Effects of Daily Incorporation of Strawberries into the Diet on Bone Biomarkers and Blood Pressure in Postmenopausal Women with Pre- and Stage 1-Hypertension

Background and Significance: The postmenopausal period is characterized by ovarian hormone deficiency which increases the production of pro-inflammatory molecules with a concomitant rise in oxidative stress levels. This cascade of events negatively affect bone metabolism with a subsequent development of osteoporosis and increased risk of fractures. Ovarian hormone deficiency is also linked to greater incidence of cardiovascular disease (CVD), in part, due to compromised endothelial function which may result in elevated blood pressure (BP), a major modifiable risk for CVD. Although osteoporosis and hypertension are commonly regarded as two separate chronic diseases, their pathophysiology appear to be linked as prospective studies have demonstrated a correlation between decreased bone mineral density (BMD) and hypertension. Even though a number of medications are available to treat these conditions, they are not free of side effects. Thus, a considerable number of people prefer alternative and/or complementary therapies such as functional foods and dietary supplements over conventional treatment options. Epidemiological studies have demonstrated that consumption of fruits and vegetables is associated with decreased incidence of chronic diseases which is mainly attributed to the polyphenol content of these foods. Strawberries are widely consumed in the United States and are reported to have a high phenolic content which confers to them strong antioxidant, anti-inflammatory, and vasodilator properties. Objective: The main objective of this eight-week randomized, double-blind, placebo-controlled clinical trial was to investigate the extent to which daily consumption of strawberries would favorably alter blood biomarkers of bone metabolism and reduce BP by improving endothelial function in pre- and stage 1-hypertensive postmenopausal women. Furthermore, we aimed to determine whether increases in antioxidant capacity and decreases in inflammatory markers were, in part, responsible for strawberries' protective effects on bone and endothelium. Methods: Sixty women (age: 59.63 ± 5.31 years; body mass index [BMI]: 31.9 ± 4.8 kg/m2; BMD: 1.15 ± 0.13 g/cm2, systolic BP: 139 ± 13 mmHg, diastolic BP: 79 ± 7 mmHg [means ± SDs]) were randomly assigned to one of three groups (n=20 per group): (1) placebo-control, (2) 25g of freeze-dried strawberry powder (FDSP; equivalent to 1½ cups of sliced fresh strawberries), and (3) 50g of FDSP (equivalent to 3 cups of sliced fresh strawberries). Body composition and BMD of whole body, lumbar spine, hip, and forearm as well as body composition were measured by dual energy x-ray absorptiometry. BP was assessed using an automatic device. Blood biomarkers of bone metabolism, endothelial function, oxidative stress, and inflammation were assessed using immunoassay and colorimetric kits. Dietary and physical activity patterns were also monitored over the course of the study using validated questionnaires. Data were analyzed using Repeated Measures analysis of variance. Results: As anticipated, there were no significant differences in baseline characteristics among groups. No changes were noted for body composition and BMD in any of the intervention groups, as expected. Serum bone alkaline phosphatase levels, a marker of bone formation, were unaltered whereas a negative time effect (p =. 002) for serum levels of tartrate-resistant acid phosphatase-5b, a marker of bone resorption, was detected in the placebo-control and 25g FDSP groups. A negative time effect was noted for systolic BP in all three groups while diastolic BP was unaltered after eight weeks of treatment. Plasma levels of nitric oxide (NO), a vasodilator molecule, increased by 23% and 56% in the 25g and 50g FDSP groups, respectively, and decreased in the placebo-control group by 3% compared to baseline levels (p =. 04). Furthermore, levels of angiotensin II, a vasoconstrictor molecule, decreased in all three intervention groups (p =. 03). Lastly, a time-by-treatment interaction (p = .034) was detected for plasma levels of adiponectin, an anti-inflammatory molecule, which increased in the 25g and 50g FDSP groups by 5 and 6%, respectively, and decreased in the placebo-control group by 30% after eight weeks of supplementation. Conclusion: Daily consumption of strawberries for eight weeks did not alter BP, but improved plasma NO levels, though not statistically significant, as well as plasma levels of adiponectin which can be beneficial for bone and endothelial health. Further studies are warranted to examine the efficacy of long-term incorporation of strawberries into the diet on a number of health outcomes including bone and vascular health.