Dietary nitrate seems to have different effects on platelet function in men than in women, inhibiting platelet activation in women but actually increasing it in men. This is one of the surprising findings of a small study from a team led by Scott J. Cameron, MD, PhD, Section Head of Vascular Medicine at Cleveland Clinic, recently published online in JACC: Basic to Translational Science.
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Nitrate is a precursor of the ubiquitous chemical messenger nitric oxide and a key component of many healthy foods such as green leafy vegetables. Many physicians encourage patients who have had heart attacks or strokes to take up a plant-based diet, in view of the many beneficial effects of such a diet, including blood pressure reduction and atherosclerosis prevention. “However, these findings raise questions about the advice we give patients, particularly whether we should be giving different advice to men and women,” says Dr. Cameron. “They also help identify some priorities for further research.”
In addition to containing various vitamins and minerals, green leafy vegetables are rich in nitrate (NO3–). In the gut, nitrate is reduced (stripped of one of its oxygen atoms) by intestinal bacteria to make nitrite (NO2–), which is absorbed into the bloodstream. In various tissues in the body, nitrite is further reduced to produce nitric oxide (NO). The effects of nitric oxide are local and short-acting, and they manifest in the body in beneficial ways, such as by relaxing blood vessels and inhibiting inflammation. Nitric oxide may also help regulate platelet function.
“Earlier studies suggested that platelet signaling differed between men and women (and between male and female mice), as well as between healthy subjects and those having a myocardial infarction,” notes Dr. Cameron. “When we heard about oral nitrate supplements, I wondered if it might affect platelets — and affect women and men differently.”
The researchers recruited 22 healthy volunteers, 12 men (mean age 34) and 10 women (mean age 41; only one was in menopause). All were white.
Each volunteer came in for two visits at least two weeks apart. At one visit volunteers received an oral dose of nitrate and at the other visit they received placebo, with the order determined by chance in a double-blind manner. Thus, each participant served as his or her own control. The oral nitrate consisted of two tablets, each containing 490 mg of potassium nitrate.
“That is a lot of nitrate, but not a ridiculous amount,” says the study’s first author, Matthew Godwin of Cleveland Clinic Lerner College of Medicine. “It is approximately the amount in half a pound of fresh spinach, for example.”
Ninety minutes after taking their placebo or nitrate pills, participants had their blood drawn and gave a saliva sample, and the samples underwent a battery of tests to measure nitrate, nitrite and nitric oxide levels and platelet reactivity.
After receiving placebo, levels of platelet reactivity were higher by several orders of magnitude in the women than in the men as measured by several methods. The reason is not clear, but Dr. Cameron notes that platelet reactivity in women may vary in different phases of the menstrual cycle, which this study did not account for, and may be related to estrogen levels.
In women, the oral nitrate supplement markedly boosted blood nitrate and nitrite levels and platelet nitric oxide production. Most importantly, it inhibited their platelet reactivity.
In men, the oral nitrate increased blood nitrate and nitrite levels to a similar extent as in women. However, it increased their platelet nitric oxide production much less, and actually increased their platelet reactivity, the opposite of the desired effect.
“The conclusions of this study are provocative, highlighting the complex interactions between dietary, hormonal and genetic factors in human biological systems,” comments A. Michael Lincoff, MD, Vice Chair of Cleveland Clinic’s Department of Cardiovascular Medicine, who was not involved in the study. “The observations suggesting a paradoxical effect of dietary nitrates on platelet reactivity in men compared with women illustrate the importance of scientific evidence in developing dietary or other medical recommendations, rather than all-too-common fads or flimsy ‘expert opinion.’ These findings await confirmation or clarification in larger and more real-world patient settings, but they reinforce why we must strive to achieve greater diversity of enrollees in clinical trials to allow us to understand potential gender and racial differences in patients’ responses to medical therapies.”
For his part, Dr. Cameron stresses that this work does not mean men should stop eating their vegetables — certainly not yet. “We would like to do another study examining both platelet reactivity and ‘hard’ outcomes, such as recurrent heart attacks in patients who are adhering to a plant-based diet after a heart attack,” he says. “We would specifically examine whether women fare better on this diet than men do.”
For now, Dr. Cameron is not changing his advice to patients regarding a healthy diet, and he is not recommending nitrate supplements. And asked what he is eating these days, he says: “Salad!”