Nitrate-rich leafy greens 'good for the heart'

Behind the Headlines

Thursday December 4 2014

Maybe our mums were right about eating up all our greens

Nitrate-rich foods include lettuce, parsley and spinach

“Leafy vegetables contain chemical nitrate that improves heart health,” the Mail Online reports. In a recent study, researchers looked at the effects of a nitrate-rich diet on rats.

Nitrate is a chemical that can react to a number of different substances in a range of ways. For example, it can be used as a fertiliser or as the active ingredient in a bomb. Some nitrates are used as medication for angina, as they dilate the blood vessels.

This study found that rats given nitrate had lower levels of red blood cells (which carry oxygen) compared to a control group. This was associated with a reduction in the hormone erythropoietin (EPO), which regulates red blood cells.

Excessive amounts of red blood cells (polycythaemia) can sometimes trigger blood clots.

Blood clots can sometimes lead to serious complications, such as a stroke.

This study found that increasing nitrate in your diet stops low oxygen levels causing the over-production of EPO. The increased nitrate optimises the production of EPO from the liver and kidneys, which in turn reduces the blood’s thickness, but without compromising oxygen supply.

While the study involved rats not people, it’s always a good idea to eat up your greens. They contain a number of nutrients thought to help prevent cancer and heart disease.

 

Nitrate-rich foods

Nitrate-rich vegetables include:

  • lettuce
  • beets
  • carrots
  • green beans
  • spinach
  • parsley
  • cabbage
  • radishes
  • celery
  • collard greens

Where did the story come from?

The study was carried out by researchers from the Universities of Cambridge and Southampton. It was funded by the British Heart Foundation, the Research Councils UK, the WYNG Foundation of Hong Kong, the European Union Framework 7 Inheritance project, the Wellcome Trust and the University of Southampton.

The study was published in the peer-reviewed Journal of the Federation of American Societies for Experimental Biology.

The reporting in the Mail Online and the Daily Express appears to be based on a press release that combined the findings of three related studies on nitrates:

  • The study we are analysing today (we chose this, as it is the most recent research).
  • study on the effects that nitrates have on how efficient the heart is in pumping blood around the body.
  • study into whether nitrates could have a protective effect against obesity and type 2 diabetes.

The reports in both the Express and the Mail overstate the results of all the studies, including the one we are discussing today. Neither paper mentioned that these were laboratory studies carried out in rats. Perhaps this is not surprising, given that the accompanying press release – and the authors quoted by it – did not mention it either.

 

What kind of research was this?

This was a laboratory study, which looked at the effect of nitrate supplementation on the red blood cells of rats.

EPO is responsible for regulating red blood cells in mammals, to meet the need for oxygen. In conditions of severe oxygen shortage, such as during critical illness and at high altitude, EPO increases, stimulating the production of more oxygen-carrying red blood cells.

While red blood cells are needed to supply enough oxygen, they can also lead to an increase in the blood’s “viscosity” or thickness, which may impair blood flow, as happens in chronic obstructive pulmonary disease, preventing it from flowing through small blood vessels in the lungs.

There is also the risk of a blood clot developing, which can lead to serious complications, such as a heart attack, stroke or pulmonary embolism.

A balance therefore needs to be met to get the optimum number of red blood cells and oxygen around the body.

Nitrate has already been shown to have beneficial effects on the heart and circulation. Here, the researchers wanted to test the theory that dietary nitrate might limit rises in the red blood cells needed for oxygen delivery by improving the efficiency of the body’s use of oxygen.

 

What did the research involve?

Two rat studies were performed to assess the effect of dietary supplementation with nitrates.

The first involved 40 rats. Half of them had nitrate added to their drinking water, while the other half acted as a control group with no supplementation. After four days, both groups were put in a chamber of low oxygen (12% rather than normal air, which is 21%). They continued to have either nitrate supplement or no supplement for 14 days.

The researchers compared their food and water intake, any change in body weight and plasma nitrate and haemoglobin (oxygen carrying component of red blood cells) levels in normal air and in low oxygen.

The second study aimed to see how fast and at what concentration the nitrate made changes to the haemoglobin levels. 24 rats were kept in normal oxygen conditions. After 12 days, half the group had their water supplemented with 0.7mm of nitrate. They measured the haemoglobin level in the blood after 0, 2, 4, 6, 9 and 12 days.

 

What were the basic results?

The researchers report that in both experiments rats given nitrate had lower concentrations of red blood cells in normal and low oxygen conditions compared with control groups.

They found that these rats also had lower levels of EPO. They say this suggests that the effects of nitrate were mediated via changes in EPO production.

The researchers found that nitrates reduced the amount of EPO released by the liver, but increased the amount released by the kidneys. They report that this balance meant that the nitrates were able to help the body produce the optimum minimum amount of haemoglobin that they required.

Nitrate supplementation did not alter the amount of food or water intake of the rats, or on their weight or growth.

 

How did the researchers interpret the results?

They conclude that nitrate acts to suppress the production of EPO by the liver, thereby lowering circulating red blood cells. Nitrate prevented an expected rise in circulating red blood cells in rats deprived of oxygen and also decreased red blood cells in rats with a normal oxygen supply.

They point out that nitrate levels used are readily achievable in humans via the diet, through eating green leafy vegetables.

In an accompanying press release, co-author Professor Martin Feelisch, from the University of Southampton, said: "These findings suggest simple dietary changes may offer treatments for people suffering from heart and blood vessel diseases that cause too many red blood cells to be produced. It is also exciting as it may have broader implications in sport science, and could aid recovery of patients in intensive care by helping us understand how oxygen can be delivered to our cells more efficiently."

 

Conclusion

It’s always a good idea to eat up your greens. This research suggests that one possible benefit is through the mechanism of nitrate “thinning” the blood and protecting against heart disease.  While the research is interesting, it’s a pity that no one thought to mention that this was a laboratory study on rats. It is important to remember that high levels of nitrates can be toxic, which is why there are safety limits for the level of nitrates in drinking water. High nitrate levels are especially harmful for infants.

A healthy diet – including plenty of vegetables – and regular exercise are important for a healthy heart and weight.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

Now there's no excuse not to eat your greens: Leafy vegetables contain chemical nitrate that improves heart health and combats diabetes. Mail Online, November 4 2014

Eat up your greens and slash the risk of diabetes and obesity. Daily Express, November 4 2014

Links to the science

Ashmore T, Fernandez BO, Evans CE, et al. Suppression of erythropoiesis by dietary nitrate. The FASEB Journal. Published online November 24 2014

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