Increased levels of the vitamin, associated with increased intake of fruit and vegetables, were found to offer significant cardiovascular benefits among the 20,649 men and women taking part in the European Prospective Investigation into Cancer.
The authors, led by Phyo Myint from the University of Cambridge, state that blood levels of the vitamin could be used as a biological marker of lifestyle used to identify people at high risk of stroke.
"An intriguing possibility is that the plasma vitamin C concentration is a good marker of a wider range of health behaviors, such as fruit and vegetable consumption, that may be protective against stroke," wrote Myint in the American Journal of Clinical Nutrition .
"It is also plausible that vitamin C may biochemically affect stroke risk."
Strokes occur when blood clots or an artery bursts in the brain and interrupts the blood supply to a part of the brain.
It is the leading cause of disability and the third leading cause of death in Europe and the US.
According to the Stroke Alliance for Europe (SAFE), about 575,000 deaths are stroke related in Europe every year.
In the US, every 45 seconds someone will experience a stroke, according to the American Stroke Association.
Myint and co-workers followed the subjects for 9.5 years and documented 448 strokes during this time.
The subjects completed a health and lifestyle questionnaire at the start of the study, and blood samples were taken to measure vitamin C levels.
The highest average blood levels of vitamin C (greater than 66 micromoles per litre) were associated with a 42 per cent lower risk of stroke, compared to the lowest average blood levels (less than 41 micromoles per litre), after adjusting the results for potentially confounding factors such as age, sex, smoking habits, alcohol consumption, blood pressure, cholesterol levels, BMI, physical activity, and use of supplements.
When the Cambridge researchers excluded participants who consumed vitamin C-containing supplements the results were the same, indicating that the benefits could have been from vitamin C-rich foods, such as fruit and vegetables.
"We believe that these findings are of interest for several reasons," stated the authors.
"First, the strong inverse association between plasma vitamin C and stroke suggests that plasma vitamin C is likely to be a good biomarker of whatever causal factors affect stroke risk, most plausibly the dietary intake of plant foods.
"Second, irrespective of any causal associations, plasma vitamin C appears to be a good predictive risk indicator of stroke, independent of known risk factors such as age, BP, smoking, lipids, diabetes, and BMI.
"Given that about half of the risk of stroke is unexplained by conventional cardiovascular disease risk factors and that the predictive validity of traditional cardiovascular disease risk factors appears to diminish with age, risk markers that may help to identify those persons at greatest risk of stroke for targeted preventive interventions with established therapies, such as BP reduction, may be of interest."
While further study is necessary, Myint and co-workers aid that it is unlikely that long-term randomised controlled trials using isolated vitamin C supplementation would be conducted using cardiovascular disease as an end-point.
"Nevertheless, the magnitude of the association between plasma vitamin C and subsequent stroke is substantial and independent of known major risk factors for stroke," they concluded.
In an accompanying editorial, Sebastian Padayatty and Mark Levine from the National Institutes of Health (NIH) welcomed the study as "refreshing in that its findings are both clear and not overstated" .
"We need readily measurable and reliable biomarkers of fruit and vegetable intake," they continued.
"Vitamin C is an attractive marker of fruit and vegetable intake because these foods are the primary sources of dietary vitamin C. "However, use of vitamin C as an intake indicator has limitations.
With ingestion of pure vitamin C, there is a steep sigmoidal dose-concentration relation in humans for doses between 30 and 100 mg.
At 100 mg, fasting steady state plasma vitamin C concentrations are about 60 micromoles per litre.
At 200 mg, corresponding to an intake of about 5 servings of fruit and vegetables, fasting steady state plasma concentrations are about 70 micromol/L and do not increase much with higher doses.
"It is unknown whether the same dose- concentration relationships hold for vitamin C in foods."
The take-home message from the study, said Padayatty and Levine, was that the public should aim for between five and nine servings of fruit and vegetables per day and to consume a wide variety of such foods.
Source: American Journal of Clinical Nutrition January 2008, Volume 87, Pages 64-69 "Plasma vitamin C concentrations predict risk of incident stroke over 10 y in 20 649 participants of the European Prospective Investigation into Cancer - Norfolk prospective population study" Authors: P.K. Myint, R.N. Luben, A.A. Welch, S.A. Bingham, N.J. Wareham, and K.-T. Khaw Editorial: American Journal of Clinical Nutrition January 2008, Volume 87, Pages 5-7 "Fruit and vegetables: think variety, go ahead, eat!"
Authors: S.J. Padayatty and M. Levine