According to findings of a randomised, cross-over trial, intakes of nitrates five times the recommended levels resulted in significant benefits to cardiovascular health with diastolic BP decreased by an average of 4.5 mmHg.
The current WHO Acceptable Daily Intake (ADI) is 3.7 mg per kg of body weight, which is equivalent to 259 mg per day for a 70 kg adult. The new study looked at consuming nitrate levels of 18.8 mg per kg of bodyweight, or 1,316 mg per day for a 70 kg adult, as part of a traditional Japanese diet.
Researchers from the Kyorin University School of Medicine in Tokyo, Japan and the Karolinska Institutet in Sweded report their findings in the journal Nitric Oxide.
“Time might have come to re-evaluate the ADI recommendations regarding nitrate consumption,” stated the researchers.
About 80 per cent of nitrates in the diet come from vegetables, while nitrites sources include vegetables, fruit, and processed meats. Nitrites are added to meat to retard rancidity, stabilise flavour, and establish the characteristic pink colour of cured meat. Studies and recommendations by health and governmental organisations ensure the safety of such products.
Observational studies, including data from the third National Health and Nutritional Examination Survey (NHANES) on 7,352 subjects over the age of 45, have suggested that increased consumption of nitrites from cured meat could increase the risk of lung disease.
However, an increasing number of studies are reporting the potential benefits of nitrates and nitrites. A recent study from researchers at Michigan State University went as far as to suggest that the compounds may actually be nutritious (American Journal of Clinical Nutrition, doi:10.3945/ajcn.2008.27131).
Twenty-five healthy volunteers with an average age of 36 were recruited to participate in the 10-day study whereby the consumption of traditional Japanese vegetables was encouraged.
At the end of the study, nitrate and nitrite levels were significantly higher in plasma and saliva, and diastolic blood pressure was reduced by an average of 4.5 mmHg. No effects on systolic blood pressure were obserevd.
Interestingly, the effects were found in people with seemingly normal blood pressure.
“Our findings further support the importance of the role of dietary nitrate on blood pressure regulation suggesting one possible explanation of healthy aspects of traditional Japanese food,” wrote the researchers.
In terms of the potential mechanism, the researchers note that nitrate can be converted to nitric oxide, a compound that plays a key role in blood vessel relaxation. Nitrate is converted to nitrite and then nitric oxide, said the researchers.
The findings that dietary intakes of nitrates may benefit blood pressure appears to go some way to answering questions raised by Martijn Katan from the Institute of Health Sciences, VU University in Amsterdam. Following publication of the Michigan State study, Dr Katan noted that, while it is “undisputed” that nitrates benefit arteries, “evidence is still scant that nitrate in the amount present in vegetables lowers blood pressure”.
“Therefore, indications that dietary nitrate or nitrite reduces cardiovascular disease risk are insufficient to relax standards for nitrate in drinking water and foods.”
Source: Nitric Oxide
Volume 22, Issue 2, Pages 136-140
“Dietary nitrate in Japanese traditional foods lowers diastolic blood pressure in healthy volunteers”
Authors: Tanja Sobko, Claude Marcus, Mirco Govoni, Shigeru Kamiya