Cinnamon is a spice that contains several bioactive agents. Cinnamaldehydes give cinnamon its aroma, Coumarins (a toxin) contribute to taste, and several compounds including MethylHydroxyChalcone polymers (MHCPs) contribute to its systemic insulin sensitizing benefits. Beyond the three unique compounds listed, cinnamon also contains tannins, flavonoids, glycosides, terpenoids and anthraquinones.
Cinnamon exerts beneficial control effects against pro-diabetic diets in a number of ways.
Cinnamon can inhibit numerous digestive enzymes, such as alpha-glucosidase, sucrase and potentially pancreatic amylase (although the only results were confounded with acarbose). Via inhibition of these enzymes, cinnamon can decrease the influx of glucose into systemic circulation and avoid overly significant insulin spikes.
In systemic circulation (beyond the liver) cinnamon also possesses anti-diabetic effects. A compound in cinnamon, methylhydroxychalcone polymer (MHCP), acts as an insulin mimetic on adipocytes. MHCP's effects as an insulin mimetic are dose dependent, and act by transphosphorlyating the insulin receptor on the cytoplasmic membrance (the same mechanism as the insulin molecule itself). Its effects on glucose uptake and glycogen, however dose-dependent, seem to be time-delayed (When insulin acts within 10 minutes of reaching the cell, MHCPs take 30-60, suggesting an intra-cellular time delay).
Cinnamon has also been implicating in aiding insulin function, potentiating its effects more than 20-fold in vitro.
When ingested in human trials, cinnamon shows much promise in reducing blood glucose levels and sometimes markers of lipid metabolism (LDL, Triglycerides, Total cholesterol). There are also intervention studies noting improved insulin sensitivity with cinnamon extract, possibly vicariously through the reduced blood glucose levels.
Coumarin is a hepatotoxic and carcinogenic phytochemical found in some plants that is present at high levels in certain variants of cinnamon. Coumarin is not the active compound that reduces blood sugar, but one that exists alongside the active ingredient(s). It initially had a TDI (Tolerable Daily Intake) of 2mg/kg bodyweight max, but was lowered to 0.5 and currently stands at 0.1mg/kg bodyweight. Although a safety buffer is included in this last recommendation, some subsets of the human population are more sensitive to coumarin toxicity due to a reduced capacity to metabolize it.
This is relevant since most of the anti-diabetic benefits with cinnamon come in a dose dependent manner, in the range of 300mg/kg bodyweight. At these doses, coumarin above the TDI can easily be ingested.
The best method of coumarin avoidance is to choose the right source of cinnamon. Ceylon cinnamon has the lowest levels of coumarin with below 190 mg/kg (some samples being below detection levels) whereas Cassia contains between 700 mg/kg on the low-end to as much as 12,230 mg/kg on the high-end.. Ceylon can be detected in stick form via its numerous thin folds, whereas Cassia has less folds and a thicker appearance. They cannot be distinguished in powder form, and Cassia is more frequently used in production and manufacturing due to its general high availability and low cost.
Via the above numbers, a 200lb human can ingest 47.8g of Ceylon Cinnamon and arrive at the 0.1mg/kg bodyweight TDI for coumarin at worst (using the highest recorded dose of coumarin in cinnamon). on the other hand, using Cassia cinnamon can easily place somebody above the TDI for coumarin with a far lesser intake.
Coumarin absorption does not seem to be dependent on the form of cinnamon ingested. Similar serum levels and excreted levels were achieved with isolated coumarin, pill form cinnamon, tea and rice pudding (solid food). These results were standardized to X dose of coumarin, so the source of cinnamon is irrelevant.
The above study, however, did note a 38.5% extraction rate of coumarin from powder to liquid when steeped (just boiling for 30 minutes); suggesting that one can tip the scales more in favor of water-soluble polyphenols and MHCPs relative to coumarins if steeped and served in tea or using said cinnamon water to mix protein shakes with, as the water-soluble components have a much higher extraction rate.