Could blackcurrant and citrus extracts help prevent type 2 diabetes?

In a recent study published in the journal Food & Function, a team of European researchers explored the role of blackcurrant (BC) extracts in reducing blood sugar or glycemic levels after meals and investigated whether fruit fiber can enhance this effect. Their results indicate that while BC reduces glycemic levels immediately after meals, higher doses are no more effective than lower doses, and fruit fibers do not augment the effect.

Study: Acute effects of drinks containing blackcurrant and citrus (poly)phenols and dietary fibre on postprandial glycaemia, gut hormones, cognitive function and appetite in healthy adults: two randomised controlled trials. ​​​​​​​Image Credit: Created with the assistance of DALL·E 3

Background

The burden of type 2 diabetes (T2D), both in terms of prevalence and associated mortality, has been rising globally. In Europe alone, one-fifth of adults are estimated to be prediabetic and could progress to T2D within the next decade. Therefore, identifying effective interventions to prevent this progression is a public health priority.

People who regularly experience elevated levels of blood glucose after meals, also known as high postprandial glycemia, are at a higher risk of developing T2D as well as certain heart diseases. Dietary adjustments, particularly by including polyphenols and fruit fibers such as pectins, may reduce postprandial glycemia and promote glycemic control in the long term. Scientists believe this is because polyphenols slow the rate at which glucose is absorbed into the blood, and fruit fibers slow the rate at which the stomach empties.

About the study

Researchers conducted two separate trials between May 2017 and January 2019.

First, they hypothesized that if combined, fruit fibers and polyphenol extracts would be more effective than either component separately in reducing blood glucose levels. This was tested in a trial called the GLU-FX study, where they examined the effect of BC and sweet orange (SO) polyphenol extracts on postprandial glucose levels, insulinemia, post-meal appetite, and gut hormone secretions.

Participants in the GLU-FX trial were randomized to a sequence of treatments, where they were assigned a placebo (0 mg polyphenols) in one period and given three out of four test drinks during the rest of the study. These drinks included (1) L-BC (800 mg polyphenols), (2) H-BC (1600 mg polyphenols), (3) L-SO (800 mg SO), and (4) H-Blend (800 mg BC and 800 mg SO). There was a ‘wash-out’ period of at least one week between treatments.

In the second trial, the GLU-MIX study, researchers hypothesized that combining fiber-rich citrus pulp and BC would reduce the area under the blood glucose curve (iAUC) between 0 and 30 minutes more than a citrus pulp individually. They tested this by examining the effect of drinks containing only fiber and BC + fiber on postprandial glucose levels, insulinemia, post-meal appetite, and cognitive functioning. The drinks provided in this trial included (1) a control with no polyphenols or fiber, (2) 1.5 g total fiber (F), and (3) 800 mg BC and 1.5 g total fiber (BC + F).

In both trials, researchers collected blood for analysis at the time participants drank the beverages and at different time points between ten minutes and two hours after consumption. Participants also filled up questionnaires with details about their mood and satiation levels at different time points and how palatable the drink was 10 minutes after they consumed it. GLU-MIX participants also completed a cognitive battery test 45 minutes and 160 minutes after consumption and were given a carbohydrate-rich meal 215 minutes after drinking the test beverages.

Participants were aged between 18 and 70 years, with a body mass index (BMI) between 18 and 35. Their good health was determined by the fact that none of them had cancer, allergies, sensitivities, or conditions such as phenylketonuria. Individuals who had donated blood in the 12 months preceding the study or had medical tests with abnormal results were excluded, as were smokers and those with a history of alcoholism or substance abuse.

Findings

In the GLU-FX trial, 29 participants completed the study, of whom 60% were female, and more than half were white. On average, they were 37 years old. Researchers found that the L-BC beverage significantly reduced blood glucose levels compared to the placebo in the first 30 minutes after consumption but not afterward.

Both L-BC and H-BC reduced blood insulin levels and C-peptide concentrations compared to the control in the first 30 minutes, but not beyond. There were no significant differences in mood and appetite scores. Participants did note that L-BC and H-blend were more difficult to drink and that the L-SO drink had a more pleasant texture.

In the GLU-MIX trial, 37 completed the trial, and participant characteristics were very similar to the GLU-FX population. Researchers found that blood glucose levels were significantly reduced by the BC + F beverage in the first 30 minutes after consumption. Similarly, BC + F was associated with lower insulin levels and C-peptide concentrations.

The BC + F beverage also reduced reaction times and accuracy in the cognitive tests. There were no significant differences in appetite, mood, or energy intake, but participants noted that they were thirstier after drinking BC + F. The test drinks were considered harder to consume and less pleasant than the control, and BC + F was considered more bitter.

Conclusions

This study aimed to assess whether combining dietary fiber and polyphenol-rich fruit extracts is more effective on blood glucose and insulin levels, gut hormones, mood, appetite, and cognitive function. Researchers found no evidence of a linear relationship between polyphenol dose and early postprandial glucose levels and no evidence that fiber-rich fruit extracts could enhance the effect of polyphenols.

However, the authors caution against treating these findings as conclusive. The fact that only healthy individuals were included in the trial is a limitation, as these populations can clear circulating glucose from their bloodstream faster. The simple meals provided in the trial did not include various macronutrients, which could enhance the effects of polyphenols on blood sugar levels. Another limitation was that individual participants did not receive every treatment.

Despite the limitations, this study provides interesting insights and avenues for further research. A key priority for future work is moving beyond healthy populations and exploring the role of polyphenols in reducing blood glucose levels in prediabetic populations.