Tuesday April 20 2010
Olive oil is a major part of the Mediterranean diet
Scientists have explained why a Mediterranean-style diet is so healthy, The Daily Telegraph reported. It said that, “consuming large amounts of olive oil suppresses genes which cause inflammation and can lead to problems like heart disease.”
Researchers gave 20 volunteers at increased risk of cardiovascular disease, meals containing virgin olive oil with either high or low levels of certain compounds, known as phenols. The effects on various genes in white blood cells were then studied. Meals with olive oil high in phenols were associated with a greater reduction in gene activity related to inflammation than meals with olive oil lower in phenols. Inflammation is involved in the build-up of fatty deposits in blood vessels, leading to heart attacks and strokes.
Due to this study’s small size and design, it is difficult to link the changes in gene activity to longer-term health conditions. Partly funded by the Spanish government’s Agency for Olive Oil, it contributes to our understanding of the effect of olive oils on our cells. However, it is only a small piece of the complex puzzle of how diet affects our health. It is not possible to say with certainty whether these changes in gene activity contribute to the reduced risk of cardiovascular disease attributed to the Mediterranean diet.
Where did the story come from?
The research was carried out by Dr Antonio Camargo and colleagues from the University of Cordoba and other research centres in Spain and the US. The study was funded by various Spanish government agencies and research centres, including the Ministry of Health, Centre of Excellence in Research on Olive Oil and Agency for Olive Oil (part of the Spanish Ministry of the Environment and Rural and Marine Affairs). It was published in the peer-reviewed open access journal BMC Genomics.
The Daily Telegraph, Independent and Daily Mail generally reported this research accurately. In claiming that the study has discovered the “secret behind health benefits of Mediterranean-style diet”, the Telegraph’s headline suggests that the research has greater significance than it does.
What kind of research was this?
This randomised controlled trial looked at the effects of particular chemical compounds, called phenols, on the activity of genes in white blood cells. Phenols are found in virgin olive oil. The researchers were interested in whether virgin olive oils with different levels of phenols had different effects on gene activity.
The Mediterranean diet, which is rich in olive oil, is associated with a reduced risk of cardiovascular disease. The researchers thought that this reduced risk might be attributed in part to virgin olive oils that are high in phenols affecting genes in the body.
A randomised controlled trial is the best way to compare the effect of different interventions. One potential design weakness is that participants received two different meals (containing olive oil either high or low in phenols) in a random order. In theory, this could result in some “carryover” of the effects of whichever intervention was received first. However, the researchers included a period of a week between the meals, which should reduce the chance of this happening.
What did the research involve?
The researchers recruited 20 volunteers (average age 56) and fed them two breakfasts, which contained virgin olive oils with either high or low levels of phenols. The researchers measured the activity of genes in the volunteers’ white blood cells to see if the meals had different effects.
The volunteers all had metabolic syndrome. This is a collection of characteristics that together predict an increased risk of cardiovascular disease, such as high blood pressure and being overweight with most of the fat carried around the waist. They did not show signs of chronic diseases of the heart, liver, kidneys or thyroid, or have a family history of early-onset cardiovascular disease. For six weeks before the study began and throughout it, the volunteers were asked to eat a similar low-fat, carbohydrate-rich diet. The day before each test breakfast, the volunteers were asked to avoid phenol-rich foods, such as juices, wine, grape juice, chocolate, coffee, tea, olive oil or soy, and not to do intense physical exercise. They also fasted for 12 hours before the test breakfasts.
The volunteers received two breakfasts on two separate days a week apart. The breakfasts constituted 60g of white bread and 40ml of olive oil that was either high or low in phenols. The low-phenol oil was made from the high-phenol oil using chemical processes to extract some of the phenols. The researchers and volunteers did not know who had received which breakfast, and the order in which they were given the high- or low-phenol breakfast was chosen at random.
Blood samples were taken before and after the two breakfasts, and a specific group of white blood cells was isolated. The researchers then looked for changes in the activity of selected genes in these cells after the different breakfasts.
What were the basic results?
The two different breakfasts were associated with differing levels of activity in 98 genes in the white blood cells. Many of these genes (39 genes) play a role in inflammation, and most of them (35 out of the 39) were less active after the breakfast containing high-phenol virgin olive oil than after the breakfast containing low-phenol virgin olive oil.
Inflammation plays a role in the build-up of fatty deposits in the blood vessels, which can lead to heart attacks, strokes and other cardiovascular problems.
How did the researchers interpret the results?
The researchers concluded that eating a breakfast containing virgin olive oil high in phenol compounds reduces the activity of several genes that promote inflammation. They say that this could partly explain the reduction in cardiovascular disease in Mediterranean countries, where virgin olive oil is the main source of fat in the diet. They acknowledge that other lifestyle factors are likely to contribute to this effect.
This small study looked at whether virgin olive oils that are high or low in phenols have different effects on gene activity in white blood cells. There are a number of points to note:
- The small size of the study and the fact that all participants had metabolic syndrome mean that results may not be representative of all individuals with metabolic syndrome or of people without the condition.
- The study only looked at gene expression after one meal. It is unclear whether the same results would be seen over a longer period, or how long after the meal these effects are sustained.
- As the participants were only fed a single meal containing the oils, and their long-term cardiovascular outcomes were not followed up, it is not possible to say whether the changes in gene activity seen would affect the risk of these outcomes.
This study contributes to our understanding of the effect of phenols on gene expression in white blood cells. However, it is only a very small piece of the complex puzzle of how diet affects our health. It is very difficult to say whether the changes seen are responsible for some of the reduction in cardiovascular disease from eating a Mediterranean diet and, if so, what the extent of their effect is.