Researchers Identify Anti-Cancer Mechanism of Beta Carotene

May 20, 1997

Although there is strong epidemiologic evidence that diets rich in carotenoids such as beta carotene are associated with a reduced incidence of cancer, the cellular mechanisms underlying this phenomenon remain unknown. A recent article describes the effect of dietary beta carotene supplementation on both the expression of functionally associated surface molecules on human monocytes and on the secretion of the cytokine tumor necrosis factor-alpha (TNF-alpha) by monocytes, all of which are involved in the initiation and regulation of immune responses involved in tumor surveillance. The article is entitled "The effect of beta carotene supplementation on the immune function of blood monocytes from healthy male nonsmokers." The article was published in the March 1997 issue of the Journal of Laboratory and Clinical Medicine [J Lab Clin Med 1997; 129: 285-7].

Researchers, led by Dr. David A. Hughes of the Department of Nutrition, Diet, and Health, Institute of Food Research, Colney, Norwich, UK, conducted a double-blind, placebo-controlled, crossover study in which 25 healthy, adult male nonsmokers were randomly assigned to receive beta carotene (15 mg daily) or placebo for 26 days, followed by the alternative treatment for a further 26 days. After dietary supplementation, there were significant increases in plasma levels of beta carotene and in the percentages of monocytes expressing the major histocompatibility complex class II molecule HLA-DR and the adhesion molecules intercellular adhesion molecule-1 and leukocyte function-associated antigen-3. In addition, the ex vivo TNF-alpha secretion by blood monocytes was significantly increased after supplemention. These findings suggest that moderate increases in the dietary intake of beta carotene can enhance cell-mediated immune responses within a relatively short period of time providing a potential mechanism for the anticarcinogenic properties attributed to beta carotene.

According to a news release issued by the VERIS Research information Service, to identify and destroy cancer cells, white blood cells, called monocytes, have to first distinguish them from normal cells. Monocytes do this with a protein called MHC II that sits on their surface and, in a sense, searches for cancer cells. When the MHC II protein notices a cancer cell, the monocyte attracts the attention of other immune system cells which move in and attack the cancer cell.

If monocytes don't have enough MHC II proteins, which are akin to biological sensors, the cancer may go unnoticed and can grow. Beta carotene supplements, however, increase the number of MHC II proteins on blood monocytes. That makes them more efficient in their ability to initiate an immune response.

The Veris news release quoted Dr. Hughes, "Our study found that beta carotene can enhance part of the immune system that's known to be involved in tumor surveillance. This could be one way in which a vegetable-rich diet helps prevent cancer. Boosting the immune system in this way could also help to fight off infectious diseases, such as colds and flu."

The amount of beta carotene used in the study was comparable to eating three to four carrots daily. As an added benefit, beta carotene also increased the subjects' production of tumor necrosis factor alpha (TNF-alpha)which zeroes in on cancer cells and helps kill them. The benefits lasted for several weeks after the men stopped taking the beta carotene.

Similar positive findings have led, in recent years, to a surge of research on a family of closely related nutrients, called carotenoids, which include a variety of little known nutrients in addition to beta carotene. Among them are alpha carotene, lutein, zeaxanthin, lycopene, and cryptoxanthin.

For example, high dietary levels of lutein and zeaxanthin appear to protect against macular degeneration, an eye disease that is the leading cause of blindness among the elderly. Similarly, lycopene seems to lower the risk of prostate and possibly other cancers.