A cup of leafy greens a day makes the heart sturdy

Eating just one cup of nitrate-rich vegetables, either cooked or raw, daily may go a long way toward lowering the risk of cardiovascular disease (CVD), according to a study.

In a cohort of 53,150 individuals residing in Denmark and CVD-free at baseline, a moderate vegetable nitrate intake quintile cut the risk of developing CVD by 15 percent compared with the lowest intake quintile (median, 59 vs 23 mg/day; hazard ratio [HR], 0.85, 95 percent confidence interval [CI], 0.82–0.89). [Eur J Epidemiol 2021;doi:10.1007/s10654-021-00747-3]

Lead researcher Dr Catherine Bondonno from New Edith Cowan University Institute for Nutrition Research in Perth, Australia, noted that the greatest reduction in risk occurred for peripheral artery disease (26 percent), “which is a type of heart disease characterized by the narrowing of blood vessels of the legs.”

Nevertheless, moderate vegetable nitrate intake also conferred protection against ischaemic stroke (17 percent), heart failure (15 percent), and ischaemic heart disease (12 percent).

Bondonno and her team followed the participants for a median of 21 years, during which 14,088 cases of incident CVD were documented. At baseline, the median age of the entire cohort was 56 years. Total median vegetable nitrate intake was 59 mg/day, while nonvegetable nitrate intake was 15 mg/day. Major individual vegetables that contributed to vegetable nitrate intake were lettuce (41 percent), potato (22 percent), celery (10 percent), carrot (5 percent), and spinach (3 percent).

Of note, participants in the highest vs lowest vegetable nitrate intake quintile (median, 141 vs 23 mg/day) had baseline systolic blood pressure (BP) that was lower by 2.58 mm Hg (95 percent CI, −3.12 to − 2.05) and baseline diastolic BP that was 1.38 mm Hg lower (95 percent CI, − 1.66 to − 1.10).

Vegetable nitrate intake was, indeed, inversely associated with CVD, with the protective effect plateauing at moderate intakes (~60 mg/day). This appeared to be more pronounced among individuals with high alcohol consumption (>20 g/day), indicating that increased nitrate levels may partially mitigate the harmful effects of alcohol.

It did suggest that more was not necessarily better, and going from about 60 mg/day to more had no big impact. Bondonno said: “People don’t need to be taking supplements to boost their nitrate levels, because the study showed that one cup of leafy green vegetables each day is enough to reap the benefits for heart disease.”

There is good evidence that nitric oxide (NO) may protect against cardiac diseases and strokes by lowering BP. NO is a compound that helps maintain vascular tone, which in turn influences blood flow and BP. [Free Radic Biol Med 2004;37:395-400]

In the present study, Bondonno explained that baseline systolic BP explained 21.9 percent of the total association between vegetable nitrate intake and incident CVD, showing that not only the effects of nitrate on BP reported in clinical trials translate to a lower risk of incident CVD, but also that other mechanisms are in play.

However, the study was unable to establish causality or rule out residual or unmeasured confounding factors, the researcher acknowledged. Also, potential interactions with known inhibitors of the nitrate-nitrite-NO pathway, including antibacterial mouthwash and proton pump inhibitors, were not investigated. Finally, since the study population was mostly Caucasian, caution should be taken when extrapolating the findings to other races and ethnicities.

Despite the said limitations, the findings drive home the message that nitrate-rich vegetables are good for the heart and people need not eat more than a cup to benefit from it. For those who are looking to make their daily leafy greens less boring and more palate-friendly, Bondonno suggested including a cup of spinach in a banana or berry smoothie. “Blending leafy greens is fine, but don’t juice them. Juicing vegetables removes the pulp and fibre.”