Cinnamon is known to contain polyphenols such as catechin, procyanidin, cinnamtannin trans-cinnamic acid and flavones (cinnamaldehyde and trans-cinnamaldehyde) [
5,
104,
105]. Eleven studies have shown positive effects of cinnamon on glycemic control. Three studies [
106,
107,
108] included subjects with T2D with baseline HbA1c levels >8%. Seven studies [
109,
110,
111,
112,
113,
114,
115] included healthy normal subjects [
111,
112,
113,
114], subjects with impaired fasting glucose [
109] and subjects with metabolic syndrome [
115]. One study showed beneficial effects on improved insulin sensitivity in patients with polycystic ovary syndrome [
105]. Two studies [
116,
117] showed positive effects on fasting blood glucose levels, but did not report baseline HbA1c levels [
116] and no significant changes of HbA1c, total cholesterol, low-density lipoprotein (LDL), high-density lipoprotein (HDL) or TG [
117]. Five studies [
118,
119,
120,
121,
122] showed no significant changes in fasting glucose, lipids, HbA1c, or insulin levels in 25 postmenopausal women with T2D taking 1.5 g of cinnamon daily for six weeks [
120], in 43 subjects with T2D receiving 1 g of cinnamon daily for three months [
118], in 60 subjects with T2D taking 1 g of cinnamon daily for 12 weeks [
121], in 72 adolescents with Type 1 diabetes (T1D) taking 1 g of cinnamon daily [
122] and in 11 healthy subjects taking 3 g of cinnamon daily for four weeks [
119]. A randomized, controlled trial [
108] (no placebo used with the concern that the strong taste of cinnamon capsules would preclude any blinding), investigating whether daily cinnamon plus usual care
versus usual care alone lowers HbA1c in 109 patients with T2D (HbA1c > 7.0), showed that supplementation with 1 g of daily cassia cinnamon did not significantly lower HbA1c over 90 days [
108]. A randomized, placebo-controlled, double-blind clinical trial of 58 subjects with T2D found that intake of 2 g daily of cinnamon for 12 weeks significantly reduced HbA1c (−0.36%;
p < 0.005), systolic blood pressure—SBP (−4 mm Hg;
p < 0.001) and diastolic blood pressure—DBP (−4 mmHg;
p < 0.001) among 58 poorly controlled patients with T2D, compared with the placebo [
107]. A randomized, double-blinded clinical study of 66 Chinese with T2D treated with gliclazide found significant reductions in HbA1c in those receiving 360 mg of cassia cinnamon extract (−0.92%;
p = 0.0004) and 120 mg of cassia cinnamon extract (−0.67%;
p = 0.003) after 90 days. No significant reductions were seen in the placebo group but the groups were not compared statistically. Fasting glucose levels were also significantly reduced over time in both treatment groups [
106]. A study [
116] in 60 subjects with T2D treated with sulfonylureas found significant reductions in fasting glucose (18%–29%), triglycerides (23%–30%), LDL (7%–27%), and total cholesterol (12%–26%), following supplementation with either 1, 3, or 6 g of cassia cinnamon daily for 40 days, while no change was observed in the placebo group but the groups again were not compared statistically. Significant effects were observed even after 20 days of washout [
116]. A double-blind study [
117] found that intake of 3 g daily cinnamon powder for 120 days significantly decreased fasting glucose but not HbA1c in 65 patients with T2D treated with oral antidiabetics or diet alone. Fasting glucose was reduced by 10.3% ± 13.2% in the treatment group and 3.37% ± 14.2% in the placebo group (
p = 0.046). No significant changes in HbA1c, total cholesterol, low-density lipoprotein (LDL), high-density lipoprotein (HDL) or triglycerides (TG) were observed. Baseline HbA1c values were below 7% [
117]. Two recently updated meta-analyses showed that the intake of cinnamon/cinnamon extracts lowered fasting glucose levels [
123,
124]. A meta-analysis of 10 randomized controlled trials (543 patients) reported that the consumption of cinnamon ranging from 120 mg/day to 6 g/day for 4–18 weeks decreased levels of fasting glucose, total cholesterol, LDL, and triglyceride and increased levels of HDL. No significant effect on HbA1c was observed [
124]
In summary, the consumption of cinnamon may have glycaemic-lowering effects, but more intervention studies covering different types and amounts of cinnamon and types of subjects, are required as not all studies have shown positive effects of cinnamon on insulin resistance and many have been poorly analysed. The usual dose of cinnamon used was 1–2 g/day but some studies used larger doses of 6 g/day which would be difficult to maintain long term.