Background: Blood glucose and insulin are elevated after intake of carbohydrate, with levels returning to normal in about 2-3 hours after ingestion. We examined the effects of daily New Zealand blackcurrant intake over 7 days on fasting glucose and insulin levels and the responses of glucose and insulin during an oral glucose tolerance test (i.e. OGTT).

Methods: Seventeen healthy participants (9 males, 8 females, age: 24±8 years, body mass: 75.4±16.4 kg, height 172±11 cm, body mass index: 25.3±3.3) consumed 6 g·day-1 New Zealand blackcurrant (NZBC) powder for 7 days. Every 6 g of the serving contained 138.6 mg anthocyanins, 49 mg vitamin C, and 5.2 g of carbohydrates with total phenolic content 271.6 mg. A cross-over design was used. Participants completed one OGTT before starting the supplementation (day 0) and another OGTT after 7 days of the supplementation (day 7). For the OGTT, participants were seated and consumed 75 g of glucose dissolved in 250 mL water. Finger prick capillary samples were taken before and every 30 minutes for a total of 120 minutes after consuming the glucose drink. Following duplicate glucose analysis, blood samples were centrifuged and then plasma was separated and frozen (-20°C) for triplicate insulin analysis using a human 96-well insulin enzyme-linked immunosorbent assay (IBL international, Hamburg, Germany).

Results: NZBC had no effect on fasting glucose (control: 4.46±0.45; NZBC: 4.41±0.44 mmol·L1, P=0.657), although there was a trend for fasting insulin to be 14.3% lower (control: 66.5±28.2; NZBC: 57.0±29.5 pmol·L-1) (P=0.091). HOMA-IR was not different between the control and NZBC (1.81±0.73 vs 1.58±0.83) (P=0.126). With NZBC during the OGTT, plasma glucose at 60 min was 8.1% lower (control: 6.68±1.13; NZBC: 6.14±1.41 mmol·L-1; P=0.016), insulin at 30 min was 18.4% lower (control: 337.1±228.3; NZBC: 275.0±136.4 pmol·L-1; P= 0.021), and insulin at 60 min was 39.2% lower (control: 297.8±154.3; NZBC: 181.2±97.4 pmol·L-1; P= 0.002). With NZBC during the OGTTs, areas-under-the-curve for plasma glucose (control: 752.6±79.4, NZBC: 709.8±93.3 mmol·L-1·120 min) and insulin (control: 28443±12816, NZBC: 20406±7985, pmol·L-1·120 min) were 5.7% (P=0.051) and 31.1% lower (P<0.001) respectively.

Conclusion: A trend for lower fasting insulin with normal glucose and lower areas under the curve for glucose and insulin suggests that repeated intake of New Zealand blackcurrant powder increases insulin sensitivity. This is the first observation of a high-anthocyanin containing berry powder to increase insulin sensitivity. Regular intake of New Zealand blackcurrant powder may be beneficial for the postprandial responses in people with type 2 diabetes or metabolic syndrome.