Monday October 8 2012
Coffee was linked with glaucoma but not other caffeinated drinks
Switching to decaf could save your sight, claims the Daily Mail. The newspaper reports that drinking three or more cups of coffee a day is linked with vision loss and blindness.
The story is based on a study that looked at the association between caffeine consumption and exfoliation glaucoma. Exfoliation glaucoma is a condition in which fluid builds up inside the eye, putting pressure on the optic nerve. This leads to a degree of sight loss and, in serious cases, total blindness.
In the study, researchers compared rates of exfoliation glaucoma with drinking habits, focusing on caffeinated drinks such as coffee, tea and cola.
The study found that drinking three or more cups of caffeinated coffee daily was linked with increased risk of developing exfoliation glaucoma. Curiously, a similar increase was not found with other caffeinated products.
This was a large, well-conducted study, but it does not directly or conclusively show that caffeinated coffee causes vision loss. This is because it relied on people recalling their coffee intake over long periods and involved relatively few people with the condition.
By far the greatest risk of exfoliation glaucoma was among those with a family history of glaucoma – which is a factor already known to be linked with developing glaucoma.
Coffee has been linked in the past with both health problems and health benefits. So, despite the headlines, there’s no need to ditch the cappuccino altogether, but it makes sense to drink coffee and other caffeinated products in moderation.
Where did the story come from?
The study was carried out by researchers from Brigham and Women's Hospital, Harvard Medical School and other US institutions. It was funded by the National Institutes of Health and other academic institutions. The study was published in the peer-reviewed journal Investigative Ophthalmology and Visual Science.
The Mail’s headline was unnecessarily alarmist. Its claim that switching to decaf could save your sight exaggerated the results of the study and is likely to be wrong because of the link between glaucoma and family history.
What kind of research was this?
The researchers drew on two large prospective cohort studies to look at the association between caffeine and exfoliation glaucoma.
While useful, cohort studies cannot prove cause and effect on their own. To do this other types of evidence are needed. It is always possible that other factors, such as diet and family history, might influence people’s health outcomes. These are called confounders. Although, the best cohort studies try to take these into account.
The researchers point out that exfoliation glaucoma is a leading cause of secondary angle glaucoma worldwide. Secondary glaucoma is when a secondary factor, such as injury or infection blocks the drainage tubes inside the eyes. As a result, ‘intraocular’ pressure builds up and this can damage the optic nerve and the nerve fibres. In cases of exfoliation glaucoma, the secondary factor is the build-up of abnormal deposits of cells, known as exfoliation deposits, inside the eye. These have been described as the “optical version of dandruff”.
This can lead to:
- deposits building up on the lens of the eye, causing an increase in intraocular pressure
- damage to the optic nerve or retina
- narrowing of the visual field, which causes a form of tunnel vision
This process is known as exfoliation syndrome (ES). If all three changes are present, a person is diagnosed with exfoliation glaucoma. If only one of the changes happens then a patient will have 'exfoliation glaucoma suspect' (EGS). The researchers say exfoliation glaucoma and ES and the changes that typify it have been found to occur at high levels in Scandinavia, where coffee consumption is also high.
They also say that coffee is associated with increased levels of a substance called homocysteine in blood, aqueous humour and tear fluid, and that this may help trigger or accelerate the development of exfoliation deposits. They say that this means caffeine or coffee consumption is “an attractive risk factor” for ES and exfoliation glaucoma.
What did the research involve?
The researchers followed two large groups of people:
- 78,977 women from a large US study called the Nurses Health Study (NHS), which began in 1980
- 41,202 men from the Health Professionals Follow-up Study (HPFS), which began in 1986
Both studies continued until 2008 and, in each, participants were asked every two years to fill in detailed questionnaires covering their health, lifestyle and diets.
For this particular piece of research, participants had to be at least 40 years old and had to have not reported glaucoma at the start of the study. They also had to report regular eye examinations.
During the study period, researchers used a validated food frequency questionnaire to collect information from participants on their dietary intake. For those in the NHS, this was conducted every two years from 1980 to 1986 and every four years after, and for men in the HPFS this was conducted in 1986 and every four years after. Participants were questioned about consumption of caffeinated coffee (in cups), caffeinated tea (in cups) and caffeinated chocolate (in 1oz servings). Later, this was expanded to include intake of decaffeinated coffee (in cups), and caffeinated and caffeine-free sodas.
For all these items the questionnaire allowed for nine responses for the frequency of intake, ranging from “Never, or less than once a month” to “Six or more times a day”. Answers were converted into average daily intake of caffeine in mg/day. The researchers assumed 137mg of caffeine per cup of coffee, with far lower amounts of caffeine assumed for tea, cola and chocolate.
During the study period, the researchers collected self-reported information about the development of glaucoma. They then asked for additional detailed information in the form of a glaucoma questionnaire from the participants’ eye care providers. This included questions on the presence of exfoliation deposits. A glaucoma specialist evaluated the returned questionnaires.
For their analysis, the researchers defined exfoliation glaucoma or EGS as the presence of documented exfoliation syndrome plus other signs of exfoliation deposits in the eye.
They analysed the association between caffeine and caffeinated drinks consumption and the risk of exfoliation glaucoma or EGS, using standard statistical methods. Since glaucoma is a slowly developing chronic condition, they say they calculated the cumulative caffeine intake, averaging the intake from all the dietary assessments made.
The researchers adjusted the results to account for various other risk factors for glaucoma, including:
- family history
- history of heart attack
- body mass index
What were the basic results?
The researchers found that, compared with people who drank no caffeinated coffee, those who drank three or more cups of caffeinated coffee daily were at increased risk of exfoliation glaucoma or EGS (risk ratio 1.66, 95% confidence interval 1.09 to 2.54). These results were not significantly altered after adjustment for total fluid intake. Associations were stronger among women with a family history of glaucoma.
In terms of lower levels of caffeinated coffee consumption they found that, compared with participants whose coffee consumption was less than 125mg/day, in those who consumed 500mg or more of coffee a day there was a trend towards increased risk of exfoliation glaucoma or EGS.
However, the increase was not statistically significant (risk ratio 1.43, 95% confidence interval 0.98 to 2.08). This means the trend could have occurred by chance.
Researchers did not find associations with consumption of other caffeinated products (caffeinated soda, caffeinated tea or chocolate) or decaffeinated coffee and the risk of exfoliation glaucoma or EGS.
How did the researchers interpret the results?
The researchers say that their study supports the hypothesis that three or more cups of caffeinated coffee daily may contribute to the gradual accumulation of exfoliation material in the eye. They point out that the previous findings that coffee consumption elevates homocysteine levels provides a plausible biological link between coffee consumption and exfoliation syndrome.
This was a large study that followed people for many years to assess the association between coffee and exfoliation glaucoma, using validated detailed questionnaires on dietary intake. However, it had several limitations, which could affect the reliability of its results:
- The researchers had to rely on information on non-standardised eye examinations from various eye care providers, to ascertain whether participants had exfoliation glaucoma or EGS, rather than having confirmed diagnoses. The fact that so few people were identified may mean that the participants did not recall their diagnoses accurately, something that would have reduced the study’s ability to detect a link.
- The researchers had to rely on people accurately recalling their intake of caffeine over the previous year.
- Their study populations were 90% caucasian, so the results may not apply to other ethnic groups. People of African or African-Caribbean origin are at increased risk of developing other types of glaucoma.
- Other factors called confounders may have affected the risk of people getting exfoliation glaucoma, although the researchers tried to take these into account.
In conclusion, the results of this study are noteworthy, but should be viewed with caution. Further study is required to look at whether caffeine consumption can really cause this disorder. It is generally not advisable to consume high levels of caffeine on a daily basis as this can cause insomnia, restlessness and irritability. But, based on the results of this study, a couple of cups a day are not going to pose a serious risk to your eyesight.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.