Thursday June 16 2011
Olive oil is an important part of the Mediterranean diet
“Olive oil staves off stroke in the elderly,” reported The Daily Telegraph. It said that a study has found that a diet rich in olive oil lowers the risk of having a stroke in later life by more than a third.
This study looked at about 8,000 people over 65 and found that those who had plenty of olive oil in their diet had a 41% lower risk of a stroke than those who never used olive oil.
This was a large, well-designed study with a long follow-up period. However, it is not robust evidence that olive oil independently lowers the risk of a stroke, and the findings should be viewed with some caution. Limitations include the fact that the participants were relied on to report their own olive oil use and only did this once at the start of the study. It's possible that their consumption could have changed over time. Their risk of a stroke may have also been affected by other factors that weren't taken into account.
Olive oil is an important constituent of the Mediterranean diet, which is associated with health benefits such as lower risk of heart disease. However, several factors lower the risk of having a stroke, including a healthy balanced diet with low levels of salt and saturated fat, and regular exercise. Adding lots of olive oil to the diet while ignoring these other factors is unlikely to bring benefits.
Where did the story come from?
The study was carried out by researchers from the French research centre INSERM and other medical and academic centres in France.
The lead researcher received financial support from Institut Carnot LISA (translated as Lipids for Industry, Safety and Health), an industry-sponsored organisation. Another researcher is a consultant for the food and beverage company Danone. The study was published in the peer-reviewed journal Neurology.
The study was covered accurately but uncritically in the press. The Daily Mail, BBC News, Daily Mirror and Daily Express all included comments from an independent expert.
What kind of research was this?
This prospective cohort study investigated whether high olive oil consumption and high levels of oleic acid in blood plasma (a marker of olive oil intake) are associated with lower incidence of stroke in people aged over 65.
The researchers point out that the Mediterranean diet is associated with a lower risk of dying from cardiovascular disease as well as a reduction in risk factors for cardiovascular disease, such as high cholesterol. High olive oil consumption is an important feature of this diet.
They say that higher consumption of olive oil has been associated with a lower risk of heart attacks and deaths from all causes, and has also been linked to lower blood pressure. They say their study is the first to test whether olive oil is associated with a lower risk of strokes, independent of other risk factors.
What did the research involve?
The participants were obtained from another, ongoing cohort study investigating vascular risk factors for dementia. This cohort study included 9,294 participants from three cities in France, who were 65 or over when the study began in 1999-2000. They were interviewed by trained psychologists and nurses at the start, and follow-up examinations were carried out at two, four and six years after the first visit, either by face-to-face interviews or through a questionnaire.
This study looked at the effect of olive oil on strokes, so the participants had to have had no history of strokes when they were first enrolled. At each follow-up, the participants were asked if they had had a stroke or stroke symptoms or been admitted to hospital for a stroke. When the answer was positive, the researchers obtained further medical data from the patient’s health services and doctors. Strokes were defined according to the diagnosis of an expert committee.
This study had two parts. The first main analysis investigated whether there was an association between reported olive oil consumption at the start of the study and the risk of a stroke over six years.
At the first interview, researchers collected dietary details from the participants, including type of dietary fats used for dressing, cooking and spreading. They divided olive oil consumption into three categories: no use, moderate use (for cooking or dressing alone) and intensive use (cooking and dressing).
The second, smaller part looked at olive oil consumption and blood levels of oleic acid (a fatty acid that is a biological marker for olive oil consumption) at the start of the study and the risk of a stroke over six years. In a sample of 1,364 people taken from those enrolled in the original study, the researchers recorded blood levels of fatty acids at the start of the study, with results for each fatty acid expressed as a percentage of the total.
Validated statistical methods were used to analyse the findings. The analyses took into account other risk factors for having a stroke, including age, gender, education, diet and physical activity, drug and alcohol use.
What were the basic results?
Of the participants in the original study, 7,625 were eligible for the main investigation into olive oil and stroke risk, and 1,245 people were available for the analysis of oleic acid levels and stroke risk.
In the main sample, there were 148 strokes. The researchers found that compared to those who never used olive oil, people who used it intensively had a 41% lower risk of having a stroke (hazard ratio 0.59, 95% confidence interval 0.37 to 0.94). This very wide confidence interval suggests that the true risk of stroke has a high chance of being lowered by somewhere between 6% and 63%.
In the secondary sample of 1,245 people whose blood levels of oleic acid were measured, there were 27 strokes. A higher level of oleic acid in the blood was found to be associated with a lower risk of having a stroke. However, there was a low association between olive oil use and oleic acid measurements. The researchers found that other dietary constituents were associated with higher oleic acid blood levels, including butter, goose and duck fat.
How did the researchers interpret the results?
The researchers say their results suggest a “protective role” for high olive oil consumption on the risk of strokes in older people. They suggest that older people should be advised to consume more olive oil.
This was a large, well-designed study with a low drop-out rate (most of the participants completed the study). However, it has several limitations that mean its findings should be viewed with caution:
- The study relied on people to report their own olive oil use, which could introduce a degree of error.
- The researchers only asked people once about olive oil use at the start of the study. It is possible that people’s consumption of olive oil may have changed over time.
- The wide confidence intervals in the main analysis raise the possibility that the findings were due to chance, or at least that the reduction in risk is less than the 41% quoted by the researchers or the “more than a third” quoted by the papers.
- There was a very low association between blood measurements of oleic acid and olive oil use. This throws doubt on the use of oleic acid as a marker for olive oil consumption and may further suggest that self-reporting of olive oil consumption itself might be inaccurate.
Olive oil has been associated with several health benefits and, as part of the Mediterranean diet, is of interest to researchers. Being composed of monounsaturated fats (80%) and polyunsaturated fats (20%), it is also a healthy substitute for saturated fats such as butter. However, further research is required before it is known for certain how much olive oil can lower the risk of strokes, independent of other risk factors.