Red meat and blindness

Friday March 20 2009

“Eating too much red meat can raise the risk of going blind by half,” the Daily Mail reported. It said that research has shown that eating red meat at least 10 times a week makes people 50% more likely to develop age-related macular degeneration (AMD), than those who eat it less than five times a week. But the newspaper said that eating chicken appears to protect against the condition. It quotes the Royal College of Ophthalmology, which says that “the evidence is still not strong enough to merit any advice to the public.”

This reliable study followed 5,600 middle-aged men and women for 13 years. The opposing effects of chicken and red meat are puzzling. The researchers say that they took into account several known major risk factors for AMD, such as smoking. They admit however, that rather than directly raising the risk of AMD, red meat consumption could actually be a marker for other lifestyle factors that are causing the damage. Meanwhile, there is no evidence that eating chicken protects against the disease. The researchers say that more research is needed.

Where did the story come from?

Dr Elaine E-W. T. Chong from the Centre for Eye Research Australia at the University of Melbourne carried out this research with colleagues. The study was supported by awards from the National Health and Medical Research Council, the Ophthalmic Research Institute of Australia and others bodies. The study was published in the American Journal of Epidemiology , a peer-reviewed medical journal.

What kind of scientific study was this?

The researchers say that age-related macular degeneration (AMD) is the leading cause of severe vision loss in people aged 50 years or older in the developed world. AMD is an eye condition that causes the gradual loss of central vision (the ability to see what is directly in front of you). It occurs when the part of the eye responsible for central vision (the macula) is unable to function as effectively as it used to.

There are two types of AMD, dry and wet, and two defined stages, early and late. Early AMD is characterised by yellow deposits and changes in retinal pigmentation, and is thought to indicate an unhealthy retina. This is thought to progress to late AMD, which is more severe and can be either dry or wet in type. Dry AMD involves thinning (atrophy) of the retina, while wet AMD involves leaking of fluid exudates. The scar tissue from both types destroys central vision.

Risk factors for AMD are known to include age, family history and smoking. Smoking is the only modifiable risk factor consistently found in previous studies. In this cohort study, the researchers wanted to see if meat consumption was linked to AMD and if it could be considered a risk factor.

The researchers obtained their data from a large study called the Melbourne Collaborative Cohort Study. This was a prospective cohort study of 41,528 Melbourne residents (17,049 men) between the ages of 40 and 69 when they were recruited between 1990 and 1994. During follow-up between 2003 and 2006, all the participants had eye examinations, and retinal photographs of both eyes were taken. These were graded in a standardised way by doctors who received additional training in the process. The extent of agreement between graders was tested statistically to ensure that the diagnoses were reliable.

When they enrolled, the participants completed a 121-item food frequency questionnaire, which asked about their eating habits. There were 18 questions relating to fresh red meat, processed red meat, and chicken. Red meat included things like roast beef, meatballs or lamb chops. The researchers also asked about demographic and lifestyle factors, including age, sex, smoking status and country of birth. Height, weight and blood pressure were directly measured.

From the 41,000 people enrolled in the Melbourne Collaborative Cohort Study, 6,734 participated in the eye study. The participants were excluded for various reasons. For example, a large number in the original cohort were the wrong age for the eye study; some had left the state (Victoria) before the examination in 2003, and some left or died during follow-up. Others were excluded if they had extreme high-energy diets or were likely to have changed diet over the 10 years since the start, or if they had missing data. This left a total of 5,604 participants for analysis.

The analysis was done by statistical modelling. The modelling was adjusted for the participants’ age, sex, smoking (current, past, never) and energy intake. The researchers also had data to allow them to test for a range of other potential risk factors that could have interfered with the result.

They modelled a range of potential ‘confounders’ (vitamin C, vitamin E, b-carotene, zinc, lutein/zeaxanthin, trans-unsaturated fatty acids, omega-3 fatty acids, saturated fat, cholesterol, total fat, alcohol, vegetable intake, fish intake, supplement use, education, body mass index, and protein intake at baseline). Only body mass index and zinc, protein, and vitamin intake were adjusted for in the final model.

What were the results of the study?

The researchers report that 1,680 cases of early AMD and 77 cases of late AMD were found from the digital macular photographs of both eyes at follow-up.

Higher red meat intake was found to be positively associated with early AMD, even after the various potential confounders were taken into account. The researchers say that someone who ate red meat 10 times a week was 1.47 times more likely to have early AMD than a person who ate it less than five times a week (OR 1.47; 95% confidence interval: 1.21 to 1.79; P for trend < 0.001.

Similar trends towards increasing prevalence of early AMD were seen among people with higher intakes of fresh and processed red meat separately. There was no significant difference in the rates of late AMD.

Eating chicken had an opposite effect. The chances of late AMD was significantly less for those who ate chicken three and a half times a week or more compared with those who ate it less than one-and-a-half times a week (OR 0.43, 95% confidence interval: 0.20 to 0.91; P for trend = 0.007).

What interpretations did the researchers draw from these results?

The researchers say their findings suggest that intakes of specific meats may have different effects on the risk of AMD. They say that these meats could be targets for lifestyle changes.

What does the NHS Knowledge Service make of this study?

Investigating new risk factors for major causes of blindness is clearly important. This study is a step forward, given the limited epidemiologic data so far on the links between eating meat and AMD. Limitations noted by the researchers include:

  • The small number of people with late AMD (77) means that the researchers were unable to examine the effect of eating red meat on the two sub-types, wet and dry AMD, separately. If eating meat only increased the incidence of one type of AMD, the overall effect would be reduced.
  • Diets were only assessed once at the beginning of the study. Although people who had extreme variation in their diets were excluded, it is still possible that consumption of meat over the participants’ lifetimes is not accurately reflected in the one-off measurement.
  • The researchers adjusted for known potential lifestyle confounders that could also contribute to the rates of AMD. However, they raise the possibility that meat intake could be a proxy for other risk factors or for other unknown substances that are associated with AMD. In the same way chicken intake may be associated with a particular lifestyle that is protective against AMD.
  • ‘Residual confounding’ due to inaccurately measured or unmeasured risk factors is always a problem for observational studies such as this, and it may have contributed to some of the difference between groups.

The authors mention that an association between red meat consumption and AMD is biologically plausible, which makes this link more robust. However, they also caution that other cohort studies need to confirm this link.

Analysis by Bazian
Edited by NHS Choices