Mixing foods may protect against cancer
Bupa investigative news - 11 April 2003
written by Rachel Newcombe, reporter for Bupa's Health Information Team
A regular intake of vitamins and minerals from dietary sources is recognised as being essential for good health. In particular certain foods have been shown to be important in preventing cancer. For example, a deficiency in the mineral selenium is associated with many types of cancer, while the presence of a chemical found in vegetables (sulforaphane) is known have a powerful role in cancer prevention.
Recently research on human cancer genes has found that foods that contain the mineral selenium and plant-based chemical sulforaphane in combination may have a 13 times greater ability to protect against cancer than when the food compounds are used separately. And, a different study has thrown light on the best form in which to buy such vegetables. Austrian research has suggested that frozen vegetables may sometimes contain more nutrients than fresh ones.
So, what foods might you eat to help prevent cancer? And in what form should these foods be bought?
KEY POINTS
Chemicals and minerals contained in certain foods have been shown to help lower the risk of developing cancer.
One such mineral is selenium. A deficiency in selenium is associated with an in increase in risk for a number of cancers, including cancer of the prostate.
Another "cancer-preventing" food component is the plant-based chemical sulforaphane.
Researchers studying human cells in the laboratory have discovered that combining the food compounds selenium and sulforaphane has an increased effect on cancer genes than when compared with each being used separately.
Selenium naturally occurs in foods such as poultry, fish, eggs, nuts and mushrooms.
Sulforaphane is found in high concentrations in broccoli, cabbage, sprouts, watercress and salad rocket.
Another study investigated which type of vegetables contained the most nutrients: frozen vegetables (which had been frozen soon after picking) or fresh vegetables that had been shipped a long way before being bought. It was found that many frozen vegetables had higher levels of nutrients than their long-distance fresh counterparts.
Experts regard the selenium and sulforaphane research as interesting and say that it adds to existing knowledge about the benefits of vegetables and fruit.
Experts also recommend eating a variety of fresh and frozen produce to increase the amount of nutrients in your diet and help you reach the target of eating five portions of fruit and vegetables a day.
What were the headlines?
The idea that certain foods could offer cancer protection gained some coverage by the UK media. Examples of headlines included, "Food combination boost cancer fight", "Food combining 'fights cancer'" and "Chickens fight cancer".
Likewise, the Austrian frozen food study gained a similar amount of attention, with headlines on this topic including "Frozen vegetables 'often healthier than fresh'" and "Frozen veg 'healthier than fresh'".
What is the bigger picture?
Selenium is a mineral that is found in nuts, poultry, fish, eggs, sunflower seeds and mushrooms. Having a deficiency of selenium in the diet is known to be associated with a number of cancers, including prostate cancer.
Sulforaphane is a plant chemical that is also associated with cancer prevention and it is being investigated as a potential cancer drug. It is found in foods such as broccoli, sprouts, cabbage, watercress and salad rocket.
The research into selenium and sulforaphane was carried out by the Institute for Food Research (IFR) http://www.ifr.bbsrc.ac.uk/ in Norwich, and was published in the journal Carcinogenesis http://carcin.oupjournals.org/cgi/content/abstract/24/3/497. The study looked at the effect of food components on the action of certain genes involved in the development of cancer. In particular, the study focused on the genes that play an important role in tumour formation, tumour progression and the spread of tumour cells.
When selenium and sulforaphane were combined, they were found to have a nearly 13 times greater impact on the genes that control cancer development than when used individually. Although the trial only used laboratory cultures of human cancer cells, the researchers believe that their findings could have implications for people's diet.
The researchers are keen to begin human trials into the effect of combining foods containing selenium and sulforaphane. One reason is that this combination effect may mean that lower doses of the components are needed. This is good news as high levels of these compounds can be harmful.
"High concentrations in the diet are normally required to protect against cancer, but when these compounds act synergistically, lower doses are needed to prevent cancer formation. This is particularly good news as selenium and sulforaphane can be toxic at high levels," explained Dr. Yongping Bao, senior researcher at the IFR.
"As a result of this research, we hope to begin a human cancer prevention trial next year," said Dr. Bao. "It opens up new possibilities for functional foods, food supplements or simply new guidelines for healthy eating."
This isn't the first time that sulforaphane has been associated with cancer prevention. Several previous American studies have found it to be useful, and one team is hoping to develop it into a tablet that could help protect women against breast cancer.
Additionally, a small study in Austria conducted by the Austrian Consumers Association http://www.konsument.at looked at the nutrient levels of frozen and fresh vegetables. They discovered that, in certain situations, frozen vegetables can be better than fresh ones. This is because frozen vegetables are processed as soon as they're picked and maintain their vitamins and minerals levels once frozen. In contrast, if the fresh produce has been imported from faraway countries, it can be some time between the vegetables being picked and finally being eaten, time in which their nutrient value has steadily deteriorated.
“Yet again eating a balanced diet has been shown to be beneficial to health. And the easiest way to achieve the best combination of vitamins, minerals and other food components for optimal health is to eat a wide range of foods as part of your daily diet, rather than resorting to supplements.”
Dr Paula Franklin,
Managing director
Bupa Lighten Up
What does this mean?
Experts were keen to point out that the research highlights the need to eat a well-balanced diet. The Government's advice to eat five portions of fruit and vegetables a day can form the basis of such a diet.
"I think this is very interesting," said Dr. Wendy Doyle, spokesperson for the British Dietetic Association (BDA) http://www.bda.uk.com. "What it's telling us is that we need to eat a healthy diet all round, because it's the synergy between nutrients and plant chemicals that may be protecting us, rather than individual nutrients. In other words, it's a holistic approach to eating."
Brigid McKevith, a nutrition scientist from the British Nutrition Foundation http://www.nutrition.org.uk, agreed. "We know that many different components in foods - particularly those in fruits and vegetables - may have a role to play in the prevention of diseases such as cancer," she said. "That is why the message of five a day is so important. It's also already known that some vitamins and minerals work together within the body and some may have a synergistic action, as seems the case here. It will be interesting to see what happens in the human trials."
Dr Doyle noted that selenium levels have been declining in the British diet, a fact that has caused some concern. "Selenium in our diet is rather low these days, because we don't import hard wheat for making bread from Canada and North America, like we used to do. The EU [European Union] put a levy on Canadian wheat and our own wheat is not rich in selenium. So, selenium levels have been going down and it's probably a nutrient that there is concern about," said Dr. Doyle.
Cancer Research UK pointed out that eating a range of fruit and vegetables not only helps to protect against cancer, but other health problems also. "A third of all cancers are thought to be linked to poor diet," said Cancer Research UK. "But healthy eating, including plenty of fruit and vegetables, fibre and starch, and low levels of fat, can reduce the risk [of cancer], as well as other diseases, such as heart disease, hypertension and diabetes."
According to the IFR's combination theory, teaming up chicken and broccoli, or mushrooms and sprouts could well be beneficial, but would frozen options be even better?
"Nutritionists and dieticians have included frozen foods in their messages to the public when speaking about fruit and vegetables because, as this and similar studies have shown, some frozen vegetables have higher nutrient levels than fresh vegetables. Frozen vegetables can also be convenient and sometimes cheaper than fresh foods," Brigid McKevith explained.
ACTION POINTS
Aim to eat a wide variety of foods and at least five portions of fruit and vegetables per day, whether fresh, frozen or canned.
Combining foods that contain selenium and sulforaphane in meals may be beneficial in helping to prevent cancer.
Good sources of selenium include nuts, poultry, fish, eggs, sunflower seeds and mushrooms.
Sulforaphane is found in broccoli, sprouts, cabbage, watercress and salad rocket.
The European Union (EU) hasn't set a recommended daily allowance (RDA) of selenium, but the upper safe limit is regarded as 200µg. If you're taking supplements, be careful not to go over this level. The average daily diet generates about 65µg.
What does this mean to me?
To reduce the risk of cancer and other diseases, "you need to eat a wide variety of foods to ensure that you get a combination of all the nutrients that are promoting health," said Dr. Wendy Doyle.
However, it's important not to eat too many selenium-rich foods, as high quantities of the mineral can be dangerous. "It's one of those nutrients where there is quite a low upper safe limit," said Dr. Doyle. It's especially important to be aware of this if you decide to take selenium supplements. "If people do take a supplement - although I wouldn't necessarily recommend it - they should make sure they don't go above the recommended intake," said Dr. Doyle. The European Union (EU) hasn't set a recommended daily allowance (RDA) of selenium, but the upper safe limit is regarded as 200µg.
"Generally, people should eat more fruits and vegetables either as fresh, frozen, dried or canned. Knowing that frozen vegetables can be as good as, and sometimes better than, fresh may make it easier for people to eat at least five servings a day," said Brigid.
"If you are going to eat fresh vegetables, it's best to shop for them on a daily basis," Dr. Doyle said. Look for items that really do look fresh - not wilted.
Or, suggests the Soil Association, buy locally from farmers' markets where produce won't have been shipped on a long journey from overseas.
Summary
The idea that selenium and sulforaphane could be a powerful combination adds to our existing knowledge of diet and disease prevention. Once human trials have been carried out, it is hoped that a fuller picture of these food components' abilities will be known, but until then continuing to eat healthily remains essential. At the moment, the UK Government is urging all people to eat at least five portions of fruit and vegetables a day to encourage healthy eating. Eating a range of vegetables, including frozen ones, may be helpful in boosting the amount of useful nutrients in one's diet.
Links for more information
- Bupa's vitamin and mineral information
- Bupa's diet and nutrition section
- Abstract of Carcinogenesis paper
- Institute for Food Research
- British Dietetic Association
- British Nutrition Foundation
- Austrian Consumers Association
- Soil Association
