According to a prospective study by the Norwich Medical School in the UK, women who had the highest intake of flavanone, a compound in citrus fruits, had the lowest risk of ischemic stroke. But the operative word in that headline is “may”.
The women took part in the Nurses’ Health Study in the U.S. More than 700,000 women were followed over 14 years. Those who consumed the highest amount of flavanones had an overall 19% lower risk of ischemic stroke, the type of stroke caused by a blood clot in the brain.
In the study, researchers looked at six different subclasses of flavonoids, including flavanones, anthcyanins, flavan-3-ols, flavonols, flavones, and polymers. The women who received the highest amount of flavanones in their stated diets had reduced risk of stroke.
But when the researchers looked for a relationship between citrus and stroke risk, there was no significant association. And there was no association between total flavonoid intake and stroke risk.
While the conclusion is interesting, it does not establish cause and effect. Food frequency questionnaires were used, which may contain inaccuracies. And studies that simply look at data don’t control other factors that affect stroke risk, such as exercise, family history, and smoking. In addition, there was no way to measure the actual flavanone content of the fruit in the women’s diets.
The researchers stated that the women who consumed the most flavanones also had a better overall diet, exercised more often, and smoked less than women who consumed less flavanones.
Just because there was a correlation between two facts (higher flavanone intake, lower stroke risk), does not mean that there is a cause-and-effect relationship. Epidemiological case-control studies, such as this one, cannot prove causation, since they simply observe facts and statistics. Double-blind, placebo-controlled, randomized clinical trials are the gold standard in medicine and nutrition used to prove cause and affect.
The summary of the study states this fact:
“Citrus fruit consumption may be associated with a reduction in stroke risk, and experimental data support these epidemiological associations that the flavanone content of citrus fruits may potentially be cardioprotective. Further prospective studies are needed to confirm these associations.”
This study can be used to generate a hypothesis, that increased citrus fruit intake may reduce stroke risk, that can then be tested in a controlled clinical trial. It’s good to remember that epidemiological research, such as the study that found eating red meat increases death rates aren’t the final word in nutrition.
