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In the past couple of days, I've repeatedly heard and read that cinnamon consumption increases the metabolic rate in humans. A Google search brings up plenty of results.

However, few of them cite any concrete evidence. In fact, even those that mention sources point to circumstantial evidence at best. For instance Livestrong.com states

A 2012 study published in the International Journal of Preventative Medicine investigated the effects of cinnamon in people with type 2 diabetes. One group took cinnamon supplements every day, while the other group took a placebo. After eight weeks, the cinnamon group lost more weight and body fat than the group taking the placebo, even though the cinnamon group did not make any changes to their usual eating habits.

Similarly, all 12minuteathlete.com put forth to back their claim is

If you’ve ever read the 4-Hour Body by Tim Ferris (and if you haven’t, I highly recommend it), you probably remember the section on his cinnamon experiments. Basically, he found that eating about one and a half teaspoons of cinnamon a day can help regulate glucose levels, which in turn helps control weight gain.

Idealbite.com goes into more detail about the purported effects of cinnamon on the body and mentions two studies but doesn't give links or any information with which to find either of them.

So basically, my question is how much evidence is there really for the metabolism-accelerating effects of cinnamon intake and how conclusive is it?

  • Excellently framed and presented question, welcome Casimir! – DoctorWhom Sep 11 '17 at 10:12
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Despite being a long known spice and its known role in traditional medicine the modern evidence for health related benefits and detriments of cinnamon are sparse.The NCCIH sums it up with NCCIH Publication No.463: Cinnamon:

Studies done in people don’t support using cinnamon for any health condition.

Cinnamon is indeed used in the way the OP phrased it, but whether it is really effective remains currently dubious.

Results in Favour of Cinnamon

There are some scientific studies showing very interesting and promising results like:

Improved Insulin Resistance and Lipid Metabolism by Cinnamon Extract through Activation of Peroxisome Proliferator-Activated Receptors

Peroxisome proliferator-activated receptors (PPARs) are transcriptional factors involved in the regulation of insulin resistance and adipogenesis. Cinnamon, a widely used spice in food preparation and traditional antidiabetic remedy, is found to activate PPAR and , resulting in improved insulin resistance, reduced fasted glucose, FFA, LDL-c, and AST levels in high-caloric diet-induced obesity (DIO) and mice in its water extract form. In vitro studies demonstrate that cinnamon increases the expression of peroxisome proliferator-activated receptors and (PPAR/) and their target genes such as LPL, CD36, GLUT4, and ACO in 3T3-L1 adipocyte. The transactivities of both full length and ligand-binding domain (LBD) of PPAR and PPAR are activated by cinnamon as evidenced by reporter gene assays. These data suggest that cinnamon in its water extract form can act as a dual activator of PPAR and , and may be an alternative to PPAR activator in managing obesity-related diabetes and hyperlipidemia.

Cinnamon polyphenols regulate multiple metabolic pathways involved in insulin signaling and intestinal lipoprotein metabolism of small intestinal enterocytes

Results Ex vivo, the cinnamon extract significantly decreased the amount of apolipoprotein-B48 secretion into the media, inhibited the mRNA expression of genes of the inflammatory cytokines, interleukin-1β, interleukin-6, and tumor necrosis factor-α, and induced the expression of the anti-inflammatory gene, Zfp36. CE also increased the mRNA expression of genes leading to increased insulin sensitivity, including Ir, Irs1, Irs2, Pi3k, and Akt1, and decreased Pten expression. CE also inhibited genes associated with increased cholesterol, triacylglycerols, and apolipoprotein-B48 levels, including Abcg5, Npc1l1, Cd36, Mttp, and Srebp1c, and facilitated Abca1 expression. CE also stimulated the phospho-p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, and extracellular-signal-regulated kinase expressions determined by flow cytometry, with no changes in protein levels.

Conclusions These results demonstrate that the CE regulates genes associated with insulin sensitivity, inflammation, and cholesterol/lipogenesis metabolism and the activity of the mitogen-activated protein kinase signal pathway in intestinal lipoprotein metabolism.

Cinnamon: Potential Role in the Prevention of Insulin Resistance, Metabolic Syndrome, and Type 2 Diabetes

Metabolic syndrome is associated with insulin resistance, elevated glucose and lipids, inflammation, decreased antioxidant activity, increased weight gain, and increased glycation of proteins. Cinnamon has been shown to improve all of these variables in in vitro, animal, and/or human studies. In addition, cinnamon has been shown to alleviate factors associated with Alzheimer's disease by blocking and reversing tau formation in vitro and in ischemic stroke by blocking cell swelling. In vitro studies also show that components of cinnamon control angiogenesis associated with the proliferation of cancer cells. Human studies involving control subjects and subjects with metabolic syndrome, type 2 diabetes mellitus, and polycystic ovary syndrome all show beneficial effects of whole cinnamon and/or aqueous extracts of cinnamon on glucose, insulin, insulin sensitivity, lipids, antioxidant status, blood pressure, lean body mass, and gastric emptying. However, not all studies have shown positive effects of cinnamon, and type and amount of cinnamon, as well as the type of subjects and drugs subjects are taking, are likely to affect the response to cinnamon. In summary, components of cinnamon may be important in the alleviation and prevention of the signs and symptoms of metabolic syndrome, type 2 diabetes, and cardiovascular and related diseases.

Metabolism Accelerator?

The most promising indicators going into the direction of the question's "metabolism acceleration" might be found in studies like these:

Cinnamaldehyde induces fat cell-autonomous thermogenesis and metabolic reprogramming:

CA activates thermogenic and metabolic responses in mouse and human primary subcutaneous adipocytes in a cell-autonomous manner, giving a mechanistic explanation for the anti-obesity effects of CA observed previously and further supporting its potential metabolic benefits on humans. Given the wide usage of cinnamon in the food industry, the notion that this popular food additive, instead of a drug, may activate thermogenesis, could ultimately lead to therapeutic strategies against obesity that are much better adhered to by participants.

Inconclusive Results

But trying to gain a systematic view on such a complex substance is quite a challenge. The Cochrane Library concludes in Cinnamon for diabetes mellitus:

There is insufficient evidence to support the use of cinnamon for type 1 or type 2 diabetes mellitus. Further trials, which address the issues of allocation concealment and blinding, are now required. The inclusion of other important endpoints, such as health-related quality of life, diabetes complications and costs, is also needed.

Another 2011 Research Summary:

One fairly well-studied supplement used to help hyperglycemia in diabetics is cassia cinnamon. Animal and laboratory studies have indicated that cinnamon may mimic the effects of insulin and make cells more sensitive to insulin (Anderson et al., 2004). In diabetic patients, some studies have shown a favorable response; some no effect. The most comprehensive review of cinnamon use in diabetics, published in 2008 by the journal Diabetes Care (Baker et al., 2008), found no metabolic benefits to the use of cinnamon by type I or type II diabetics. Specifically, no benefits to fasting blood glucose, lipids, or cholesterol were observed in a meta-analysis of five small clinical trials. [Ronald Ross Watson and Victor R. Preedy: "Bioactive Food As Dietary Interventions For Diabetes", Academic Press: San Diego, London, 2013, p377.]

Whereas an article in Annals of Family Medicine sees it in more positive light:

Cinnamon use in type 2 diabetes: an updated systematic review and meta-analysis.

The consumption of cinnamon is associated with a statistically significant decrease in levels of fasting plasma glucose, total cholesterol, LDL-C, and triglyceride levels, and an increase in HDL-C levels; however, no significant effect on hemoglobin A1c was found. The high degree of heterogeneity may limit the ability to apply these results to patient care, because the preferred dose and duration of therapy are unclear.

Summary

Please note that most of the positive results where obtained in beakers and rats, while studies of its effect in humans are mostly either negative, not replicated or inconclusive. Citing a single study that is not a fundamental and large breakthrough is usually a good indication that the newspapers have filled their bogus pipeline.

The effect on metabolic rate seems to be out of focus for the interest in cinnamon and its most promising fields of application as a medicine are directed to diabetes and metabolic syndrome. Whether those sources cited by the OP have any background in valid and reliable scientific evidence seems unlikely. More likely is that they are all examples of 'journalism' (this is meant as an insult) and bad science communication: Taking preliminary findings and blowing them out of proportion, misrepresenting the facts given to them.

Cinnamon in moderation as a spice is nice. If used as an agreeable herb that makes fibrous ingredients more attractive without adding much calories it is superb. If taken as a medicine or supplement it can be quite ineffective and at the same time dangerous. This is especially true if one does not differentiate between cinnamon and cassia. If using large amounts of cinnamon might still be a goal then there are some more things to consider:

How much cinnamon is too much?

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