The positive findings of the present study seem promising. Cinnamon showed various beneficial effects on lipid profiles, glycemic status, blood pressure, and waist circumference. However, the attentive reader may raise some justified questions: If cinnamon has such beneficial effects on these risk markers, why was there so much controversy regarding its efficacy? How does the present study stand concerning previous meta-analyses? And how relevant are the measured effects in clinical practice? Answering these questions requires diving deeper into the study under review in order to assess it in relation to the current body of literature.
The present study is in good agreement with two previous meta-analyses from 2017 (in adults) and 2020 (in participants with type 2 diabetes). All three meta-analyses found that cinnamon significantly reduced total cholesterol (-12 to -14 mg/dL), triglycerides (-16 to -26 mg/dL), and LDL (-6 mg/dL). However, the two previous meta-analyses did not find any statistically significant improvements in HDL-C, while the present study reports a significant increase in HDL-C (+1.35 mg/dL). That said, the 95% confidence intervals from each study shown in Figure 3 indicate that the studies are still roughly compatible with each other, even if the previous two didn’t find statistically significant results.
Figure 3: HDL-C changes in this study compared to two previous meta-analyses (with 95% confidence intervals)
So how relevant are these effects? Given the American Heart Association’s recommendations (see Table 1), the effects of cinnamon on cholesterol and triglycerides may be clinically relevant. Notably, cinnamon reduced triglycerides most significantly by 16 to 26 mg/dL, which could make a clear difference in participants with dyslipidemia, characterized by triglyceride levels of more than 200 mg/dL, aiming for levels of less than 150 mg/dL. Cinnamon’s effect on total cholesterol (-16.3 mg/dL) and LDL-C (-6.4mg/dL) is also considerable and could make a difference. Based on a recent cohort study of 12.8 million adults, a change in total cholesterol of -16 mg/dL in adults with blood levels of more than 200 mg/dL would be associated with approximately 5% lower all-cause mortality risk. Similarly, the reduced LDL-C levels found for cinnamon could lead to approximately 8% lower risk for coronary artery diseases (CAD), based on a recent Mendelian randomization analysis. The small yet significant increase in HDL-C caused by cinnamon may not be as clinically relevant as its lipid-lowering effects. Overall, the beneficial effects of cinnamon on lipid profiles are promising, but must be interpreted with caution. As decreasing risk markers do not necessarily lead to improved clinical outcomes, future RCTs investigating the effect of cinnamon on hard clinical outcomes such as decreased mortality rates are needed.
Besides blood lipids, cinnamon could be beneficial to improve the glycemic status of people with type 2 diabetes. Animal studies showed that cinnamon could inhibit numerous digestive enzymes, such as alpha-glucosidase and sucrase, and decrease the influx of glucose into the systemic circulation, thus avoiding high insulin spikes. Like previous meta-analyses, the present study found a significant reduction in fasting serum glucose (-11.4 mg/dL) caused by cinnamon. If supplemented for more than 8 weeks, this glucose-lowering effect of cinnamon was even more pronounced (-15.0 mg/dL). As diabetes is diagnosed when fasting serum glucose levels are above 126 mg/dL (see Table 2), cinnamon could be clinically relevant for diabetes management. A recent Mendelian randomization study and meta-analysis found that a 18 mg/dL higher fasting glucose was causally linked to an 48% higher risk of ischaemic stroke. That means it’s possible that the fasting glucose-lowering effect in the present study could meaningfully lower stroke risk by up to 40%, if cinnamon is supplemented for a prolonged period of time, although this is not confirmed.
In contrast to the previous meta-analyses, which probably lacked statistical power, the present study also found significant improvements for HbA1c (-0.23%), serum insulin (-7.6 pmol/L), and HOMA-IR (-0.59). These effect estimates could be clinically relevant, e.g., by improving the parameters of a participant with diabetes from moderate to slight insulin resistance. However, it remains an open question as to whether cinnamon could improve clinical outcomes such as disease progression or mortality rates. The positive findings of the present study could incentivize scientists to conduct further research on this topic.
Cinnamon may also have an effect on blood pressure. In 2020, two meta-analyses were published that assessed the anti-hypertensive effects of cinnamon in adults. The findings were somewhat incompatible and created some controversy: While the first study found significant improvements for both systolic (-6.2 mmHg) and diastolic blood pressure (-3.9 mmHg), the other reported only a slight effect on diastolic blood pressure (-0.9 mmHg) and no significant effect on systolic blood pressure. The present study now provides a third perspective: cinnamon improved both systolic (-4.0 mmHg) and diastolic blood pressure (-3.4 mmHg) significantly. The effect was even more pronounced in studies of more than 8 weeks in duration (-6.7 and -8.9 mmHg, respectively). These findings are in good agreement with the other meta-analysis that showed positive effects of cinnamon on blood pressure. Also, both studies found that longer supplementation periods yielded more pronounced blood pressure improvements, indicating that cinnamon may exert biological effects, which emerge only after long-term administration.
Based on current evidence, the blood pressure-lowering benefits of cinnamon may have clinical relevance (see Table 3 for reference values). A meta-analysis of 123 studies including more than 613,000 participants found a 13% risk reduction in all-cause mortality for a 10 mmHg decrease in systolic blood pressure. The anti-hypertensive effects of cinnamon could thus lead to an approximately 5–9% reduced risk for all-cause mortality, assuming a linear dose-response relationship between blood pressure and all-cause mortality risk. However, due to the high heterogeneity (i.e., the included studies varied a lot in design and findings) and unclear risk of bias, the effect estimates of cinnamon on blood pressure cannot be taken for granted and need to be viewed with caution.
Overall, the present meta-analysis has several strengths. Most notably, the present study is one of the most extensive meta-analyses of 35 RCTs that investigated the effects of cinnamon in 2,282 participants with different metabolic diseases. The sample size of the present meta-analysis allowed the researchers to identify significant changes where previous meta-analyses may have lacked the statistical power to do so.
However, there are three important limitations to consider when interpreting the reported effect estimates. First, the heterogeneity of the overall effect estimates was high, thus limiting the explanatory power of the results. A possible explanation that could account for part of the high heterogeneity may be the broad range of metabolic diseases analyzed. Apparently, the effects of cinnamon may vary for different metabolic diseases. Second, the researchers did not specify the type of cinnamon (C. verum or C. cassia), probably because most RCTs did not report on the type of cinnamon used. However, the type of cinnamon is a crucial factor that could account for some of the heterogeneity observed. In addition, different types of cinnamon differ by their bioactive compound content, most notably the potentially toxic coumarin. In fact, the non-linear dose response and upper limit of no more than 1.5 grams of cinnamon daily could be explained by most RCTs using C. cassia (high in coumarin) instead of Ceylon cinnamon. Future RCTs should thus carefully outline which type of cinnamon was used to make subgroup analyses and thus more specific supplement recommendations popular. Third, the complex multitude of biological mechanisms cinnamon is involved in makes it difficult to study the effects of cinnamon, especially in the context of such complex diseases as metabolic syndrome. Future studies may shed more light on the complex mechanisms and effects of cinnamon that could provide important information about general efficacy, the kind of people who benefit most, optimal dosage, and maybe even ideal timing.
The present study is one of the most comprehensive meta-analyses on cinnamon and metabolic diseases to date. Overall, the present study’s findings are in broad agreement with previous research but provide higher statistical power. This may be why the present study found significant effects that were previously only assumed, but not established. Cinnamon’s positive effects on lipid profiles, glycemic status, and blood pressure may have clinical relevance, especially if taken in the optimal dose (no more than 1.5 grams) over more extended periods (more than 8 weeks). However, the present study’s findings are limited by the high heterogeneity of the effects, unclear risk of bias for most the included RCTs, and unknown type of cinnamon used in the studies. Future studies should address these limitations to shed more light on the possible positive effects of cinnamon to treat metabolic diseases.