Could an Orange a Day Keep BP Down?

January 06, 2009

January 6, 2009 (Berkeley, California) — A new observational study has found that plasma vitamin-C concentrations were inversely associated with blood pressure in young women [1]. The results suggest that this nutrient may favorably influence blood pressure in healthy young adults, say the researchers, led by Dr Gladys Block (University of California, Berkeley), who report their study online December 17, 2008 in the Nutrition Journal.

Block told heartwire : "These findings are very relevant. Many of these young women were already obese, and we know this is pertinent to the subsequent development of frank hypertension." Young people should thus be encouraged to eat healthier diets, particularly more fruit and vegetables, she notes. But she adds that the sad reality is that the diets of many people of this age "are horrendous," and she therefore advocates that such people be encouraged to take a simple multivitamin a day.

And she acknowledges that many people are skeptical of vitamin C: "I recognize that no one is going to be convinced about this vitamin-C hypothesis until I can get funding to do a randomized trial, and I have applications in to get funding to do this."

The study of Block et al certainly is provocative.

Dr Franz Messerli (St Luke's Roosevelt Hospital, New York), a hypertension expert who was not connected with this study, told heartwire : "In humans, the cardiovascular effects of vitamin C have been disappointing." Nevertheless, he says, "The study of Block et al certainly is provocative in that it documents an inverse correlation between blood pressure and vitamin-C levels in young women. Clearly, this is an observational study and may at best serve to generate a hypothesis that could be tested in a prospective trial.

"However, any extrapolation from these observations to the use of vitamin C as a therapeutic agent in hypertensive cardiovascular disease is a non sequitur and should be considered scientific hogwash," he adds.  

Young Participants Already Overweight

In their study, Block et al followed 242 girls aged 18 to 21 who were participants in the National Heart, Lung, and Blood Institute Growth and Health Study, a 10-year longitudinal study to investigate the development of obesity in black and white girls during adolescence. For this particular research, Block et al examined only girls in the Richmond, CA, cohort who had plasma ascorbic acid (vitamin-C) samples obtained at the 10th annual visit. Of the subjects, 155 were black and the remainder white.

Block points out that the average body-mass index (BMI) in this study was 26, "so the mean was already in the overweight section, and we know that people who are overweight at this age are going to be even more overweight at higher ages, portending the development of hypertension."

In cross-sectional analysis, after adjustment for race, BMI, education, and dietary intake of fat and sodium, plasma ascorbic acid at year 10 was inversely associated with systolic and diastolic BP. Those in the highest quartile of plasma vitamin C had 4.66-mm-Hg lower systolic BP and 6.04-mm-Hg lower diastolic BP (p=0.0002) than those in the lowest quartile.

And in analysis of the change in BP, plasma ascorbic acid was also inversely associated with change in systolic BP and diastolic BP during the previous year. While diastolic BP among those in the lowest quartile of plasma vitamin C increased by 5.97 mm Hg from year 9 to 10, those in the highest quartile increased by only 0.23 mm Hg (p<0.0001). A similar trend was seen for change in systolic BP (p=0.005).

"It appears that the BP is less likely to rise if people have a good level of plasma vitamin C," Block noted.

She and her colleagues conclude: "This study suggests that vitamin C may be an important factor in BP regulation even among healthy young adults and that further study is warranted."

Young People Are Not Eating Fruits and Vegetables

Messerli told heartwire : "To my way of thinking, plasma vitamin-C level in this population could be simply a good biomarker for intake of fruit and vegetables. Thus, the more fruits and vegetables these young women ate (ie, the healthier their diet), the lower their salt intake and not surprisingly, the lower their blood pressure."

Block said that she believes the lower half of the plasma vitamin-C concentrations seen in the study participants "probably represents that coming from the diet" but that the upper one-quarter "likely represents those getting [vitamin C] from supplements.

"The blood plasma levels of vitamin C are high enough in the upper quartile that it's unlikely they are getting it from food." She also points out that the data were adjusted to take into account sodium and fat intake.

And she says that while it is laudable to advocate better diets for young people, the reality is that this is a difficult thing to achieve. "I've published lots of papers, I'm known as the fruit-and-veg woman, and obviously we should definitely be encouraging people to eat more fruit and vegetables. But I've also done a lot of research looking at national data to see what people actually are eating, and they are not eating fruits and vegetables.

I've also done a lot of research . . . to see what people actually are eating, and they are not eating fruits and vegetables.

"The proportion of people who are eating a good diet--not just in terms of vitamin C, but in terms of a whole range of nutrients, is actually quite low," she continues. "And in this particular population--young people, many of whom are on low incomes--their diets are horrendous. Other researchers in my team have published quite shocking information about what people in this age range are eating, and it's horrible--they are drinking Coke for breakfast!

"So I say, let's be realistic about what people are actually doing, and let's get these nutrients into them, one way or another. It seems to me a perfectly prudent recommendation that everyone ought to be taking a multivitamin--they cost pennies a day."

Randomized Trial Needed To Counter Skepticism

Block says she has recently authored two papers detailing possible mechanisms by which vitamin C could lower BP. These show that vitamin C significantly lowers F2-isprostane, a marker of oxidative stress, and CRP, a marker of inflammation, but, crucially, only in those who already have elevated levels of these biomarkers [2,3]. "Both inflammation and oxidative stress are pretty well established as having a role in hypertension," she points out.

Messerli says, "Vitamin C is a powerful antioxidant in vitro, and in some animal studies it has been shown to act as a vasodilator, possibly by enhancing the bioavailability of nitric oxide. But a recent study has failed to show an effect of the acute oral ingestion of vitamin C on oxidative stress, arterial stiffness, or blood pressure in healthy subjects."

Block realizes she will have to conduct a randomized trial to convince others of her enthusiasm for vitamin C but says that she has been appalled at the amount of skepticism she has encountered.

"I submitted a grant proposal to the [National Institutes of Health] and was amazed at some of the comments I got back from reviewers of the proposal. Some of the comments were, 'Well, we have good medication [for hypertension] that works, so why even bother with this hypothesis?' or 'We know the DASH diet works, so why bother withthis hypothesis?'

"But this attitude overlooks the fact that we are just not getting people to eat the DASH diet," she says. "We've talked about getting people to eat more fruit and vegetables. Well, DASH is five to nine servings of fruits and vegetables a day, as well as low sodium, high calcium, and low saturated fat, but the proportion of the population who are eating that is vanishingly small," she concludes.

  1. Block G, Jensen CD, Norkus EP, et al. Vitamin C in plasma is inversely related to blood pressure and change in blood pressure during the previous year in young black and white women. Nutrition J 2008; DOI: 10.1186/1475-2891-7-35. Available at: http://www.nutritionj.com. Abstract

  2. Block G, Jensen CD, Morrow J, et al. The effects of vitamins C and E on biomarkers of oxidative stress depend on baseline level. Free Radic Biol Med 2008; 45:377-384. Abstract

  3. Block G, Jensen CD, Dalvi TB, et al. Vitamin C treatment reduces elevated C-reactive protein. Free Radic Biol Med 2009; 46:70-777. Abstract

 

The complete contents of Heartwire , a professional news service of WebMD, can be found at www.theheart.org, a Web site for cardiovascular healthcare professionals.

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