This open access 2018 review by Paolo Governa and co-workers from the University of Siena and the Italian Society of Phytotherapy provides an overview of the use of medicinal plants in the management of diabetes, with particular regard to evidence of clinical effectiveness of medicinal plants in controlling diabetes-related symptoms.
The authors emphasise the following species enlisted in WHO monographs with indication of use for diabetes:
Holy basil, Ocimum tenuiflorum L., leaves.
Fenugreek, Trigonella foenum-graecum L., seeds.
Onion, Allium cepa L., bulb.
Neem, Azadirachta indica A. Juss., leaves.
Bitter melon, Momordica charantia L., fruit.
Korean ginseng, Panax ginseng C.A. Meyer, roots.
American ginseng, Panax quinquefolius L., roots.
Rehmannia glutinosa (Gaertn.) DC., roots.
Many of these are used of diabetes in traditional medical systems or described in pharmacopoeias for this use. The authors consider that for the first two, holy basil and fenugreek, their use in diabetes is supported by clinical data.
The authors also discuss some other species which are attracting the interest of the scientific community for their promise in treating diabetes.
The authors comment that there is a crucial need for stronger evidence-based data.
Governa P, Baini G, Borgonetti V, Cettolin G, Giachetti D, Magnano AR, Miraldi E, Biagi M. Phytotherapy in the Management of Diabetes: A Review. Molecules. 2018; 23(1):105. https://doi.org/10.3390/molecules23010105
This intriguing double-blind, randomized, placebo-controlled trial appears to indicate that propolis as an adjunct to selective serotonin reuptake inhibitors (SSRIs), a type of antidepressant medication, improves depressive symptoms over and above SSRIs alone.
Caveat: I have only been able to access the abstract of this study, so have not been able to assess the methodology. The abstract may be accessed here.
Varzaghani, V., Sharifi, M., Hajiaghaee, R., Bagheri, S., Momtaz, S., Tarassoli, Z., & Razmi, A. (2022). Propolis add-on therapy alleviates depressive symptoms; A randomized placebo-controlled clinical trial. Phytotherapy Research, 1– 10. https://doi.org/10.1002/ptr.7380.
Medour et al. (2002) carried out an ethnobotanical survey of medicinal plant use in two rural municipalities in the province of Bouira, Northern Algeria. This is a biodiverse, mountainous, Berber-speaking region with a rich ethnobotanical knowledge system.
Data were compiled from 69 informants among the local population and on 136 plant species. Data were gathered and data analyses were provided on:
Socio-demographic profile of the informants.
Diversity of medicinal plants.
Plant parts used, mode of preparation and administration.
Diseases groups, treated diseases and number of use reports.
Relative frequency of citation of the plant species recorded.
Frequency of use of the plant species recorded.
The percentage of informants claiming the use of a certain plant species for the same major purpose.
Consensus among informants for plant use for the different disease categories recorded.
Among the many interesting data provided in this quantitative survey, the most interesting for me were those on the most used medicinal plants for various types of symptoms or conditions. These are summarised in the table below, taken from the cited paper .
Of particular interest to me personally are the reported uses of several plants commonly found in my own bio-region of Ibiza, Balearic islands, Spain, namely:
 Meddour, R., Sahar, O., Abdoune, N., & Dermouche, M. (2022). Quantitative ethnobotanical investigation of medicinal plants used by local population in the rural municipalities of Haizer and El Asnam, province of Bouira, Northern Algeria. Mediterranean Botany, 43, e71190. https://doi.org/10.5209/mbot.71190
This interesting paper presents an overview of the existing literature published since the year 2000, seeks to identify some repeatedly found seasonal trends and discusses some possible explanations for these trends.
3. Physiological Effects of Phytochemicals from C. sativum
Flavonoids: A flavonoid-rich fraction was found to have hypotensive activity.
Quercetin (a flavonoid): A quercetin-rich aqueous ethanolic extract inhibits α-amylase, α-glucosidase and lipase, and thus potentially has antidiabetic and anti-obesity effects.
Polyphenols: A polyphenol-rich extract inhibits angiotensin-converting enzyme thus potentially has a antihypertensive effect.
Isocoumarins: Isocoumarin aglycones and (to a lesser degree) isocoumarin glycosides (cilantroside A and B) have been found to have neurotrophic / neuroprotective effects by stimulation of nerve growth factor. The aglycones of isocoumarins also showed anti-inflammatory effects.
Phenolic glycosides: The phenolic glycosides daphnin and benzyl-O-β-d-glucoside have also been found to stimulate nerve growth factor.
Sterols: Plant sterols have hypocholesterolaemic effects.
Essential oil: Prominent activities against diabetes, microbial infections, and inhibitory to acetylcholinterase.
Other: A linalool, ascorbyl palmitate and petroselinic acid-rich petroleum ether extract of coriander seeds reduces oxidative stress, is hypolipidaemic, hypoglycaemic, and preventative against diabetic nephropathy.
4. Cardiovascular Benefits of C. sativum
A systematic review was carried out of studies investigating the potential cardiovascular benefits of C. sativum.
Studies have demonstrated the cardioprotective benefits of C. sativum. These include its effect as an antioxidant, antihypertensive, anti-atherogenic, antiarrhythmic, as well as the improvement of other factors that may lead to cardiovascular disease (CVD), such as altered lipid profile, hyperglycaemia and cardiac biomarkers or enzymes.
Most of the studies included in the review were in vivo studies carried out on laboratory animals. Only two were human studies. These latter demonstrated hypolipidaemic, hypocholesterolaemic, hypotensive and antioxidant effects of coriander seed powder. As to plant parts, the majority of the studies included investigated the effects of the seeds. The two studies on the leaves showed hypolipidaemic, hypotensive, normoglycaemic and antioxidant effects.
The authors comment that more in vitro studies are needed to elucidate mechanisms of action.