“5 a day” is the German Society for Nutrition’s famous campaign slogan. This refers to the five servings of fruits and vegetables which should be consumed per day for a healthy diet – and to reduce the risk of cancer. But who really keeps up with that? The amount of consumption of fruit and vegetables has continually diminished over the past few decades. Concomitantly, the proportion of fibre has dropped, while carbohydrates and fat uptake has increased.
Link between asthma and diet
Likewise, more and more people have been falling ill in the last decades from allergic asthma. But what does fruit and vegetable consumption have to do with allergic asthma? Researchers led by Prof. Benjamin Marsland from the University Hospital in Lausanne have demonstrated in experiments on mice that these two developments do not just coincide in time, but are also causally linked. The results have now been published in Nature Medicine.
Intestinal bacteria have effects all the way to the lungs
The fact that the microbial composition of the gut plays a part in the prevention of colon cancer is known. That fermented fibre from the intestinal bacteria can also enter the bloodstream and affect other organs, however, is something newly known. “We show for the first time that the influence of intestinal bacteria goes much further, namely as far as the lungs”, says Marsland.
His team looked at mice, what sort of influence fibre in the diet has on immunological reactions in the lungs, and on the composition of the intestinal flora. To do this, mice were fed either a standard recommended diet with an amount of fibre of four percent or a low-fibre diet with 0.3 percent fermentable fibre. This diet component is about that of the Western diet, which contains an average of only 0.6 percent fibre. Following that, the animals were exposed to a dust mite extract. The mice that had received the low-fibre diet developed a stronger allergic reaction and significantly more mucus in the lungs than animals that had received a standard recommended diet. The concentration of various interleukins which were emitted due to the increased lymphocyte infiltration was elevated. Similarly, the levels of total IgE and of the house dust mite-specific IgG were elevated.
Direct effect on the immune system
The reverse experiment, namely the comparison between standard diet and fermentable fibre-enriched diet, provided evidence for the protective effect of this type of fibre. The researchers also examined which mechanism might be behind the effect. It is a multi-step reaction chain. First, the fibres pass into the intestines, where bacteria ferment them into short chain fatty acids (SCFA) such as acetate, propionate and butyrate. These among other things are emitted into the bloodstream and affect the maturation of immune cells in the bone marrow, the so-called dendritic cells. Attracted by the house dust mite extract, they migrate to the lungs, where they eventually trigger a lesser immune response. The fatty acids also lead to a reduced formation of TH2 cells which, through the spreading of cyktokines, can fire up an allergic reaction. The low-fibre diet resulted in the opposite effect, namely an increased formation of TH2 cells.
Inflammation subsides faster
Marsland and colleagues compared the inflammatory responses to the house dust mite extract after administration of drinking water with either saline or propionate added. The first reaction was the same in both groups. And then the big difference appeared. In animals which had in addition received propionate, the inflammation decreased rapidly, whereas with the control animals this effect made itself strongly observable for up to six days. The measurement of the immunological parameters (cytokine levels) confirmed this observation.
Diet affects microbial flora
The investigation also shows that the number and diversity of intestinal bacteria is reduced by a low-fibre diet compared to the standard recommended diet. The further addition of pectin – the fibre used in this study in opposition to the indigestible cellulose – to the standard recommended diet did not yield any further effect with respect to the number of bacteria. Nevertheless, several families of bacteria changed in the gut. A high fibre diet has the consequence that the proportion of bacterioidaceae and bifidobacteriaceae increased. With bifidobacteriaceae it has already been shown that short-chain fatty acids are growth stimulators. The low-fibre diet on the other hand leads to the dominance of firmicutes, especially erysipelotrichaceae, which have been shown to occur increasingly more often in mice feeding on a typical diet in the western world.
Is man like the mouse?
Marsland thinks that the results of his group are clinically relevant, because on the one hand the proportion of vegetable fibre of the low-fibre mice is comparable to the Western diet, and secondly because the investigated aspects of the immune system in mice and man hardly differ. Whether nutrition in humans has a similar effect on allergy events in the body is however still uncertain. “We are now planning clinical studies to investigate how a diet enriched with fermentable fibre has an influence on allergies and inflammation”, explains Marsland. As with all complex conditions, the diet is not the only determining factor. How else can it otherwise be explained that there were fewer allergies and asthma in the GDR, where vegetables, but especially fruit, was not available in the variety and quantity as it was in the rest of Germany? Nevertheless, the results are convincing and could be a good occasion to again look at the “5 a day” idea.