Blueberries are tasty, but are they good for our hearts?
For that reason, the United States Highbush Blueberry Council helped fund a study to investigate blueberries' potential benefit to heart health.
Researchers from the University of East Anglia in the United Kingdom teamed up with scientists from Harvard University in Cambridge, MA.
In particular, they wanted to understand whether regularly consuming blueberries could alter the metabolic profile of people with metabolic syndrome.
Metabolic syndrome describes a cluster of conditions that include high blood pressure, excess body fat around the waist, high blood sugar levels, and abnormal cholesterol and triglyceride levels. Together, these factors increase the risk of cardiovascular disease and type 2 diabetes.
Blueberries and anthocyanins
"Previous studies have indicated that people who regularly eat blueberries have a reduced risk of developing conditions including type 2 diabetes and cardiovascular disease," says lead researcher Prof. Aedin Cassidy.
"This," she says, "may be because blueberries are high in naturally occurring compounds called anthocyanins."
Anthocyanins are water soluble pigments that can appear red, black, blue, or purple. These flavonoids are present in the tissues — including the stems, leaves, flowers, roots, and fruits — of many higher plants.
However, to date, much of the research has taken place over a relatively short period; in fact, some studies looked at the consumption of just a single portion of blueberries.
There have also been no randomized controlled trials investigating blueberries' potential to protect against disease in a population with a higher risk of developing type 2 diabetes and cardiovascular disease.
Prof. Cassidy says, "We wanted to find out whether eating blueberries could help people who have already been identified as being at risk of developing these sort of conditions."
Testing blueberry powder
To investigate, the team recruited 115 participants, ages 50–75, all of whom were either overweight or obese and had metabolic syndrome. The study ran for 6 months, making it the longest of its kind.
Importantly, the scientists used "dietarily achievable levels" of blueberries rather than expecting the participants to consume an unsustainable and unrealistic amount of blueberries each day.
They split the participants into three groups:
- One group consumed freeze-dried powdered blueberries equivalent to 1 cup (150 grams) of fresh blueberries per day.
- Another group consumed freeze-dried powdered blueberries equivalent to half a cup (75 grams) of fresh blueberries per day.
- The final group acted as a control group; they received a powder that looked similar to blueberry powder but which primarily contained dextrose, maltodextrin, and fructose.
At the start and end of the trial, the researchers assessed biomarkers for insulin resistance, lipid status, and vascular function. They recently published their findings in the American Journal of Clinical Nutrition.
"We found that eating 1 cup of blueberries per day resulted in sustained improvements in vascular function and arterial stiffness — making enough of a difference to reduce the risk of cardiovascular disease by between 12 and 15%."
Co-lead study author Dr. Peter Curtis
Interestingly, the scientists only saw the benefits in the group consuming 1 cup of blueberries per day — not in those consuming half a cup.
Dr. Curtis believes that this is because "higher daily intakes may be needed for heart health benefits in obese, at-risk populations, compared with the general population."
It is also worth noting that the blueberry intervention did not alter the other parameters the scientists measured. The authors write:
"No favorable effects of the intervention were shown for the primary endpoint [insulin sensitivity] or indices of glucose control. [...] The intervention had no effect on [blood pressure] or other biomarkers of vascular function."
Explaining the benefits of blueberries
The scientists believe that the cardiovascular benefits they saw are primarily due to the presence of anthocyanins in blueberries.
In the lower intestine, the body metabolizes anthocyanins to produce a range of chemicals; some of these chemicals provide sustenance to the resident gut bacteria and are "likely play a key beneficial metabolic role," say the study authors.
They offer some examples. For instance, some researchers have shown that syringic acid, which is a chemical that the metabolism of anthocyanin produces, benefits vascular endothelial cells in the laboratory.
Dr. Curtis concludes, "The simple and attainable message is to consume 1 cup of blueberries daily to improve cardiovascular health."
At this point, it is worth mentioning that other foods contain anthocyanin, including blackcurrants, black and red raspberries, blackberries, red cabbage, plums, red radish, black carrot, and purple potato.
Although this project was the first long-term, placebo-controlled study to look at blueberries and cardiovascular and metabolic health, it is important to remember that only 115 participants completed this trial.
By the end, only 37 participants remained in the group consuming 1 cup of blueberries per day.
Blueberries are likely to be a healthful addition to any diet, as are most other fruits and vegetables. However, scientists will need to carry out larger studies to confirm the clinical benefits of blueberries.
Because the U.S. Highbush Blueberry Council are dedicated to "driving consumer demand," more research is likely to follow.