Low levels of vitamin C (ascorbic acid) in the bloodstream can identify people with a high risk of stroke, a large study in the United Kingdom finds.

Although many risk factors for stroke are known, about half of all cases are unexplained. An independent biomarker would allow clinicians to identify patients in the most danger and intervene with preventive therapies like BP reduction, the researchers note.

Using data from the European Prospective Investigation into Cancer, investigators at Cambridge University followed 20,649 people aged 40-79 years (mean age approximately 58 years), and without a history of stroke at baseline. During 9.5 years of follow-up, 448 strokes were recorded, about a third of them fatal.When ranked according to their level of plasma vitamin C, those in the top quartile (≥66 µmol/L) had a 42% reduced risk of stroke compared with those in the lowest (<41 µmol/L).

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That inverse relationship held despite biologic, behavioral, and social risk factors. Individuals in the top quartile also had the lowest BMI, systolic BP, and cholesterol concentrations. They were less likely to smoke and more likely to be active, to eat more fruits and vegetables, and have white-collar jobs. They also had a lower prevalence of heart attack, cancer, and diabetes at baseline (Am J Clin Nutr. 2008;87:64-69).