Essential oils (EOs) are mixtures of volatile molecules endowed with health-promoting biological activities that go beyond their role as aromas and natural preservatives and can be exploited to develop functional foods and diet supplements. Some of the potential health benefit of human diet supplementation with EOs are in the area of: (1) irritable bowel syndrome; (2) inflammatory bowel disease; (3) regulation of microbiota; (4) gastroprotection; (5) hepatoprotection; (6) protection of the urinary tract and diuresis; (7) management of metabolic disorders including hyperglycemia and hyperlipidemia; (8) anti-inflammatory and pain control; (9) immunomodulation and protection from influenza; and (10) neuroprotection and modulation of mood and cognitive performance.
Inhibition of carbohydrate hydrolyzing enzymes α-amilase and α-glucosidase is a key target for oral hypoglycemic treatments for type 2 diabetes mellitus (T2DM). While pancreatic α-amylase hydrolyzes starch into oligosaccharides, these are further hydrolyzed by intestinal α-glucosidase into glucose, which is finally absorbed into systemic circulation, raising the plasma glucose level. In T2DM subjects with reduced glucose tolerance, postprandial plasma glucose rise can be smoothed by enzyme inhibitors, which slow down carbohydrate digestion, ameliorating their condition. In a comparative study on several terpenoid EO components, citral was the only one that showed mild inhibition of α-amilase compared to the reference drug acarbose. Instead, several monoterpenes (10 mM) showed inhibition of α-glucosidase with the following order of potency: (R)-(+)-limonene = (S)-(−)-perillyl alcohol > α-terpineol, while (R)-(+)-β-citronellol, terpinolene, citral, (R)-(−)-linalool, nerol, geraniol, and (S)-(−)-β-citronellol exerted weaker inhibition, and (L)-menthol and γ-terpinene did not show any significant α-glucosidase-inhibitory activity [31]. The same terpenes were tested for stimulation of glucose uptake in 3T3-L1 adipocytes, which represents another strategy to reduce glucose plasma levels. Geraniol, citral, limonene, and (R)-(+)-β-citronellol (1 μM) had the highest activity, while nerol, (S)-(−)-perillyl alcohol, γ-terpinene, and α-terpineol were weaker stimulants; and (S)-(−)-b-citronellol, terpinolene, and linalool did not affect the glucose uptake [32]. Thus, to a different degree, several terpenes of widespread presence in EOs, such as limonene, citral, and citronellol, have hypoglycemic activity, which helps rationalize the efficacy recorded for the whole oils.
High levels of circulating total cholesterol (TC), of low-density lipoprotein-cholesterol (LDL-C), and of plasma triglycerides along with low levels of high-density lipoprotein-cholesterol represent well-established risk factors for cardiovascular disease and might be associated with diabetes or dysregulated glycemic parameters in metabolic syndrome. Treating these unbalances with suitable dietary approaches is not less important than addressing hyperglycemia alone. Some EOs, most notably green and black cumin, show parallel reduction of lipidemic factors along with glycemic.
A recent literature survey concerning the immunostimulant activity of EOs pointed out some differences among studies depending on their type, distinguishing in vitro, pre-clinical, clinical, or those based on diet supplementation of animals. It suggested that further studies are needed to clarity the matter although a few EOs were found to stand out in all families of studies: eucalypt and ginger [42]. However, other EOs have accumulated evidence for their efficacy in many studies, including human; these are thyme, lavender, clove, tea tree, and citruses (lemon, orange, and bergamot) [36]. The mechanism of immune stimulation is often complex to rationalize, as the same EO typically also expresses anti-inflammatory activity through the down-regulation of pro-inflammatory cytokines, and these, in principle, should stimulate immune response. Although the modulation varies with the actual EO and, even within the same botanical species, with the actual composition of the EO used in the study (i.e., with the chemotype), often, immune stimulation is linked to boosting innate immune response, and it is associated with interfering with the NFkB, p38, or ERK/MAPK signaling pathways [36]. Eucalyptus EO, with prevalent 1,8-cyneole content, was found to increase phagocytosis by inducing monocyte-derived macrophages with increased phagocytic activity. The activity of tea tree oil rich in tepinen-4-ol was attributed to activation of NF-kB factor, which increases phagocytic activity. Both tea tree oil and terpinen-4-ol induce the differentiation of immature myelocytes into active phagocytizing monocytes and increase the expression of CD11b, a receptor that is partially responsible for the phagocytosis of opsonized bacteria and fungi by leukocytes [36].
With the global increase of life expectancy, the prevalence of age-related diseases is also increasing, particularly including neurodegenerative conditions such as Alzheimer’s disease (AD). Neurodegenerative diseases and other chronic inflammatory conditions characterized by cognitive impairment are related to lifestyle and to the diet; hence, a dietary intervention can help their course and patients’ quality of life. In AD, the two main microscopic hallmarks of disease, the abnormal accumulation of extracellular protein material in the amyloid–beta plaques and the formation of intracellular neurofibrillary tangles, are associated with abnormal expression of acetylcholinesterase (AChE), which leads to decreased levels of neurotransmitter acetylcholine (ACh), synaptic alteration, and impaired memory and learning [43]. Increased AChE expression appears to be related both to the formation of plaques and to the impaired cognitive function; hence, inhibiting AChE is currently the main pharmacological strategy to treat AD [43][44]. On the other hand, research efforts are clarifying the association between AD, inflammation, and oxidative stress, outlining that antioxidants are key to effective AD treatment strategies [44][45][46]. One additional point is the association between AD and impaired glycemic control, with increased incidence of T2DM in patients with dementia [43]. The above constitutes a solid rational basis to understand the potential of dietary EO supplementation in protecting from neurodegenerative conditions such as AD because (1) many EOs have excellent anti-inflammatory activity with low incidence of side effects; (2) many EOs have good antioxidant activity; (3) many EOs are effective in improving glycemic control and T2DM parameters; and (4) EOs are small, lipophilic molecules with high diffusivity, and hence, they have excellent ability to cross the blood–brain barrier and reach the central nervous system. A fifth reason is that some EOs [47][48] and EO components [47][48][49] have been demonstrated to possess good inhibition activity toward AChE, thereby representing a promising aid to tread AD and other cognitive-impairment conditions [49]. Therefore, several essential oils have the potential to ameliorate the cognitive function and protect from AD; these include thyme, sage, eucalypt, lemon balm, peppermint, oregano, rosemary, lavender, basil, and citrus EOs (rich in limonene and citral) owing to the combination of glucose-lowering, anti-inflammatory, antioxidant, and anti-AChE activities [43][47][48][49].
This entry is adapted from the peer-reviewed paper 10.3390/molecules28020901