Reviewed: Allen RW, Schwartzman E, Baker WL, Coleman CI, Phung OJ. Cinnamon use in type 2 diabetes: an updated systematic review and meta-analysis. Ann Fam Med. 2013;11(5):452-459.

Type 2 diabetes results in impaired glycemic control and potential complications from cardiovascular disease. The main forms of treatment for type 2 diabetes include medication, lifestyle adjustments, and dietary modifications. Dietary supplements are not generally recommended as a treatment by the American Diabetes Association because the organization believes that the clinical efficacy of supplements has not been sufficiently established and because there is a lack of “standardized formulations.” (Presumably, this refers to the fact that there are various types of cinnamon formulations, for example, that have shown some degree of clinical efficacy, and — unlike single chemical entity pharmaceutical medications — cinnamon dietary supplements contain numerous naturally occurring phytochemicals, more than one of which may contribute to the formulation’s physiological effect.) 

Cinnamon (Cinnamomum spp., Lauraceae) has been demonstrated in basic research studies to lower serum lipids and blood glucose, promote insulin release, enhance insulin sensitivity, increase insulin disposal, and regulate protein-tyrosine phosphatase 1β and insulin receptor kinase. In studies with diabetic rats, cinnamon extracts were shown to increase the expression of peroxisome proliferator-activated receptors (PPARs). PPARs are targeted in therapies for diabetes and hyperlipidemia (high levels of blood lipids, e.g., cholesterol) and have been shown to increase insulin sensitivity and high-density lipoprotein cholesterol (HDL-C) levels in addition to decreasing triglyceride levels.

Evidence from previous human clinical trials has shown conflicting effects of cinnamon on blood glucose and lipids. The authors of the this review found no significant relationship between cinnamon and measures of type 2 diabetes in a previous meta-analysis published in 2008.1 For their current paper, they repeated the meta-analysis with the addition of five new randomized, controlled studies.

The databases MEDLINE®, Embase™, and Cochrane Central Register of Controlled Trials (CENTRAL) were searched through February 2012 using the keywords cinnamon and diabetes. Studies were considered for inclusion in the analysis if they were randomized, controlled trials that investigated type 2 diabetes using cinnamon supplements in any dose or form. The studies had to have one of the following dependent variables measured to be considered for analysis: glycosylated hemoglobin (hemoglobin A_1c), fasting plasma glucose, total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), HDL-C, or triglycerides (TGs). All of the potentially relevant articles were reviewed by two of the investigators. Author identification, year of publication, study design, source of study funding, study duration and follow-up, study population, sample size, time since diagnosis, cinnamon dose, product name and brand, formulation used, and cinnamon species were recorded. Baseline data for hemoglobin A_1c level, fasting plasma glucose level, body weight, body mass index, TC, LDL-C, HDL-C, and TGs were recorded. The studies were checked for bias with the Cochrane Risk of Bias Tool. The mean change from baseline for levels of hemoglobin A_1c, fasting plasma glucose, TC, LDL-C, HDL-C, and TGs were analyzed with Comprehensive Meta-Analysis software, version 2 (Biostat). Both heterogeneity among studies and publication bias were calculated. The effects of cinnamon dosage and form were calculated with subgroup analysis.

Ten studies met the criteria imposed by the authors (n=543 subjects). Of these, eight trials had measures of hemoglobin A_1c, LDL-C, and HDL-C, and eight trials had measures of fasting plasma glucose, TC, and TGs. Eight of the studies used formulations of C. aromaticum (syn. C. cassia) and two did not state the species of cinnamon. The dosage of cinnamon ranged from 120 mg per day to six grams per day. There was variation among the studies as to when the cinnamon supplements were administered relative to meals. In seven studies, participants took the cinnamon supplements with a meal. In the remaining studies, participants took supplements either before (one study) or after a meal (two studies).

Cinnamon supplements significantly reduced fasting plasma glucose by a weighted average of 24.59 mg/dL. This is a slightly greater reduction than that seen with the antihyperglycemic drug sitagliptin (-16 to -21 mg/dL), but considerably lower than that found with metformin monotherapy (-58 mg/dL). LDL-C and TGs also were reduced in patients who took cinnamon supplements when compared to control patients (-9.4 mg/dL and -29.6 mg/dL, respectively). Again, these reductions are considerably less than in patients on conventional medications; pravastatin and gemfibrozil have been shown to decrease both LDL-C and TGs by -50 mg/dL. HDL-C was significantly increased (1.66 mg/dL) in patients who took cinnamon supplements. There was no effect of cinnamon on hemoglobin A_1c levels; however, when only capsule formulations of cinnamon were considered, hemoglobin A_1c was significantly decreased (-0.27%).

There was a high level of heterogeneity among the studies for hemoglobin A_1c, fasting plasma glucose, TC, LDL-C, and TGs. This may be the result of variation among the studies in the patients’ age and health and in the dosage and form of cinnamon supplements used. There was also potential publication bias for fasting plasma glucose. No difference was seen in glycemic or lipid measures with cinnamon dosage.

These trends are similar to the authors’ findings from their 2008 meta-analysis, but the five additional randomized, controlled trials have allowed the authors to conclude that cinnamon does, indeed, have a positive, significant effect on some measures of glycemic and lipid metabolism in patients with type 2 diabetes. This publication is consistent with another previous meta-analysis by other researchers of eight randomized, controlled trials on cinnamon preparations, which showed that cinnamon supplementation resulted in a statistically significant reduction of fasting blood glucose in people with type 2 diabetes and prediabetes.2

—Cheryl McCutchan, PhD

References

1.         Baker WL, Gutierrez-Williams G, White CM, Kluger J, Coleman CI. Effect of cinnamon on glucose control and lipid parameters.Diabetes Care. 2008;31(1):41-43.

2.        Davis PA, Yokoyama W. Cinnamon intake lowers fasting blood glucose: meta-analysis. J Med Food. April 2011; [epub ahead of print]. doi:10.1089/jmf.2010.0180.