What causes asthma? Bad genes? Or something in the air?
Pollen, pets, air pollution, smoke, even chickpea dust… all these are known to trigger asthma attacks.
Makes sense right? Asthma is a lung disease in which the airways become narrowed in response to things you breathe in.
But this logical conclusion, that in genetically susceptible individuals asthma is caused by things we inhale, may be missing the target by miles.
Let’s take a closer look. What happens in an asthma attack?
In an asthma attack, the lining of the airways becomes inflamed. That means swollen and covered in thick mucus. Also the smooth muscle in the airway walls contracts. All these things work together to narrow the airways, making breathing difficult.
Traditional thinking states that in asthmatics airway inflammation is triggered by things we inhale, or sometimes by cold or emotional stress. Which is true.
But rarely do we discuss why some people react to these things and others don’t. A few other facts also hint at the limitations of this way of thinking.
Asthma prevalence (how many people have it at any given time) varies greatly around the world, from 1-7% in Eastern Europe, China and Indonesia to 17-30% in the UK, New Zealand and Australia (1). While this could be explained by differences in genetics, it is notable that asthma appears to be a predominantly Western disease.
Of particular interest is the observation that asthma prevalence has risen dramatically in the last few decades (and it is generally agreed that this rise is real and not just the result of increased rates of diagnosis). For example in Aberdeen in 1964 only 4.1% of children were affected, compared to 19.6% in 1994 (2). This parallels the rise in adult chronic diseases such as heart disease, diabetes and some cancers, which are beginning to be recognised as dietary in origin. A similar rise has also been seen in other atopic conditions (eczema, allergic rhinitis), and I hardly need to mention the obesity epidemic that has been developing over the same time period.
This rate of rise more or less excludes the possibility that susceptibility to wheeze is mainly determined by your genes, which do not change so quickly. That only leaves environmental factors as the probable culprit.
So could it be air pollution? Well, back in 1994 Professor Anthony Seaton from the Department of Environmental and Occupational Medicine at the University of Aberdeen wrote a hypothesis paper entitled “Increase in asthma: a more toxic environment or a more susceptible population?” (3). In it he states that:
“it is not surprising that the attention of researchers should have focused on air pollution […] There is no doubt that many [outdoor air pollutants] can induce bronchoconstriction in susceptible individuals in the laboratory setting […] However air pollution in general in the UK has lessened substantially since the smogs of the 1950’s […] there has been a well documented fall in levels of smoke and sulphur dioxide. [Urban] ozone levels have not altered since the early 1970’s […] Peak urban levels [of oxides of nitrogen] do not appear to have risen. While these data over an adequate time and range of sites are sparse, there is no evidence of the significant rises in any pollutants that would have been necessary were they to have been responsible for the increase in asthma and hayfever. In support of this conclusion, no association between exposure to particulates, nitrogen dioxide, or sulphur dioxide and prevalence of asthma was found in the American six cities study, nor in a more recent study which compared schoolchildren in the heavily polluted city of Leipzig and the cleaner city of Munich; there was significantly more hayfever and rhinitis in Munich children, while the prevalence of airway hyperreactivity did not differ significantly between the two populations.
Meanwhile there is no reason to think that exposure to animal dander, cigarette smoke, house dust mite, cold and stress have increased in recent times, and if anything they are more likely to have fallen with the invention of vacuum cleaners, hypoallergenic duvets and central heating, and with greater passive smoking awareness, certainly around children.
Why are more and more people developing airway inflammation in response to environmental triggers? Why do people in large parts of the world hardly suffer from asthma at all, and why did people in the UK hardly suffer from asthma in the past? Something we are doing (or not doing) here and now is making us ill, regardless of our genes.
Back to the pathophysiology. We are all being exposed to foreign inhaled particles all of the time. Healthy bodies do not develop an abnormal inflammatory response to those triggers. We are designed to be able to cope.
The key to this puzzle may lie in a chemical process known as oxidation. Put simply, oxidation is what happens when a chemical combines with oxygen. Or it can refer to the loss of electrons, but I’m not going to go into the details of that here. What is important to know is that oxidation reactions are occurring all the time in our bodies, producing toxic metabolites called oxygen free radicals. These are highly chemically reactive and have been implicated in the aging process. If not removed, free radicals cause cellular damage, and they are important mediators in airway inflammation.
What our bodies need, in order to clear free radicals from our systems, are antioxidants. These include enzymes our bodies make themselves, and complex chemical compounds derived from plant foods in the diet: vitamin C, vitamin E, carotenoids, flavonoids and lycopene to name just a few.
So reduced dietary antioxidant intake may result in increased susceptibility to oxidant attack and airway inflammation. Could this be the key? After all, we know that only around 20% of UK children aged 5-15 currently consume the recommended five portions of fruit and vegetables per day (4), and over 50% of a Scottish sample of adults consumed less than the recommended daily intakes of vitamins C, E and carotene (3).
Well, consumption of fruits rich in Vitamin C (kiwi and citrus) has been shown to be a highly significant protective factor for wheeze in children in one Italian study that asked about intake and then followed up 18,737 children (5).
The International Study of Asthma and Allergies in Childhood (ISAAC) analysed the global prevalence of wheeze, atopic eczema and allergic rhinoconjunctivitis in 6-7 year olds and 13-14 year olds in relation to diet. Symptom prevalence was plotted against food intake derived from UN FAO (Food and Agricultural Organisation) data for 53 of the 56 countries that took part, and a consistent inverse relationship was seen between prevalence rates and the intake of starch, cereals and vegetables (6).
This means that in countries where a high proportion of calories are derived from grains, tubers and vegetables, asthma prevalence is low, and vice versa. It does not tell us that the people in those countries who have asthma are the ones eating the least amounts of these foods, and so it is impossible to say from this that the relationship is causal, but it is one piece of the jigsaw.
What you need to do in order to draw conclusions about causation, is take a group of asthmatics and a group of matched non-asthmatic controls, and either add or take away the dietary components of interest (in this case antioxidants from fruits and vegetables), and see if they get better, or worse, over time.
As long ago as 1985 (7) it was shown that treatment of 35 adult patients with severe asthma with a vegan diet for one year, resulted in medication being withdrawn or drastically reduced in almost all cases. 24 patients completed the treatment, and of these, 92% reported improvement at one year. There was also significant improvement in a number of objective measures of lung function and of biochemical indices measured in the blood.
But this was not a randomized controlled trial. That took another 27 years…
At this point I would just like to acknowledge Dr Michael Greger for his excellent video series: Preventing Asthma with Fruits and Vegetables, Treating Asthma with Fruits and Vegetables, Treating Asthma with Plants vs. Supplements, and Treating Asthma and Eczema with Plant-Based Diets.
If your head is hurting by now, I suggest you take a break, have a stretch and watch these videos that introduce the study we will be discussing in my next post. Then we’ll move on!
1 – The burden of childhood asthma. E. von Mutius. Archives of Disease in Childhood 2000;82(Suppl II):ii2-ii5
2 – Worldwide variations in the prevalence of atopic symptoms: what does it all mean? J.O. Warner. Thorax 1999;54(Suppl 2):546-551)
3 – Increase in asthma: a more toxic environment or a more susceptible population? Seaton A et. al. Thorax 1994;49:171-174
4 – British Heart Foundation Heart Statistics: http://www.bhf.org.uk/research/heart-statistics.aspx Accessed on 20th September 2014
5 – Consumption of fresh fruit rich in vitamin C and wheezing symptoms in children. Forestiere F et. al. Thorax 2000;55:283-288
6 – Diet and asthma, allergic rhinoconjunctivitis and atopic eczema symptom prevalence: an ecological analysis of the International Study of Asthma and Allergies in Childhood (ISAAC) data. P. Elwood et. al. European Respiratory Journal 2001;17:436-443
7 – Vegan regimen with reduced medication in the treatment of bronchial asthma. Lindahl O. et al. Journal of Asthma 1985;22(1):45-55