Thursday June 18 2009
“Carrots cooked whole are ‘better at fighting cancer’,” The Independent reported. The newspaper said that a study has found that when carrots are cooked whole, they contain 25% more of the “anti-cancer compound” falcarinol than if they are chopped up first. It also reported that boiling carrots whole retains more of their natural sugars, making them taste better as well.
This report is based on a presentation at a nutrition conference, which described the optimal method for cooking carrots. It highlights the findings of a study, which has not yet been published.
There are several steps in the chain of logic that implies “whole carrots fight cancer”, and although the concentration of the chemical falcarinol may be maintained in carrots cooked whole, it is yet to be proven that falcarinol can actually prevent cancer in humans. The researchers quote an animal study that was conducted four years ago, which showed that rats fed on a diet containing carrots or isolated falcarinol were a third less likely to develop tumours than those in a control group.
A serving of carrots counts towards the recommended target of five portions of fruit and vegetables a day and is known to be healthy. While waiting for further research, taste may be a reason to cook carrots whole.
Where did the story come from?
This research was carried out by Dr Kirsten Brandt and colleagues from the School of Agriculture, Food and Rural Development at Newcastle University. Sources of funding were not reported for this study, which has not yet been published.
What kind of scientific study was this?
This was a conference presentation of a portion of ongoing research looking at the nutrient falcarinol, which is commonly found in carrots. This presentation specifically addressed how falcarinol levels in carrots are altered by different cooking methods, while the ongoing research focuses on the wider properties and availability of falcarinol. This research is due to be fully published later in 2009.
The researchers say that although carrot intake is strongly linked to with reduced risk of cancer, the active ingredient is unknown and that the common belief that beta-carotene in carrots prevents cancer is untrue. The researchers say that their previous experiments have shown that falcarinol slowed the growth of isolated cancer cells and tumours in rats, and that this may be the active ingredient in carrots.
Carrots were boiled or steamed before or after being cut into 1cm cubes. The researchers then compared the four types of carrot: those that were boiled then cut, those that were cut then boiled, those that were steamed then cut and those that were cut then steamed.
They measured the water lost in cooking, the loss of sugar and the falcarinol and beta-carotene concentrations at five-minute intervals for up to twenty minutes of cooking. They also carried out a blind taste test on almost 100 people to compare the taste of boiled-then-cut versus cut-then-boiled carrots.
What were the results of the study?
The researchers found that when carrots are heated, their composition changes. This is because heat destroys the normal structure of the cells, allowing water and water-soluble nutrients to leak out. Since falcarinol is soluble in water, the concentration of falcarinol decreases as the carrots lose water.
Cooked carrots weighed about 10% less than they did before being cooked, even when they were boiled in water. The carrots boiled in cubes lost more sugar due to the higher surface area to volume ratio. Water-soluble falcarinol was lost in all groups except the steamed-then-cut group. The cut-then-boiled group lost the most falcarinol, almost 25% more than the boiled-then-cut group.
Seventy per cent of people said that they preferred the carrots cooked whole before being cut, compared to thirty per cent who said the cut-before-boiled carrots tasted better.
What interpretations did the researchers draw from these results?
The researchers say that heat softens cell walls and allows water-soluble compounds, such as sugar and vitamin C, to be lost through the surface of the tissue. This also results in the leaching out of other compounds, including falcarinol.
If the carrot is cut before being boiled, the surface area becomes much greater. This leads to a greater loss of nutrients and taste when cooking compared to carrots that are boiled whole.
What does the NHS Knowledge Service make of this study?
This appears to be a simple study that could potentially have a big impact on how people cook carrots and possibly other vegetables. The methods advocated by the researchers require virtually no extra effort, appear to produce more flavoursome carrots and might potentially allow produce to retain more of its natural nutrients during cooking.
However, as this was a conference presentation, the level of information available means that it is impossible to draw firm conclusions on the study’s methods and findings, and therefore the validity of the claims made by newspapers. An in-depth evaluation of this work and its implications will require further detail, which will only come through full publication of the work.
For example, the number of carrots tested was not reported, nor is any statistical significance of the differences between cooking groups. Publishing these would allow greater confidence that the reported results had not occurred by chance.
The link between increased levels of falcarinol and reduced human cancer also needs proof. The researchers illustrate this point by citing a body of research into beta-carotene. While early studies suggested that the supplement prevented cancer, later studies in large numbers of smokers found that it actually contributed to cancer.
The published results of this study and any further trials of falcarinol in humans will be awaited with interest. Until then, it would be wise to base choice of fruit and vegetables on taste and to aim to eat a minimum of five portions a day.