Research Review: How diet affects asthma | Precision Nutrition

Research Review: How diet affects asthma

By Helen Kollias


Take a deep breath in. Now exhale all the air. Take another deep breath in and exhale. Now one more time – deep breath in and out. Feel better, more relaxed and even a little less foggy-headed?

Most people take breathing for granted. You breathe in and out without a second thought, but for more and more people, trying to breathe can be a regular struggle. They can’t always take that deep, cleansing breath. There are over 100 million asthma sufferers worldwide, and that number’s increasing [1].

My brother’s an asthmatic. I asked him what an attack was like. He said, “It’s like trying to breathe through a long thin straw. The worse the attack, the longer and thinner the straw.”

Not fun.

Researchers are struggling to figure out why people get asthma and why more and more people, mostly in the Western world, are getting asthma.

By now you’re probably wondering where I’m going with this and how diet or exercise fits in. This week’s review covers a study that has found a link between how we eat and rates of asthma.

What is asthma

Before I get into the study, let’s get acquainted with asthma.

MDs define asthma as “chronic inflammatory disease of the airways.” This inflammation causes one or more symptoms: shortness of breath, tightness in the chest, coughing and wheezing.

In many ways asthma is just like getting punched in the eye.

  1. First, you need an irritant. With asthma you have things like smoke and dust; with a swollen eye you have a fist.
  2. Then, in both cases you get inflammation. With asthma, the inflammation is in the tubes of your lungs (bronchioles); with a swollen eye the inflammation is the eyelid and the tissue around the eye that was hit with the fist.
  3. Next you get blockage. With asthma you block air from going in and out of the lungs, and with a swollen eye you block light getting to the eye.
  4. Finally in asthma you can’t breathe; with a swollen eye you can’t see.

With asthma there is a constant amount of swelling that, when irritated, causes the muscles around the bronchioles to twitch. This narrows the already-narrow tube and causes even more swelling.


Asthma on the rise

In Canada, as of 2005, there were about 3 million asthmatics over 12 years old, or 8.4% of Canada’s population. In children, rates of asthma are higher – 12%.

Yet in 1979 there were only 2.3% asthmatics (over the age of 15). Less than 10 years later, in 1988, that number jumped to 4.9%. By 1994 it was 6.1%. And in the US, the number of asthmatics has gone up 60% since the early 1980s.

Problem is, doctors and researchers don’t know why. Family history, high exposure to dust mites as a baby, and tobacco smoke exposure are the strongest risk factors, but they don’t explain the increase in asthma.

What’s causing the increase? We can cross off family history, since in 30 years family history can’t change enough to account for the increases. And I’m pretty sure that there have been more dust mites in the past and tobacco use has been going down, not up in the last 30 years.

The hygiene hypothesis

Back in 1989, researcher David Strachan found that children of larger families were healthier and less likely to have allergies than children of smaller families [3]. He figured that more exposure to viruses and bacteria in larger families improved health and kept allergies away. Basically, large families have more germs that keep the immune system functioning properly – think of germs as your immune system’s gym.

The hypothesis — known as the hygiene hypothesis — is that if the environment is too clean then the child’s immune system doesn’t have to do anything. It just sits around until it perceives a threat (likely not a real threat) and then it over-reacts because it hasn’t had to deal with any real threats. In extreme cases, for instance, the slightest whiff of a relatively harmless substance such as peanuts leads to a full blown immunological meltdown called anaphylactic shock.

Asthma was thrown into the hygiene hypothesis in the 90s in a study that compared rates of asthma and allergies between East and West Germany [4]. The researchers figured that cleaner, more affluent West Germany would have fewer allergies and lower rates of asthma compared to the dirtier, poorer, and generally less healthy environment of East Germany, but that didn’t happen. In fact, East Germany had lower rates of asthma and allergies. Huh.

While the hygiene hypothesis may explain some differences in allergies and asthma, it doesn’t explain why there are more asthmatics today than thirty years ago – are we that much cleaner than in 1979? Probably not.

Research question

This week’s review explores other possible factors — in this case, food.

Nagel G, Weinmayr G, Kleiner A, Garcia-Marcos L, Strachan DP*; ISAAC Phase Two Study Group. Effect of diet on asthma and allergic sensitisation in the International Study on Allergies and Asthma in Childhood (ISAAC) Phase Two. Thorax. 2010 Jun;65(6):516-22.

*Yes, the same Strachan of the hygiene hypothesis fame.


This is a questionnaire-based study done over 10 years, starting from 1995. Parents of 50,004 schoolchildren aged 8-12 years, from 20 countries, were asked about asthmatic symptoms (wheezing) and eating habits. 10 years and a small city of schoolchildren for a study. Now that’s a sample size!

Some of the schoolchildren also got to have the not-so-fun skin prick or scratch test that involves being pricked or scratched on the back or forearm with up to 25 possible allergens. The test shows a subject’s response to an allergen.

Let’s say you’re pricked with a needle with cat dander. If you’re allergic to cat dander, then you get a red itchy bump. This isn’t too bad if you’re allergic to one or two things, but if you’re allergic to 20 of the 25 scratches… well, there goes your fun afternoon.

The study also grouped the schoolchildren into affluent and non-affluent countries. Affluent countries had a gross national income (GNI) of more than $9200 per year per capita.



Eating more fruit was correlated to less wheezing across all countries.

Okay that makes sense, because really why should income matter, right?

But it does.

  • Eating more fish in affluent countries was correlated to less wheezing. In non-affluent countries, eating more or less fish had no correlation with wheezing.
  • Conversely, eating more cooked green vegetables in non-affluent countries was correlated to less wheezing, but not in affluent countries.

If you throw out the economics, eating more fruit, vegetables and fish correlated to less wheezing – basically the Mediterranean diet. Eating more burgers — yes, burgers specifically — correlated to a higher chance of getting asthma during their lifetime. In other words, children who lived on basically the McDonald’s diet and ate burgers more than 3 times a week were more likely to wheeze.

Neither meat in general nor soda pop seemed to affect wheezing, however.

Allergies vs asthma?

Here’s something strange. In affluent countries, even though there were foods that correlated to wheezing (and asthma), none of the foods were associated to allergic sensitization.

In other words, if food X caused you to wheeze, you wouldn’t show a reaction to food X in the pinprick test. Since most asthmatics have and allergies you’d figure that if certain foods were associated with asthma these same foods would be associated with allergies, but that’s not the case in this study.

Even weirder, in non-affluent countries children with allergies that ate fish, raw vegetables, fruit juice and burgers were less likely to be wheezers (yup they used wheezers in the article.)


The big finding for this study was that eating burgers more than 3 times a week was associated with higher rates of children developing asthma while eating fruits, vegetables and fish were associated with lower rates of children developing asthma.

As with any questionnaire based study, you have a few limitations.

First, anything you find is just a correlation, so you can’t say that anything causes or prevents something else. While eating fruits, vegetables and fish are correlated with lower rates of asthma it doesn’t necessarily mean they prevent asthma. It could be that people who like the taste of broccoli, eat more broccoli and just happen to have lower rates of asthma. Or, people who eat broccoli might have other relevant health habits too.

The second problem with questionnaire-based studies is that it relies on people’s ability to accurately report what’s going on. Honesty, memory and perception of reality all affect the outcomes.

Despite these problems, this study is the first step in finding diet-based approaches to preventing asthma. Since it doesn’t contradict what you should be eating anyway, you don’t have to make any hard choices. Eat your fruits and vegetables – gee, grandma was right again.


Bottom line

If you didn’t have enough reasons to eat your fruits and vegetables while avoiding fast food you can add one more — it may decrease your risk of asthma.


Click here to view the information sources referenced in this article.

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