Cinnamon

Two distinct species are sold as cinnamon, and the difference between them is the single most useful thing on this page.

Ceylon cinnamon, Cinnamomum verum, is sometimes called true cinnamon. It is lighter in colour, mild in flavour, and forms brittle multi-layered quills. Cassia cinnamon, from related species, is darker, stronger, cheaper, and forms a single thick curled layer. Almost all supermarket cinnamon in the UK, and most supplement cinnamon, is cassia.

The reason it matters is coumarin. Cassia contains it in substantial amounts; Ceylon contains only traces. Coumarin places a load on the liver at sustained intakes, and a tolerable daily intake has been set on that basis. Regular daily cinnamon supplementation using cassia can exceed it more easily than people expect, particularly at the doses used in blood sugar research.

Cinnamon's effects on blood sugar are attributed to several actions: improving how effectively cells respond to insulin, slowing gastric emptying so carbohydrate arrives more gradually, and inhibiting enzymes that break down starch in the gut.

The compounds credited include cinnamaldehyde, which gives cinnamon its flavour and aroma, alongside various polyphenols.

Coumarin, the compound of concern in cassia, is unrelated to any of these effects. It is simply present in the plant, which means you take on the liability without it contributing to the benefit — a good argument for Ceylon if you intend to take cinnamon daily.

Meta-analyses of cinnamon supplementation have generally reported reductions in fasting blood glucose, with effects clearer in people whose readings were elevated to begin with than in those with normal glucose handling.

Findings on longer-term glucose markers have been less consistent, and reviews have noted considerable variability between trials in species used, dose and duration — which is unsurprising given that many did not specify which cinnamon they used.

There is a smaller literature on lipid markers with modest findings. Evidence for weight loss is weak. Overall this is a genuine if modest effect, most relevant to people whose glucose handling is already less than ideal.

Cinnamon as a culinary spice is safe at ordinary amounts. Daily supplementation at gram doses is a different exposure and the coumarin content of cassia becomes relevant.

Sustained high coumarin intake places a load on the liver. This is well characterised enough that a tolerable daily intake exists, and it is the reason to use Ceylon or a water-extracted product for regular use rather than the cassia in your cupboard.

Because cinnamon may lower blood sugar, take care if yours is being managed. Speak to your doctor before use if you take regular medication or have been told your liver is not working normally. Some people react to cinnamon on the skin or in the mouth.