‘Just two apples a day could cut risk of heart disease by cutting cholesterol levels’, the Daily Mail tells us.
The news follows a trial in which post-menopausal women who ate either dried apples or prunes (dried plums) every day for a year had their blood cholesterol measured regularly. Researchers found that cholesterol levels were significantly lower in the women who ate dried apples than those who ate dried plums, but only at six months, not at any other time they were measured.
Before Granny Smith rushes out to buy a pound of pippins, it is important to remember that though women who ate dried apples had a reduction in their cholesterol levels, the study only found significant differences between their cholesterol levels and those in the prune group at six months.
This relatively small trial also suffered from high drop-out rates, which limits the reliability of results as the women who dropped out may have had different results to those who stayed in the trial. The high drop-out may also suggest that eating a daily dose of dried fruit for a year may not be to everyone’s taste.
While high cholesterol is a known risk factor for heart disease, the media have assumed that this difference in cholesterol will cut risk of heart disease, and we cannot be certain that this would be the case.
Nevertheless, the study supports the general health message that fruit is an important part of a healthy, balanced diet. Together with a healthy lifestyle and regular physical activity, this is the best way to stay healthy and reduce the risk of heart disease.
Where did the story come from?
The study was carried out by researchers from The University of North Carolina at Chapel Hill and other institutions in the US. It was funded by the National Research Initiative of the US Department of Agriculture Cooperative State Research, Education, and Extension Service. The plums were provided by the California Dried Plum Board.
The study was published in the peer-reviewed Journal of the Academy of Nutrition and Dietetics.
The Mail highlights the benefits of two apples a day, but doesn’t make it clear that this study was of dried apples, not fresh apples (though the study does say that the 75g is about the equivalent of two medium sized apples). Also, while the reported drop in cholesterol is accurate, we don’t know that this will definitely cut the risk of heart disease.
What kind of research was this?
This was a randomised controlled trial which aimed to see whether eating dried apples or plums reduced risk factors for cardiovascular disease in post-menopausal women. The authors say that prior animal and human research studies have demonstrated that polyphenolic compounds and fibre in certain foods can regulate fat metabolism and reduce the production of inflammatory molecules in the body - factors that are associated with risk of cardiovascular disease. As apples are a good source of polyphenolic compounds and fibre, the researchers aimed to investigate their effect on these factors in their study.
A randomised controlled trial is the best way of investigating whether a particular intervention (in this case dried apple) affects an outcome (cholesterol and inflammatory molecules) compared to a comparator (dried plum). With any self-administered food trial, making sure people eat what they are supposed to may be an issue.
What did the research involve?
The researchers recruited post-menopausal women from Tallahassee, Florida, during 2007 to 2009. Eligible women were not using hormone replacement therapy (HRT), and had not received any other drug treatments, including cholesterol-lowering medications, for at least the past three months. Researchers also excluded women who were heavy smokers or who had chronic disease, including cardiovascular disease and diabetes.
The women underwent a medical and nutrition assessment and 160 were included in the trial. The women were randomly assigned to eat either 75g of dried apple daily or 100g of dried plum daily for 12 months. To monitor compliance, the women were given calendars and asked to mark the days they missed eating the apple or plum and to record or return any unused portion.
At the start of the study, and then at 3, 6 and 12 months, fasting blood samples were taken in order to measure cholesterol and inflammatory molecules in the blood. At the same time points body measurements, physical activity recall and seven-day dietary recall were also obtained.
Women were not blinded to their allocation (as they obviously knew if they were eating apples or plums), but researchers analysing the results were.
What were the basic results?
At three months 82% of the apple group and 73% of the plum group had continued the trial and were analysed. At six months this dropped to 68% of both groups, and at the final 12-month follow-up had dropped to 63% of both groups. A common reason for drop-out was non-compliance with eating the dried fruits.
In the apple group:
- at three months total cholesterol had reduced by 9% and LDL cholesterol (sometimes called ‘bad’ cholesterol) by 16%
- at six months total cholesterol had reduced by 13% and LDL cholesterol by 24%
- at 12 months, total cholesterol was still 13% down and LDL cholesterol was still 24% down
These reductions were large enough to be statistically significant.
In the plum group, total cholesterol was reduced by 3.5% and LDL ‘bad’ cholesterol by 8% at 12 months. This reduction was not large enough to be statistically significant (the difference could have just occurred by chance).
The only difference found between the groups was that total cholesterol levels were significantly lower in the dried apple group compared with the dried plum group at six months, but not at 3 or 12 months. There was no difference between dried apples and dried plums in LDL ‘bad’ cholesterol at any time-point.
Both dried fruits reduced the inflammatory molecule C-reactive protein in the women’s blood. Levels of c-reactive protein were significantly lower in the dried plum group compared with the dried apple group at three months.
The average body weight of women in both groups was not significantly different at study start, or at 3, 6, and 12 months. Analysis of the seven-day dietary recall and physical activity recall also found no between-group differences in reported food intakes and physical activity levels at any time-points.
How did the researchers interpret the results?
The researchers conclude that there was a significant difference between the dried apple and dried plum groups only in total cholesterol level at six months. However, within-group comparisons showed greater reductions in cholesterol levels in the apple group as early as 3 months. Both dried apple and dried plum reduced certain inflammatory markers.
This 12-month trial in 160 post-menopausal women has found that eating dried apples daily reduces total cholesterol and LDL ‘bad’ cholesterol at three months and further reduces it at six months. Many news stories on the benefits of fruits and vegetables are based on laboratory or animal studies using chemicals extracted from fruits and not fruits themselves. This study is a commendable attempt to use a robust study design to look at the direct effects of fruit in people. Its strengths are that it is a well-designed, randomised controlled trial, which made careful attempts to consider the potential confounders of physical activity and other diet, and to assess the compliance of women to their assigned group.
However, there are limitations to this trial to be considered before post-menopausal women rush out to buy dried apples:
- Though within their group the women eating the apple had greater reductions in their cholesterol, the only significant difference between the groups was a greater reduction in total cholesterol at the six-month time point only.
- The trial was relatively small to start with and suffered from high drop-out rates: only 68% of women in both groups continued to six months and this had dropped to only 63% at 12 months. This limits the reliability of results, as the women who dropped out could have had differing results.
- Most importantly, though high cholesterol is a known risk factor for cardiovascular disease, the study has not measured health outcomes, so although the media may have assumed that this difference in cholesterol will cut risk of heart disease, we do not know that this would definitely be the case.
- The study only included post-menopausal women, and findings may not apply to children, men or pre-menopausal women.
Nevertheless the study supports the general health message that a balanced diet high in fruit and vegetables, a healthy lifestyle and regular physical activity, are the best ways to stay healthy and reduce the risk of heart disease.