Monday March 29 2010
It's unclear how the blackcurrant chemicals affect humans
Eating blackcurrants could “help millions of people with asthma”, according to the Daily Express. The newspaper said that the “superfruit” could help by “working with the body’s immune system to reduce inflammation in the lungs”.
The news is based on a laboratory study in New Zealand, which tested blackcurrant extracts on human lung cells in culture. Its findings have shed some light on the complex immune responses to allergens (substances that trigger allergic reactions), in particular those associated with inflammation of lung tissue seen in some asthma attacks. However, as this was a laboratory study on extracted cells, it is too soon to know whether the type of exposure these lung cells had to the blackcurrant extracts (namely incubating the cells with purified blackcurrant compounds) equates to how the body may access them after consumption of blackcurrants.
This is very early research. While the findings may help explain why diets high in some fruits seem to reduce the incidence and prevalence of asthma, it remains to be seen which exact chemical reactions may be responsible. It is also not yet clear whether the substances tested can be purified and turned into a safe and effective form of treatment for certain types of asthma.
Where did the story come from?
The study was carried out by Dr Suzanne Hurst and colleagues from the Plant and Food Research Institute of New Zealand. It was funded by the Foundation for Research Science and Technology of New Zealand and published in the peer-reviewed medical journal Molecular Nutrition and Food Research.
What kind of research was this?
In this laboratory study, which was conducted in cultures of human lung cells, researchers aimed to identify whether the polyphenol compounds found in blackcurrants could target particular cellular activities, thereby complementing the body’s own immune actions.
In allergy-induced asthma, white blood cells called CD4+ T-helper type 2 cells are activated. These cells interact with other types of white blood cells called eosinophils to promote the lung inflammation that is associated with the classic symptoms of asthma. Two particular chemical messengers released by the T-helper 2 cells, called interleukin 4 (IL4) and interleukin 13 (IL13), are responsible for switching on a chemical called eotaxin, which is known to recruit the eosinophil white blood cells to the lungs.
One of three types of eotaxin (known as CCL26) seems to be the most important chemical in recruiting eosinophil white blood cells to the airways. In this study, the researchers investigated whether chemicals extracted from blackcurrants could interrupt the recruitment of eosinophils through the disruption of CCL26 production. They say that recent epidemiological studies (not assessed here) have shown that an increased intake of fresh fruit and vegetables is linked with lower levels of respiratory symptoms and non-specific lung disease. They say that these studies suggest that certain fruits may contain chemicals that can reduce allergen-induced asthma and that they designed this study to see whether this was true of blackcurrants.
What did the research involve?
The researchers conducted several experiments using polyphenols sourced from New Zealand blackcurrants and cultures of human lung cells growing in a special growth medium.
The researchers first exposed cultured cells to different blackcurrant extracts or to a control substance to see whether the substances had any harmful effect on the cells. They then exposed the lung cells to different blackcurrant extracts or to a control in the absence or presence of IL4 or IL13 for 24 hours and measured the effects this had on levels of CCL26. The researchers then investigated the effects of exposure to “total polyphenols” (the mix of polyphenols naturally found in plants) and then to two specific polyphenol chemicals called anthocyanin (BC-A) and proanthocyanidin (BC-P).
The researchers then undertook further experiments to determine the exact action of the polyphenols on the cells and the time it took for the cells to recover from their effects. Further biochemical characterisation was undertaken to determine the exact chemical components of the polyphenols.
In a second set of experiments, the researchers assessed whether the blackcurrant extracts would affect the role of interferon-y (another chemical messenger) in reducing the secretion of CCL26. Interferons are secreted by a different type of CD4+ T-helper cell called type 1 cells. While their action would be helpful for asthmatics, there are fewer of these cells in allergic lung tissue.
What were the basic results?
There were no adverse effects on the cultured cells when they were incubated in polyphenols extracted from blackcurrants. Initial exposure to the IL4 and IL13 chemical messengers led to secretion of CCL26 from the cells. Incubating lung cells with proanthocyanidin (BC-P) and IL4 or IL13 inhibited the secretion of CCL26 that would normally occur. However, no inhibiting effect was seen when the cells were incubated with anthocyanin (BC-A) and IL4 or IL13. The inhibitory effect of BC-P was no longer present 24 hours after the cells were washed and incubated with IL4 again.
The blackcurrant extract proanthocyanidin (BC-P) enhanced the action of interferon-y (INF-y) in suppressing CCL26 secretion, with the combination of BC-P and INF-y being more effective than either on its own. A chemical called epigallocatechin (EGC) seemed to be the active component of BC-P.
How did the researchers interpret the results?
The researchers concluded that their results demonstrate that an extract from blackcurrants can suppress the CCL26 secretion that is stimulated by IL4 and IL13, both on its own and in conjunction with interferon-y. They say the fact that BC-P but not BC-A had an effect on this pathway suggests that they may be involved in similar but distinct events in the cells.
This laboratory study has revealed how certain chemicals can affect the complex pathways that underpin the response that lung cells have to known chemical messengers. The study has shown that, when lung cells were incubated with certain blackcurrant extracts, they were able to inhibit the expected release of a substance that is known to lead to the lung inflammation seen in the human allergic asthma response. The authors discuss the findings of some epidemiological studies that show that consumption of selective fruits lowers the incidence and prevalence of asthma, particularly in children. The findings from their research may help explain why that may be the case. However, the results come from tests on lung cells in the laboratory, meaning that their applicability to living systems, whether human or animal, is currently uncertain and that the study should be viewed as very preliminary research into the potential pathways for asthma treatment.
The researchers raise the important issue of the “bioavailability” of the plant-derived phytochemicals, i.e. how and at what rate a substance can enter the circulatory system in a human and, therefore, become available for the body to use. They say that these chemicals are found as complex compounds in plants, but that other studies have found that chemicals and enzymes in the gut can break the large molecules down into smaller molecules, which can be more easily absorbed. Whether this process happens in humans, and how the by-products of digestion will affect the immune responses in living people’s lung tissue, will need to be the subject of further study.
A healthy, balanced diet is important for a number of well-established reasons. Asthma medication should not be replaced by blackcurrants until this research has moved on further.