Cacao And Iron Absorption: What The 1999 Trial Measured
The research people are reaching for when they ask about cacao and iron absorption is a single human trial from 1999, and it measured cocoa as one of nine drinks. What it actually did, and in whom, is a good deal more specific than the headlines built on it.
The category manages to assert two opposite things about the same food, often on the same website: that cacao is a rich source of iron, and that cacao blocks iron. Neither claim usually arrives with a figure attached. The trial does have figures.
Haem and non-haem, in one line each
Haem iron is the iron bound inside haemoglobin and myoglobin, so it comes from meat and fish. Non-haem iron is everything else: plants, eggs, dairy and fortified foods. The two are absorbed by different routes, and only the non-haem route is the one that polyphenol research is about.
The trial everybody is citing
Hurrell, Reddy and Cook published "Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages" in the British Journal of Nutrition in 1999, volume 81, pages 289 to 295.
Seventy-seven volunteers took part, 23 men and 54 women, aged 19 to 40, split across eight separate studies of eight to ten people each. The test meal was a 50 g iron-fortified bread roll with 10 g of butter, eaten with 275 ml of either water or one of the test beverages. Absorption was measured using extrinsic radio-iron tags, which is the standard method for this kind of single-meal work.
The cocoa serving delivered 116 mg of polyphenols. Eaten with the bread roll, iron absorption was 1.29% with cocoa against 4.46% with water. That is a ratio of 0.29, or roughly a 71% reduction. In the eighth study, where the drinks were given without the bread, absorption was 1.61% against 28.5% with water.
Cocoa was not the strongest inhibitor in the set. Black tea was.
The full table from that study
| Beverage | Polyphenols per 275 ml | Reduction in iron absorption |
|---|---|---|
| Assam tea B | 396 mg | 79 to 94% across tea studies |
| Assam tea A | 274 mg | 79 to 94% across tea studies |
| Peppermint B | 209 mg | 84% |
| Peppermint A | 177 mg | 84% |
| Pennyroyal | 121 mg | 73% |
| Instant coffee | 120 mg as chlorogenic acid | Not reported against water |
| Cocoa | 116 mg | 71% |
| Vervain | 116 mg | 59% |
| Lime flower | 58 mg | 52% |
| Camomile | 52 mg | 47% |
Note the two rows at 116 mg. Cocoa and vervain delivered the same polyphenol dose and produced different results, 71% against 59%, which tells you the total polyphenol figure is not the whole explanation. Which polyphenols, and in what company, matters.
What the trial does not tell you
It was single-meal work. Each measurement captured one breakfast, not a week of eating. The design cannot say anything about anyone's iron status over months.
The participants were 19 to 40 and two thirds of them were women. The vehicle was an iron-fortified white bread roll, which is a deliberately simple matrix chosen to make the measurement clean. And the cocoa was a 275 ml drink prepared for the study. It was not a chocolate, not a bar, and not any retail product.
The paper is also 27 years old. It has been cited heavily, but a 1999 single-meal study is a starting point for a question, not an answer to it.
How much iron is in play
For context on the other side of the equation, the NHS gives the following reference amounts: 8.7 mg a day for men aged 19 and over, 14.8 mg a day for women aged 19 to 49, and 8.7 mg a day for women aged 50 and over, with the note that women over 50 who still have periods may need the higher figure. Its guidance also says that most people should be able to get all the iron they need from a varied and balanced diet, and that doses above 20 mg from supplements can cause constipation, nausea and stomach pain. That page was last reviewed on 1 August 2019.
The NHS page does not discuss what affects absorption, so it does not speak to the 1999 findings in either direction. The two sources answer different questions and should not be stitched together.
Where the figures for a specific product come from
They usually do not exist. Polyphenol content in cacao varies with genetics, fermentation, roast and processing, which is why the useful comparisons in this category are about method rather than about brands. Alkalisation in particular changes the number substantially, and we cover that in the difference between raw cacao and cocoa powder. For a neighbouring compound question, what oxalates in chocolate are covers similar ground, and the whole set is indexed at the Chaski guides hub.
What is in a Chaski tube
We do not publish a polyphenol, flavanol, magnesium or iron figure for Chaski, because none of them has been measured. Printing an estimate would be worse than printing nothing.
What we do publish is the composition. A 600 g tube of 77% dark chocolate buttons, with four buttons as the daily dose on the pack. Single-origin cacao from Peru and Ecuador, ground at low temperature. Coconut sugar as the sweetener. No emulsifiers, no seed oils, no palm oil, no added cane sugar. Lion's Mane as a fruiting body extract, Cordyceps sinensis as mycelium, and the Ginkgo Biloba component being replaced by 200 mg of Tremella fuciformis, a 10:1 fruiting body extract, per daily dose, with tubes in that transition now. Every batch is tested for lead, cadmium and pesticides. The full list sits on the Chaski Cacao product page.
If iron intake is something you are tracking for a medical reason, the useful next step is a conversation with your GP or a registered dietitian, who can look at your bloods rather than at a 1999 breakfast study.
Frequently Asked Questions
How much did cocoa reduce iron absorption in the 1999 study?
Hurrell, Reddy and Cook reported that a 275 ml cocoa drink containing 116 mg of polyphenols, taken with a 50 g iron-fortified bread roll, gave iron absorption of 1.29% against 4.46% for water. That is a ratio of 0.29, or about a 71% reduction. In a separate study within the same paper, where beverages were given without the bread roll, absorption was 1.61% against 28.5%. The study involved 77 volunteers aged 19 to 40 across eight sub-studies, and it measured single meals using extrinsic radio-iron tags rather than long-term iron status.
Was cocoa the strongest inhibitor the study measured?
No. Black tea was. The 1999 paper reports inhibition of 79 to 94% for black tea, 84% for peppermint tea, 73% for pennyroyal and 71% for cocoa, then 59% for vervain, 52% for lime flower and 47% for camomile. The polyphenol doses ranged from 52 mg for camomile to 396 mg for the stronger Assam tea preparation, with cocoa at 116 mg. Vervain delivered the same 116 mg and produced a smaller reduction, which suggests the total polyphenol figure does not fully explain the result.
Does the NHS say anything about cacao and iron?
Not on its iron page. The NHS sets reference amounts of 8.7 mg a day for men aged 19 and over, 14.8 mg for women aged 19 to 49 and 8.7 mg for women aged 50 and over, lists good sources such as red meat, beans, nuts, dried fruit and fortified cereals, and warns that supplement doses above 20 mg can cause side effects. It does not discuss what affects absorption, so it neither supports nor contradicts the 1999 beverage findings. The page was last reviewed on 1 August 2019.
Does Chaski publish a polyphenol or iron figure?
No. Neither polyphenol content nor mineral content has been measured for Chaski, so there is no figure to publish and we do not estimate one. The facts we will state are compositional: a 600 g tube of 77% dark buttons, four buttons as the daily dose on the pack, single-origin Peruvian and Ecuadorian cacao ground at low temperature, coconut sugar, no emulsifiers, no seed oils, no palm oil and no added cane sugar, with every batch tested for lead, cadmium and pesticides.
Sources
- Hurrell R F, Reddy M and Cook J D, British Journal of Nutrition, 1999
- National Health Service, nhs.uk, 2019
See what is in the tube
Chaski Cacao — 600 g tube, 77% dark, single-origin Peruvian and Ecuadorian cacao