Thursday December 6 2007
Tomatoes contain lycopene
“Ketchup is sauce of wellbeing” is the punning headline for a story in The Sun today. The report says a “daily dollop” of tomato ketchup “slashes” cholesterol levels, by reducing the "bad" cholesterol in the blood (LDL-cholesterol) which is linked to heart disease and strokes.
The story is based on a small study in volunteers with “normal” cholesterol levels who consumed 30g (1oz) of ketchup and drank tomato juice daily, for three weeks. The researchers report that average LDL (bad) cholesterol levels dropped by about 13 per cent in the three weeks that the volunteers were on the high tomato diet.
The design of this small study had some limitations, but these were countered to some degree by the objectivity of the measures that were used. It is unclear whether these findings in young, healthy people apply to people at greater risk of heart attack, and there may have been some bias in the trial.
It is only common sense to warn against making major dietary changes based on a short study in 21 people. In addition, the food to which ketchup is added may also determine an individual’s cholesterol levels.
Where did the story come from?
Dr Marja-Leena Solaste and colleagues from the University of Oulu in Finland carried out this research. The study was supported by the Research Council for Health of the Academy of Finland, the Finnish Foundation for Cardiovascular Research, the Paulo Foundation and the Sigrid Juselius Foundation.
The study was published in the (peer-reviewed) medical journal: British Journal of Nutrition.
What kind of scientific study was this?
This was a crossover trial that was not randomised or blinded. This means that both the participants and researchers knew when people were having low or high amounts of tomato in their diets, and the volunteers were each switched from a low to a high diet during the course of the experiment.
The researchers recruited 21 volunteers (5 men and 16 women, aged about 30) who were all non-smokers and who worked at the university. They were all young and of a normal weight (mean BMI 23.5 at the start of the study) and, apart from the contraceptive pill, none were taking any sort of medication. During the run-in period the volunteers were told how to follow the low and high tomato diets.
The researchers were interested in the effects of lycopene, a constituent of tomatoes and tomato products that is the source of their red colour. Lycopene is also present in watermelon, apricot, rosehip and guava.
During the low-tomato diet period the volunteers were told not to consume tomatoes, tomato ketchup, sauces, juices or the other lycopene containing vegetables or fruit. After three weeks they were all switched to a high tomato diet for a further three weeks, during which they consumed 400 ml of tomato juice and 30g (1oz) of ketchup daily. This resulted in an average intake of 27mg of lycopene a day.
The researchers performed laboratory measurements on the volunteers throughout the study; including blood tests for plasma lipids (measuring total and LDL- cholesterol) and other measures of cholesterol metabolism such as LDL oxidation, apoB, and vitamin levels, beta-carotene, vitamin C, and the lycopene level itself. The results were analysed statistically to look for any significant differences between diets.
What were the results of the study?
The blood plasma levels of lycopene and other organic pigments found in fruit and vegetables all decreased during the low tomato diet and then increased in response to the high tomato diet.
The total cholesterol level was significantly lower (4.19 mmol/L) during the high tomato diet phase when compared to both the low tomato diet (4.50 mmol/L) and to the baseline phase (4.43 mmol/L). The LDL cholesterol (‘bad’ cholesterol) level was also significantly lower (2.18 mmol/L) during the high tomato diet phase when compared to both the low tomato diet (2.56 mmol/L) and to the baseline phase (2.44 mmol/L). They reported this 0.38 mmol/L fall in ‘bad’ cholesterol between the high tomato diet and the low tomato diet as statistically significant.
The largest difference between the measurements was a 12.9% reduction in "bad" cholesterol between the high tomato diet and the low tomato diet. No significant changes were found between diets in the "good" cholesterol (HDL- cholesterol) levels. Vitamin C levels increased significantly on the high tomato diet compared to the low tomato diet, as did a measure of LDL oxidation.
What interpretations did the researchers draw from these results?
The researchers concluded that, in healthy adults with normal cholesterol levels, increasing the daily consumption of tomato juice and tomato ketchup resulted in a significant decrease in LDL (‘bad’) cholesterol concentration and improved the ability of LDL particles to resist oxidation, a process that occurs when LDL cholesterol invades the lining of blood vessels.
Simultaneously they saw increases in plasma levels of lycopene, beta-carotene and gamma-carotene, which are all organic pigments responsible for some of the colours of vegetables and fruit. Vitamin C levels also increased.
What does the NHS Knowledge Service make of this study?
This authors propose two mechanisms by which a high intake of tomato products may protect individuals against heart disease and stroke. The first is that tomato products, or lycopene, could have a direct effect of causing a reduction in LDL-cholesterol levels. Alternatively, they say that dietary supplementation with processed tomato products (tomato juice and ketchup) could increase the resistance of LDL particles to oxidation.
However, the researchers suggest that it is speculation to suggest that the decrease in cholesterol levels and in lipid oxidation seen in this study was specifically due to the lycopene. This is due to the fact that the people in the study with the greatest increase in lycopene in their blood were the ones with the smallest reductions in their cholesterol levels and there are also many other constituents of tomatoes that could have antioxidant potential.
The objective measures the researchers used partly counteract the possible bias in such unblinded and non-randomised studies. However, it is still possible that some other aspect of the participants diets changed over the study period, and it is actually this unknown factor that accounts for the changes in cholesterol seen. It is also possible that there are harms from consuming high amounts of tomatoes that the researchers did not investigate. It is also clear from other research that LDL-cholesterol is only one of the many factors that contribute to an individual’s risk of heart disease.
The results from this short eight week study in normal healthy, young people are not necessarily relevant or applicable to outcomes such as heart attack and stroke, or to people at higher risk or older people. For instance, it is not possible to definitely state that a high tomato diet will reduce heart disease over five years in people with high levels of cholesterol or those who have already had a heart attack.
The authors appropriately call for larger controlled clinical trials of tomatoes as a way of preventing heart disease.
Sir Muir Gray adds...
There are already plenty of reasons to eat tomatoes. They are a fruit after all and contribute to the 5-a-day health habit. Whether they can lower the risk of heart disease and stroke needs further research.