Curtis PJ, van der Velpen V, Berends L, et al. Blueberries improve biomarkers of cardiometabolic function in participants with metabolic syndrome—results from a 6 month, double-blind, randomized controlled trial. Am J Clin Nutr. 2019;109:1535-1545.
To examine the effect of 2 dietarily achievable levels of 6 months of blueberry intake vs placebo on insulin resistance, vascular function, lipid status, and anthocyanin metabolism in adults with metabolic syndrome (MS)
Double-blind, placebo-controlled, parallel study
The study included 138 men and women, aged 50-75 years, with BMI≥25 and 3 or more metabolic syndrome (MS) components (impaired fasting glucose, hypertension, central adiposity, hypertriglyceridemia, low HDL). Exclusion criteria were history of smoking, vascular disease, cancer, digestive, hepatic, or renal disorder, hypoglycemia, vasodilator use, or hormone replacement therapy. However, antihypertensive (≥6 mo) or statin (≥3 mo) therapies were allowed following habituation.
Participants were randomly divided into 3 groups: one group (n=37) consumed the equivalent of 1 cup blueberries per day; a second group (n=39) consumed the equivalent of ½ cup blueberries per day; the third group (n=39) consumed a placebo. Each intervention was isocaloric and carbohydrate-matched (glucose 31%, fructose 30%, sucrose 0%) and formulated as a milled, freeze-dried powder. The powder was provided in 26 g single-serve sachets, and participants were instructed to consume 1 sachet per day.
The sachet for group 1 contained 26 g blueberry powder, equivalent to 1 cup blueberries, providing 364 mg anthocyanin and 879 mg phenolics; sachet for group 2 contained 13 g blueberry powder + 13 g placebo, equivalent to ½ cup blueberries, providing 182 mg anthocyanin and 439 mg phenolics; and the sachet for group 3 contained 26 g placebo (produced by US National Food Lab). Placebo consisted of dextrose, maltodextrin, and fructose as a purple powder with blueberry aromatics and artificial color and flavor, and contained no anthocyanins or phenolics.
Participants were given 8 standardized recipe ideas for daily use, which included creating a blueberry drink or smoothie (using sachet powder added to a liquid); adding sachet powder to cereal, yogurt, dessert, or banana toast; or adding powder to salad dressing or vinaigrette. Compliance was calculated from returned wrappers, unused sachets, and questionnaire at 6 months.
Typically one thinks of blueberries as beneficial for macular degeneration and eye health, but this positive study should expand our horizons to include metabolic syndrome and patients at higher risk for cardiovascular events.
During the 21 days prior to start of trial and throughout the trial, dietary restrictions to limit anthocyanin-rich foods were incorporated; only 1 serving portion was allowed per week. Participants were advised to avoid strenuous exercise for 48 hours and anthocyanin-rich or nitrate-rich food, caffeine, or alcohol within 24 hours prior to each assessment visit. A standardized evening meal low in anthocyanin and flavonoids was provided prior to the overnight fast (≥10 hours) that preceded urine and blood tests.
Study Parameters Assessed
To monitor dietary adherence, participants completed a 131-item validated food frequency questionnaire at baseline, midpoint, and 6 months. Insulin resistance and insulin sensitivity were calculated from fasting insulin and glucose using homeostatic model assessment of insulin resistance (HOMA-IR) and quantitative insulin sensitivity index (QUICKI); a 2-step hyperinsulinemic euglycemic clamp substudy was also performed (at Cambridge University Hospital). Automated triplicated blood pressure (BP), 3-lead ECG brachial artery flow-mediated dilation (FMD), aortic distensibility, and arterial stiffness were also assessed, along with anthropometric measurements (weight, height, waist and hip circumference). Venous blood after ≥10-hour overnight fast was tested for glucose, lipoproteins, HbA1c, apolipoprotein A-I (apoA-I), apoB, plasma nitrite (NO2)-free and nitrate (NO3)-free thiols, cyclic guanosine monophosphate (cGMP), and fasting plasma insulin (measured in the clamp study). Anthocyanin-derived phenolic acids in serum and 24-hour urine sample were also measured; 72 metabolites were quantified.
Participants (N=138) were both male (68%) and female (32%), with a mean BMI of 31.2 and mostly having 3 (53%) or 4 (40%) MS components; 38% were on statins (for hyperlipidemia) and 24% were on antihypertensives. Intervention adherence was 82%, with only 18% correctly guessing their treatment allocation; 115 participants completed the study.
The interventions had no significant effect on HOMA-IR, QUICKI, HbA1c, or peripheral, hepatic, and adipose insulin tissue sensitivity.
At 6 months there were significant changes in the following measures:
- FMD increased (+1.45%) in the 1-cup group, remained unchanged in the ½-cup group, and fell (−0.39%) in the placebo group (P=0.003);
- Arterial stiffness decreased (−2.24%) in the 1-cup group and increased in the ½-cup (+0.45%) and placebo (+0.24%) groups (P=0.04).
- Plasma cGMP increased (+0.99 pmol/mL) in the 1-cup group, and fell in the ½-cup (–6.15 pmol/mL), and placebo (−6.75 pmol/m) groups (P=0.04).
There was no significant effect on BP, vascular function measurement, systemic redox status, or total free thiols.
High-density lipoprotein (HDL)-cholesterol significantly increased in the 1-cup group vs placebo (+0.04 mmol/L vs −0.02 mmol/L; P=0.03). When statin use was excluded there was a significant difference again in 1 cup vs placebo (+0.05 mmol/L vs −0.03 mmol/L; P=0.03). Nuclear magnetic resonance (NMR) spectroscopy analysis in statin nonusers revealed a significant increase in apoA-I and HDL-P at 6 months in 1 cup vs placebo, (P=0.002 and P=0.013, respectively). Triglycerides increased in ½ cup vs placebo (P=0.01), and this remained in statin nonusers (P=0.01). There was no significant effect on total cholesterol, low-density lipoprotein (LDL), and total cholesterol:HDL ratio.
Blueberry intake increased concentrations of anthocyanin-derived phenolic acid metabolites in serum and 24-hour urine (P<0.01 and P<0.001, respectively) in a dose-dependent manner. The change in individual metabolites was considered the byproduct of individual microbial catabolism and human phase II metabolism.
This is the longest blueberry randomized controlled trial (RCT) to date. It resulted in clinically relevant improvements in endothelial function, systemic arterial stiffness, and HDL-cholesterol, in both statin users and nonusers. It is presumed that increased cGMP, HDL particle density, and apoA-I levels were key to the improved vascular and lipid findings. Unfortunately, insulin resistance and peripheral, hepatic, and adipose tissue insulin sensitivity were unchanged. The 1.06% improvement in FMD vs placebo, based on meta-analyses, translated into a 13% reduction in risk for future cardiovascular events.1 Reduction in systemic arterial stiffness along with HDL improvements in statin nonusers equaled a 6.2%-9.3% reduced risk of coronary heart disease and a 11.4%-14.5% lower risk of cardiovascular disease in men and women, respectively.2 The JUPITER and the Kadoorie Biobank studies confirmed that a 1 standard deviation–increase of very large, large, and medium HDL particles significantly reduced myocardial infarction (MI) risk 13% and 20%, and a 1 standard deviation–increase in apoA-I reduced MI risk 11%.3
The greatest clinical benefits in this trial were from participants consuming the 1-cup blueberry equivalent. The ½-cup dose may have been insufficient to overcome the inflammatory effects of obesity in 6 months. It may also not be sufficient to overcome the gut microbiome associated with the inflammatory nature of obesity. Anthocyanins are extensively metabolized primarily in the lower bowel to which the serum and urine downstream metabolites responded in a dose-dependent manner. In cell and animal studies several of these metabolites increase endothelial levels of nitric oxide (NO) and have improved animal health and vasodilation when the animals are hypercholesterolemic and hypertensive,4-7 the so-called French paradox; high saturated fat intake yet low coronary heart disease and high wine intake that are rich in anthocyanins.
A key positive feature of this study is the inclusion of higher-risk patients,3 or more metabolic syndrome components, the 6 months’ duration, and the large number of factors evaluated, including secondary metabolites in blood and urine. This is one of the best thought-out and conducted studies I have recently read. In addition, to reduce confounding dietary factors there was excellent control of other sources of anthocyanidins in participants’ diets. We all consume, or should consume, anthocyanins daily.
A weakness of this study is that most patients were male and Caucasian. Also, the authors argue that 1 cup of blueberries per day is realistic, especially as a powder. One overlooked constituent not in the powder is fiber; fresh blueberries contain about 3.6 g fiber per cup, or 1.8 g per ½ cup. The total calorie content of 1 cup fresh blueberries is 85 (43 for ½ cup).8 Thus blueberries are concentrated in beneficial anthocyanins and other constituents compared to calories and compared to other fruit sources, and can easily fit into the 400 g fruit and vegetable per day recommended for a healthy diet. Blueberries also provide at least 13 minerals, depending on the soils on which they grow, especially manganese, potassium, and chromium.9 Typically one thinks of blueberries as beneficial for macular degeneration and eye health, but this positive study should expand our horizons to include metabolic syndrome and patients at higher risk for cardiovascular events.
Circulating cGMP represents the activity status of soluble guanylate cyclase in vascular smooth muscle coupling that is stimulated by endothelial NO. Anthocyanins enhance NO bioavailability. This finding was supported by a prior study of 12 weeks’ intake of 320 mg of purified anthocyanin in hypercholesterolemic patients, comparable to the 364 mg anthocyanin in the 1-cup group of this study.10
Prior short-term studies have had mixed results for HbA1C, insulin sensitivity, and glucose metabolism using 742 mg, 668 mg, and 581 mg of anthocyanins, respectively.11-13 The authors of this paper assumed the absence of significant results for HbA1c, insulin sensitivity and glucose metabolism was due to the higher risk patients in their trial compared to previous trials.
Blueberries as a powder in 1-cup and ½-cup dose equivalents were compared to placebo in mostly Caucasian male patients with obesity and 3 or more metabolic syndrome components for 6 months, including some patients on statin and antihypertensive therapy. The 1-cup dose containing 364 mg of anthocyanins and 879 mg of phenolics significantly increased HDL-cholesterol and improved vascular endothelial function, arterial stiffness, cGMP, and apoA-I, which could potentially reduce cardiovascular event risk by 11% or more. There was no effect on HbA1c, insulin sensitivity, fasting glucose, or other lipids. No adverse events were reported and the dose was considered to be achievable within a normal diet.