Abstract
Seeds play important roles in human nutrition and health since ancient time. The term “specialty” has recently been applied to seeds to describe high-value and/or uncommon food products. Since then, numerous studies have been conducted to identify various classes of bioactive compounds, including polyphenols in specialty seeds. This review discusses nutrients, fat-soluble bioactives, polyphenols/bioactives, antioxidant activity, bioavailability, health benefits, and safety/toxicology of commonly consumed eight specialty seeds, namely, black cumin, chia, hemp, flax, perilla, pumpkin, quinoa, and sesame. Scientific results from the existing literature published over the last decade have been compiled and discussed. These specialty seeds, having numerous fat-soluble bioactives and polyphenols, together with their corresponding antioxidant activities, have increasingly been consumed. Hence, these specialty seeds can be considered as a valuable source of dietary supplements and functional foods due to their health-promoting bioactive components, polyphenols, and corresponding antioxidant activities. The phytochemicals from these specialty seeds demonstrate bioavailability in humans with promising health benefits. Additional long-term and well-design human intervention trials are required to ascertain the health-promoting properties of these specialty seeds.
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Key Points
A systematic review conducted in 2017 identified 22 human intervention studies on flaxseed relevant to MetS risk. Trials have been conducted in healthy individuals or those with MetS, obesity, T2D, hypercholesterolemia or other chronic diseases. Interventions spanned 2 weeks to 12 months and utilized flaxseed‐enriched baked products or ground, milled, or raw flaxseed powders. Among these studies, there were no consistent effects of flaxseed consumption on MetS risk factors, possibly due to the varying doses, forms, and participant populations. A systematic review and meta‐analysis of RCTs reported that flaxseed consumption reduced both systolic blood pressure (SBP) and DBP. Among the 15 trials evaluated, the SBP WMD was −2.85 mmHg (95% CI: −5.37 to −0.33; P = 0.027); DBP WMD was −2.39 mmHg (95% CI: −3.78 to −0.99; P = 0.001). These trials evaluated flaxseed powder, oil, and lignan extracts, and flaxseed powder had the most consistent effects on SBP, whereas powder and oil reduced DBP.
Another systematic review and meta‐analysis of RCTs conducted with flaxseed for inflammation reported significant reduction of high‐sensitive C‐reactive protein (hs‐CRP) (n = 15 studies) and tumor necrosis factor‐alpha (TNF‐α) (n = 12), but not of IL‐6 (n = 11) and CRP (n = 19). The WMD of hs‐CRP was −0.75 (95% CI: −1.19 to −0.30; P < 0.001), whereas TNF‐α WMD was −0.38 (95% CI: −0.75 to −0.01; P = 0.04). Participants in these trials included healthy and overweight individuals, as well as individuals with obesity, T2D, polycystic ovary syndrome, NAFLD, prostate cancer, dyslipidemia, rheumatoid arthritis, and being treated with hemodialysis or recovering from burns. In subjects with BMI greater than 30, the changes reported hs‐CRP, TNF‐α, and CRP were all significant. These results are consistent with another systematic review and meta‐analysis of CRP.
Another systematic review and meta‐analysis of flaxseed consumption for glycemic control concluded that whole flaxseed consumption improved glycemic control. This effort identified 25 RCTs with a WMD for blood glucose of −2.94 mg/dL (95% CI: −5.31 to −0.56; P = 0.015). In subgroup analysis, whole flaxseed interventions were associated with improvements in blood glucose, whereas flaxseed oil and lignan extracts were not.
Among recent RCTs, consumption of 31.5 g of ground flaxseed with a high‐glucose beverage inhibited postprandial glucose in 15 healthy participants relative to the control. Ground flaxseed also increased postprandial fullness by 24% and decreased overall appetite by 8% relative to the control beverage. However, in another RCT, when 65 g of crushed and roasted flaxseed was incorporated into a mixed meal, it did not inhibit postprandial glucose relative to the control meal and beverage. Flaxseed contain several polyphenols, most of which render health promotion and possess strong antioxidant activities.