A 2013 meta-analysis in the Annals of Family Medicine pooled 10 randomized trials with 543 people with type 2 diabetes. Cinnamon lowered fasting plasma glucose by 24.59 mg/dL on average.
Total cholesterol dropped 15.60 mg/dL, LDL fell 9.42 mg/dL, and triglycerides came down 29.59 mg/dL. HDL rose 1.66 mg/dL.
The trials tested doses from 120 milligrams to 6 grams a day for 4 to 18 weeks. But the same analysis found no meaningful change in hemoglobin A1c, the blood test that reflects average blood sugar over about three months.
That gap is why the story is not settled. NIH's National Center for Complementary and Integrative Health says it is unclear whether cinnamon supplementation is helpful for diabetes or weight loss.
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An earlier Cochrane review of 10 randomized trials with 577 patients also concluded there was insufficient evidence to support cinnamon for type 1 or type 2 diabetes.
What short-term trials in healthy people show
A 2009 study in the European Journal of Applied Physiology gave 3 grams of cinnamon a day to healthy young men for 14 days.
After two weeks their insulin response to an oral glucose tolerance test fell by about 27 percent compared with baseline, and insulin sensitivity improved.
A separate 40-day trial in 41 healthy adults compared 1, 3 and 6 grams a day.
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The 6-gram group showed the largest changes, with a 5.92 percent drop in fasting blood glucose and a 12.71 percent drop after meals. HbA1c did not shift significantly at any dose.
The coumarin problem with everyday cinnamon
Most cinnamon sold in supermarkets is Cassia cinnamon, which contains a compound called coumarin. At high doses coumarin can strain the liver in sensitive people.
The European Food Safety Authority has set a tolerable daily intake of 0.1 milligrams of coumarin per kilogram of body weight, which works out to about 6 milligrams a day for a 60-kilogram adult.
Germany's Federal Institute for Risk Assessment says consumers who use large quantities of cinnamon as a condiment should opt for the low-coumarin Ceylon cinnamon instead.
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NCCIH notes that prolonged use of Cassia cinnamon could be an issue for sensitive people, such as those with liver disease.
What to do with the evidence
Cinnamon is a plausible dietary complement, not a substitute for prescribed diabetes care.
The pooled numbers on fasting glucose and LDL are real, but the mixed HbA1c results mean cinnamon has not been shown to change long-term blood sugar control. No medical body currently recommends it as a treatment.
Anyone taking blood sugar medication or drugs that are hard on the liver should talk with their doctor before adding a daily cinnamon supplement, and choosing Ceylon cinnamon lowers the coumarin exposure that comes with daily Cassia use.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.