Consumption of plant foods is inversely associated with cancer. The exact reason for this is not known, but it is hypothesised that plant contain many chemicals that may confer health benefits in humans. The polyphenols are one group of plant chemicals, found in most fruits and vegetables, tea and red wine, that have known health benefits in humans. The carotenoids are another group that may be particularly protective of cancer.
Red, Orange and Yellow
Carotenoids are responsible for many of the red, orange and yellow colours in fruits and vegetables. Plants synthesise carotenoids and use them in times of low illumination to harvest light, and during times of high illumination to protect the cells from ultraviolet rays. Carotenoids are bioavailable in humans and increasing dietary intakes causes increasing plasma and tissue levels of most carotenoids.
Cooking Increases Carotenoid Bioavailability
Like all plant nutrients, carotenoids are present in plant cells. This is problematic because the cells of the plant are surrounded by a cellulose cell wall, and this cannot be digested by humans. Therefore if the cell walls are not mechanically damaged during chewing, the carotenoids are not able to be absorbed. Cooking plant foods breaks down the cellulose cell walls, exposing the carotenoids to the absorptive process.
Carotenoids And Cancer
The free radical theory of cancer suggests that oxidative stress, caused by free radicals, initiates cancer. Because carotenoids are antioxidants, this might be one way they help lower cancer risk. However, carotenoids may have other beneficial mechanisms by which they inhibit cancer cells. In particular, carotenoids can increase the communication between cells (gap cell communication), which increases the likelihood of the body detecting a cancerous cell and killing it.
Types Of Carotenoids
There are over 500 known carotenoids, but only some are commonly eaten in the human diet. The most common dietary carotenoids are beta carotene and lycopene which are found in spinach and tomatoes, respectively. While some carotenoids such as beta carotene can be converted to vitamin A in humans (provitamin A activity), others such as lycopene cannot. However, all carotenoids appear to have some antioxidant activity.
Synthetic Versus Natural Carotenoids
A number of studies have investigated carotenoids and found them to increase cancer rates in certain groups of people. For example, it is claimed that beta carotene increases lung cancer rates in smokers. While this may be true in a clinical setting such data does not translate to the real world. This is because studies tend to give high concentrations of a single carotenoid (usually beta carotene), often in a synthetic version.
However, plants contain mixtures of carotenoids. It is rare to find isolated carotenoids in foods because they often appear as mixtures, and combining foods increases the number of different carotenoids consumed. In addition the total amount of carotenoids is often much lower in plant foods compared to synthetic supplements. Therefore carotenoids should not be taken in isolated supplements, but only as mixtures in foods as nature intended.