The effect of cooking on dietary nitrate content
While I work on my follow up article to food variety and over consumption of calories, I figured I would cover the effect of cooking and dietary nitrate. This is an issue I have given a lot of thought to as we continue to work on our Beet Strong bars- a high protein, high fiber, beet-infused nutrition bar (see here).
Dietary nitrate can be converted to nitric oxide in the body, a gaseous compound that helps blood vessels dilate and plays a role in the prevention of atherosclerosis and heart disease . The best sources of dietary nitrate are generally leafy green vegetables , which happen to be the vegetables with highest negative relation to heart disease . In other words, the more green leafy vegetables you eat, the less likelihood you could have of developing heart disease. Considering a recent meta-analysis also found that green leafy veggies reduced risk of cardiovascular disease, this is probably not a coincidence .
The problem is Americans don’t like to eat their fruits and vegetables. So researchers have started using enriched products to help deliver high doses of nitrate without the stigma associated with vegetables as being earthy and flavorless. Researchers most commonly use beet root juice sweetened with apple juice to investigate dietary nitrate [5-8], and a recent study used beet-enriched bread . In most investigations on the use of dietary nitrate the dose used is about 250 mg of nitrate or more, however the beet bread in this study used just over 100 mg in the bread.
Subjects were randomly assigned to one of three different bread products: equal amounts of a control white bread, or breads enriched with either white or red beetroot. The bread was served in the form of a sandwich with Philadelphia cheese spread. The white and red beets used in the bread were grown on the same farm under identical conditions and soil so the mineral content (i.e. potassium) was probably similar. Blood pressure (BP) was measured for 24 hours.
The authors reported BP started to decrease about an hour after eating the bread. The reported results show cumulative reductions in blood pressure exposure at 6, 13, and 24 hours (see figure 2 if interested). The reductions were statistically significant for diastolic BP and only for the red beetroot bread. However, the changes appear to be clinically meaningful for systolic BP, and white beet root bread as well.
This is where my interest was piqued. The beet-enriched bread appeared to provide some beneficial health benefits, even at a low nitrate dose. Furthermore, I didn’t mention that the study participants were young, healthy males (average age of about 25). The findings may have been more pronounced had the researchers used older participants or people with elevated blood pressure. The key takeaway here is that these findings indicate that fortifying “normal” foods with fruits and vegetables may be a clever way to get people to increase their fruit and vegetable consumption. However, I was personally intrigued that the findings also indicate that the cooking process did not negate the health-promoting effects of the dietary nitrate. This led me to further wonder how cooking affects dietary nitrate.
Apparently there has not been a ton of research done on this topic. However, a lot of the findings thus far do make sense intuitively. In a study on the effects of cooking and wilting* on nitrate and nitrite, the content of each compound in seven varieties of commonly consumed vegetables in East Africa were determined before and after ten minutes of boiling or wilting (eight hour wilting with measures of nitrate/nitrite being taken every two hours) .** Boiling the veggies reduced the nitrate levels in all the samples but nitrite levels surprisingly increased in all samples. Wilting also decreased nitrate levels. Interestingly, nitrite either increased or stayed the same in all of the vegetables. The ramifications of this finding are unclear.*** Another study described the effects on the nitrate content of vegetables boiled in water for five or fifteen minutes. Certain vegetables showed a slight increase at five minutes but all of the vegetables examined (celery, cabbage, spring greens, leeks, and turnips) displayed a decrease in nitrate levels at fifteen minutes. The nitrate content of the cooking water used demonstrated a sustained increase as the cooking time went on . While this is a small sample, I think the finding that all of the vegetables displayed a decrease in nitrate content at fifteen minutes is pretty compelling.
In summary, it appears that cooking products containing nitrate could reduce nitrate content, but this would likely vary considerably between cooking methods. The two studies discussed here whereby nitrate levels were reduced both used boiling. In the study that used bread, a baked product, it appears the nitrate content remained intact as indicated by the reduced blood pressure measures. However, this was not a direct measure of nitrate content, so we can’t rule out that nitrates were reduced. An interesting future study may evaluate the effects of various common methods of cooking on dietary nitrate. The final take away here would be that consuming fresh veggies may be the way to go if maximizing dietary nitrate is the goal. This was indicated by the study showing decreased nitrate levels as the veggies sat out (wilting). Dietary nitrate may lower the likelihood of developing heart disease, and eating fresh veggies may be the best way to maximize nitrate in your diet. So until next time, enjoy your vegetables!
Thanks to Sofie and Vik for editing the article.; thumbnail photocredit to Desertrose 7 @ pixabay.
* Wilting is when a plant or leaf becomes limp due to heat or loss of water.
**Anyone with access to the full text and interested enough to convert the nitrite concentrations reported in the study will note that the nitrate concentrations were low. However, I would attribute this to simple methodological differences. Most studies now use chemiluminescence.
***In order to determine if the gain in nitrite outweighs the loss the loss of nitrate a study comparing the vascular effects of cooked vs raw forms of the vegetables would have to be conducted.
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