Cooking and In Vitro Digestion Modulate the Anti-Diabetic Properties of Red-Skinned Onion and Dark Purple Eggplant Phenolic Compounds

Abstract

The intake of phenolic-rich foods is an emerging preventive approach for the management of type 2 diabetes, thanks to the ability of these compounds to inhibit some key metabolic enzymes. In this study, the influence of cooking and in vitro digestion on the α-glucosidase, α-amylase, and dipeptidyl-peptidase IV (DPP-IV) inhibitory activity of red-skinned onion (RSO) and dark purple eggplant (DPE) phenolic fractions was assessed. The applied cooking procedures had different influences on the total and individual phenolic compounds gastrointestinal bioaccessibility. DPE in vitro digested phenolic fractions displayed no inhibitory activity versus α-amylase and DPP-IV, whereas the fried DPE sample exhibited moderate inhibitory activity against α-glucosidase. This sample mainly contained hydroxycinnamic acid amides that can be responsible for the observed effect. Contrariwise, raw and cooked in vitro digested RSO phenolic fractions inhibited all three enzymes but with different effectiveness. Fried and raw RSO samples were the most active against them. Statistical analysis pointed out that quercetin mono-hexosides (mainly quercetin-4′-O-hexoside) were responsible for the inhibition of α-glucosidase, whereas quercetin di-hexosides (mainly quercetin-3-O-hexoside-4′-O-hexoside) were responsible for the DPP-IV-inhibitory activity of RSO samples. An accurate design of the cooking methods could be essential to maximize the release of individual phenolic compounds and the related bioactivities.

Examination of Primary and Secondary Metabolites Associated with a Plant-Based Diet and Their Impact on Human Health

“…Glucosinolates Anti-inflammatory activity [83] Eggplant (Solanum melongena) Phenolic compounds Antidiabetic activity [84] Solasodine Anticancer activity [85] Ginger (Zingiber officinale) and mulberry fruit (Morus alba) Extract Brain health and oxidative stress [86] Grape (Vitis spp.) Phenols, proanthocyanididns Gut health [87][88][89] Hemp grain (Cannabis sativa) Phenolics Antioxidant activity [90,91] Lettuce (Lactuca sativa) Polyphenols Antidiabetic activity [92] Manchurian walnut (Juglans mandshurica) Hydrolyzed peptides Antidiabetic activity [93] Oats (Avena sativa) Phenolics Antioxidant, prebiotic [12,94] Olive oil (Olea europaea) Polyphenols Cardiovascular activity [95] Oolong tea (Camellia sinensis) (−)-epigallocatechin Gut health [96] Quinoa (Chenopodium quinoa) Bioactive peptides Antidiabetic and anticancer [97,98] Seaweed (Porphyra tenera) Extract Gut health, antioxidant [99,100] Sorghum (Sorghum bicolor) Phenolic Anticancer activity [101,102] Soybean (Glycine max) Isoflavones Anticancer activity [103,104] Isoflavonoid Insulinotropic activity [105] Oolong tea (Camellia sinensis)…”

“…Red-skinned onion and purple eggplant are other sources of phenolic compounds with possible antidiabetic properties. Cattivelli et al [84] found that cooking increased the bioavailability of phenolic compounds from the onion and eggplant, which improves their action against the risks of type 2 diabetes. The study found that certain compounds in the onion inhibited three of the key enzymes that are involved in diabetes pathogenesis [84].…”

“…Cattivelli et al [84] found that cooking increased the bioavailability of phenolic compounds from the onion and eggplant, which improves their action against the risks of type 2 diabetes. The study found that certain compounds in the onion inhibited three of the key enzymes that are involved in diabetes pathogenesis [84]. Another study was conducted on the effects of novel peptides derived from quinoa.…”

Impact of cooking methods of red-skinned onion on metabolic transformation of phenolic compounds and gut microbiota changes

“…Generally, onion can be consumed, in the daily diet, either raw or cooked through different methods such as grilling and frying, which are the two most usual cooking procedures applied to onion. Several studies evidenced that heat treatments during cooking may modify the phenolic profile and structures of vegetables foods resulting in a different and sometimes higher bioactivity, and improved bioaccessibility after in vitro gastrointestinal digestion 1,6,7 .…”

“…Several studies evidenced that heat treatments during cooking may modify the phenolic profile and structures of vegetables foods resulting in a different and sometimes higher bioactivity and improved bioaccessibility after in vitro gastro-intestinal digestion. 1,6,7 Additionally, these modified compounds undergo an ulterior transformation when they enter the colon, where they can feed the colon microbiota to foster beneficial bacteria and inhibit opportunis-tic microbes. 8 Unfortunately, this improvement can also have the opposite impact on the colon microbiota, adversely affecting sensitive beneficial bacteria, such as bifidobacteria.…”