This release from the American Academy of Ophthalmology describes a study on citrus foods as a possible nutritional cataract intervention. Because this study focused on 1,000 sets of female twins, the release offers some interesting observations about the role genetics may play in cataracts compared to diet and exercise. The body of the news release did not go further than the study itself in linking diets rich in Vitamin C to reduced risk of cataract formation and progression. But the headline blew past those constraints to assert a cause-and-effect relationship that the study itself did not.
Cataracts are a clouding of the lenses in your eyes. They affect vision and are very common in older people. More than 22 million Americans have cataracts, according to the National Institutes of Health. The Institute further states that “By age 80, more than half of all people in the United States either will have a cataract or have had cataract surgery.” According to the World Health Organization, cataracts cause most of the world’s blindness, some 20 million cases, as well as cases of low vision. If a simple dietary intervention can delay or prevent such vision loss, that would be important information for individuals as well as for public health organizations.
No mention is made of the cost of adding more Vitamin C rich foods into the diet, but that would be tough to do, since the cost of foods rich in Vitamin C varies widely across the world.
The news releases notes that, “During the baseline measurement, diets rich in vitamin C were associated with a 20 percent risk reduction for cataract. After 10 years, researchers found that women who reported consuming more vitamin C-rich foods had a 33 percent risk reduction of cataract progression.”
The release would have been stronger if it had used absolute numbers (the actual total rate of cataracts in women with vitamin C-rich diets vs. those with diets low in vitamin C) in addition to the relative risk figures.
The news release makes no mention of harms that may come with increasing the amount of Vitamin C one consumes in their diet. While mega doses of Vitamin C supplements are known to be harmful, this release and study stress that the focus here is on obtaining Vitamin C through food sources.
Prospective cohort studies tend to be stronger than retrospective studies. But this one was limited by its dependence on a questionnaire that asked subjects to recall dietary intake, an often unreliable way of gathering information. In addition, the release doesn’t really describe how the slowing of cataracts was measured.
The news release cites “diets rich in Vitamin C” as possibly protective, but fails to say what that means. What and in which quantities does a person have to eat in order to be eating a diet rich in Vitamin C?
The news release notes further that “Genetic factors accounted for 35 percent of the difference in cataract progression. Environmental factors, such as diet, accounted for 65 percent.” But it does not say how those numbers were calculated. And we are told that there is a 33% reduced risk of cataract progression 10 years after the baseline studies were performed, but never learn why the number of pairs of twins who were tested dropped from 1,000 to 324.
None of the study limitations were mentioned in the release. Here’s a summary of those limitations:
1. This was an observational study. There was no step for controlling for possible confounders (extraneous variables that could bias the study).
2. There was a large loss to follow up.
3. Volunteers were asked to recall their diets at only one point in time.
4. With the number of variables studied (25+) they were bound to find a difference by chance alone.
The news release does not engage in disease mongering.
The release doesn’t mention its funders or potential conflicts of interest. Just because a funder has no apparent conflicts of interest does not mean that that information need not be included. In this case, inclusion of the fact that the Wellcome Trust and the Guide Dogs for the Blind Association paid for the work would have strengthened the release’s credibility. Instead, a reader is left to wonder whether perhaps a group representing citrus growers is behind the study (it wasn’t).
In the study (as opposed to the news release) the authors say that they “also found dietary manganese to be protective against cross-sectional nuclear cataract independent of vitamin C,” but found no association between dietary magnesium and nuclear cataract progression and the lack of a dose response.”
That muddies the waters a bit, but would have been nice to include — if only to underscore the fact that much about science is not clear. And we never learn from the release that other interventions, like reducing cigarette smoking and ultraviolet light exposure, may prevent or delay the development of cataract, or that diabetes and overweight are risk factors, too. The study itself underscored these points.
It’s common knowledge that Vitamin C-rich foods are widely available, although in different types and costs, so we rated that Not Applicable.
The news release says, “A diet rich in vitamin C could cut risk of cataract progression by a third, suggests a study being published online today in Ophthalmology, the journal of the American Academy of Ophthalmology. The research is also the first to show that diet and lifestyle may play a greater role than genetics in cataract development and severity.”
Elsewhere in the release is another claim to novelty, “These results make the study the first to suggest that genetic factors may be less important in progression of cataract than previously thought.”
In the study the authors gave themselves wiggle room: “This study is the first, to our knowledge, to show that dietary vitamin C intake protects against progression of nuclear lens opacity.”
The release includes an unsupported claim in the headline, “Eating Foods High in Vitamin C Cuts Risk of Cataract Progression by a Third.” This is an observational study — a type of research that can’t determine cause and effect. And so the active verb used here “cuts risk” is not appropriate to describe the results.
With that being said, this does not rise to the kind of flagrant language that we flag under this criterion.