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How food affects your FeNO score

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Testing FeNO can help your doctor decide whether you have asthma and manage your asthma more effectively. Several factors can influence the level of FeNO that your lungs produce, including your age, sex, weight and height. This short article explains how some types of food can affect your FeNO score too, and what you can do before a FeNO test to make sure you get the best results.

How does a FeNO test work?

A FeNO test measures the amount of nitric oxide - known as NO - in your breath. Nitric oxide is important for a healthy body but too much of it from your lungs can indicate that you might have asthma. To find out your personal FeNO level, you can take a FeNO test. The test is quick and easy to do during a check-up.

Some devices, like NIOX VERO®, have a breathing handle that filters out any nitric oxide that is in the air around us. This means your test result will only show how much nitric oxide you breathe out so you receive the right FeNO score for you.

After you breathe in through the handle, you exhale back through the handle for 10 seconds (or 6 seconds for anyone under 10 years old if they can't manage 10 seconds, but most people find they can). The screen on the device has helpful animations that can let you know when to breathe out harder or more gently so you can achieve a stable flow. You only need to use a gentle exhalation, like blowing bubbles in a drink. When the time is up, the device analyses your breath and, around one minute later, it will show your FeNO score on the screen. Your doctor will be able to tell you what your FeNO number means and how likely it is that you have asthma.

Which foods increase your FeNO level?

Foods that contain a lot of nitrate can increase your FeNO level so you should avoid those for a couple of hours before a test. This will ensure the test result reflects your normal FeNO level. Nitrate-rich foods include green, leafy vegetables, like spinach and kale.1 Other sources of nitrate include cured meat, fish and dairy products. Beetroot can also increase FeNO. One study showed that people who drank beetroot juice had more than 20% higher FeNO levels 45 minutes later and the effects were still there after 90 minutes, so it's best to wait and have these items after your test.2

Can any foods make your FeNO level go down?

Some types of food and drink are thought to decrease your FeNO level. These include caffeinated drinks and alcohol, so it's wise to avoid those for a couple of hours before a test too.1 Tobacco, second-hand smoke and vaping have also been found to have an effect on FeNO. When scientists studied the effect of smoking a cigarette just before a FeNO test, they saw a significant drop in the smoker's FeNO number.3 They believe that both current and ex-smokers have lower FeNO, so the effects of tobacco can last for some time.4

Before you take a FeNO test, your doctor might ask about your food and drink so they can use your FeNO score correctly to give you the best treatment. Let your doctor know if you have been exposed to caffeine and food or drink with a high concentration of nitrate for a few hours before the test.

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Supporting evidence:

1. Dweik RA et al. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FeNO) for clinical applications. Am J Respir Crit Care Med. 2011;184(5):602-15.
2. Kroll JLet al. Acute ingestion of beetroot juice increases exhaled nitric oxide in healthy individuals. PLoS ONE. 2018;13(1): e0191030.
3. Jacinto T et al. Differential effect of cigarette smoke exposure on exhaled nitric oxide and blood eosinophils in healthy and asthmatic individuals. J Breath Res. 2017 21;11(3):036006.
4. Malinovschi A et al. Effect of smoking on exhaled nitric oxide and flow-independent nitric oxide exchange parameters. Eur Respir J. 2006;28(2):339-45.

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