthocyanins, on glycaemic control as well as other MetS components including insulin dynamics, vascular resistance, and lipid profiles. A final aim was to evaluate the feasibility of a longer-term functional food intervention in individuals with prediabetes. A narrative review established that BR and BC have mechanistically plausible effects on glucose and lipid metabolism via enhanced nitric oxide (NO) bioavailability, α-glucosidase inhibition, increased insulin sensitivity, and antioxidant activity. However, human trials often lack robust outcome measures or clearly defined bioactive content in interventions and frequently exclude at-risk groups such as those with prediabetes or MetS. Due to observed methodological discrepancies between different intervention trials investigating acute glycaemic control, we aimed to determine optimal methodology for capturing intervention effects when targeting peripheral glucose uptake. A repeated-measures, crossover trial using an oral glucose tolerance test (OGTT) compared capillary, venous whole blood, and plasma glucose after acute beetroot juice intervention (BRI) ingestion and placebo (PL). Capillary sampling revealed significantly reduced 30 min postprandial glucose (BRI = 2.51 ± 1.29 mmol/L vs. PL = 3.40 ± 1.47 mmol/L; p = 0.002) and incremental area under the curve (iAUC₀₋₃₀: BRI = 52.2 mmol·min/L, PL = 66.5 ± 27.2 mmol·min/L; p = 0.017) in the BRI group vs PL. These effects were not detected in plasma or venous samples. The data suggested enhanced peripheral glucose uptake, likely via glucose transporter type 4 (GLUT4) translocation in skeletal muscle stimulated by NO, is best captured via capillary measures. A randomised crossover trial in healthy adults assessed the acute effects of BRI (600 mg nitrate) and the blackcurrant juice intervention (BCI; 120 mg anthocyanins) on postprandial glucose and insulin responses. Compared with PL, BRI significantly reduced capillary glucose early-phase iAUC₀₋₃₀ (BRI = 52.24 mmol·min/L, PL = 66.50 mmol·min/L; p = 0.017) and iAUC₀₋₆₀ (BRI = 95.3 mmol·min/L, PL = 122.9 mmol·min/L; p = 0.010) and insulin iAUC₀₋₆₀ (BRI = 836 ± 480 vs. PL = 848 ± 446 μU·min/mL). In contrast, BCI did not significantly reduce blood glucose concentrations but showed a trend toward lower insulin iAUC (BCI = 772 ± 365 μU·min/mL), suggesting improved insulin sensitivity. These findings support complementary mechanisms: BRI enhances glucose uptake via NO-dependent vasodilation or increased GLUT4 translocation; BCI may modulate insulin signalling or incretin response. To isolate the effects of betalains from nitrate in BR, a 6-day intervention compared two beetroot juice formulations with differing phytochemical profiles: UKBR (1280 mg nitrate, 294 mg betalains) and NZBR (1120 mg nitrate, 506 mg betalains). Plasma nitrate and nitrite concentration, systolic and diastolic blood pressure (SBP, DBP), systemic vascular resistance (SVR), fasting glucose and lipid profiles were assessed in a fasted state (overnight) after 6 days of supplementation and acutely at 2 h and 5 h following ingestion on day 7. Baseline plasma nitrate concentrations after 6 days were 267.3 µmol/L for UKBR, 163.5 µmol/L for NZBR, and 47.1 µmol/L for PL. Plasma nitrite increased significantly with UKBR (Δ +96 ± 94 nM; p < 0.001), but not with NZBR, suggesting differential NO bioavailability. Reductions in SBP were observed after 6 days in NZBR vs PL (-5.3 mmHg for NZBR vs. PL, p = 0.038) and at 2 h post-ingestion for both UKBR (-5.7 mmHg, p = 0.003) and NZBR (-5.6 mmHg, p = 0.029) compared with PL. The NZBR formulation elicited a greater reduction in systemic vascular resistance relative to baseline after 6 days’ intake compared with PL (NZBR = –219 dyne·sec·cm⁻⁵, PL = –25 dyne·sec·cm⁻⁵; p = 0.024). Both PL and UKBR showed modest reductions in high density lipoprotein cholesterol (HDL-C) compared with baseline (PL -0.13 mmol/L, p = 0.047; UKBR -0.13 mmol/L, p = 0.011) whilst NZBR trended lower (NZBR, -0.13 mmol/L, p = 0.082). These results indicate that betalains may exert independent vascular effects beyond those attributable to nitrate. A 12-week feasibility trial evaluated the practicality of long-term beetroot juice (BRJ) and blackcurrant juice (BCJ) supplementation in adults with prediabetes (n = 12 enrolled; n = 8 completed), as well as establish common challenges with recruitment and retention to longer-term nutritional intervention studies in individuals with prediabetes. Despite strong interest during recruitment (45 screened), only 12 participants were randomised primarily due to having a HbA1c result at screening outside the prediabetes range (41 – 49 mmol/mol). Reasons for participant attrition included time constraints and lack of immediate benefit from the intervention. Nevertheless, compliance among completers exceeded 90% and participant feedback favoured encapsulated or powdered formats, with gastrointestinal issues reported by 3 participants resulting from the intervention. Although statistical power was insufficient to detect significant changes in fasting glucose, lipids, or blood pressure, metabolic outcomes varied among individuals and further investigation with larger sample sizes is recommended. Collectively, these studies demonstrate that both BRJ and BCJ intake exert beneficial effects on early-phase postprandial glucose regulation and vascular function. Acute BRJ ingestion significantly reduces postprandial glycaemia in healthy adults, effects likely mediated by NO-induced improvements in skeletal muscle perfusion and glucose uptake. Acute BCJ intake may enhance insulin sensitivity, though higher anthocyanin doses may be required to achieve statistical significance. Importantly, the vascular effects of BRJ intake are not solely nitrate-dependent; betalains appear to play a previously understudied role in modulating SVR, potentially via redox-sensitive mechanisms affecting endothelial function. These findings also highlight critical methodological considerations for future trials. Capillary blood glucose sampling provides enhanced sensitivity for detecting peripheral glucose clearance, a key parameter in early insulin resistance. Bioactive content should be quantified and standardised, and intervention formats adapted to population preferences to improve long-term compliance. Recruitment of at-risk individuals requires culturally appropriate strategies and simplified intervention protocols. In conclusion, this thesis supports the inclusion of nitrate-, anthocyanin-, and betalain-rich functional foods as part of early dietary strategies for reducing T2DM risk. Future work should prioritise longer-term trials in prediabetic cohorts, with mechanistic endpoints to clarify bioactive action, and optimised delivery systems to enhance adherence. These data contribute to the growing evidence base supporting functional foods as viable tools in metabolic disease prevention.