Modified citrus pectin (MCP) is an altered form of pectin, a type of soluble dietary fiber found in most plants and used in making jellies and in canning foods. The peel and membranes of citrus fruits, apples, and plums contain the highest amounts of pectin. Our bodies can't absorb pectin in its natural state, but we can absorb citrus pectin that has been chemically modified by breaking its molecules into smaller pieces. MCP is available as a dietary supplement in capsule or powder form.
According to the American Cancer Society (ACS), MCP has shown some activity in preventing the spread of prostate cancer, breast cancer and melanoma (the potentially lethal form of skin cancer) to other organs, but only in animal studies. Similar effects have been seen in a few uncontrolled human studies, but so far, we have no proof that MCP reliably slows cancer spread in humans. Some evidence indicates that MCP works by making it difficult for cancer cells that break off from the main tumor to adhere to each other and grow in other organs. However, the ACS notes that in most animal studies, MCP had no effect on the main tumor, suggesting that it may only be useful for preventing or slowing the growth of metastatic tumors in very early stages of their development.
The ACS has also reported that lab studies of both human and animal cells supported the theory that MCP may slow the spread of cancer by attaching to galectin-3, a chemical present in abnormally high levels in many cancers. It plays a key role in the growth, survival and spread of cancer cells.
Despite the encouraging findings in animals and test tubes, information from well-designed trials with MCP in humans remains scant. In a study published in 2003, 10 men with prostate cancer were treated with MCP after standard treatment failed. Afterward, blood tests showed that in seven of the men, the doubling time for prostate specific antigen (PSA), a marker for prostate tumor growth, improved - an indication that MCP may slow the cancer's growth. Unfortunately, in this study there was no control group of men who didn't get MCP, so we can't say for sure that the change seen was due to it.
At this point, we need larger, randomly controlled trials of MCP to determine whether it is effective. MCP appears to be safe, although it can cause stomach upset and allergic reactions in individuals sensitive to citrus. In a few instances, powdered MCP was associated with the development of asthma.
With regard to breast cancer, the only evidence we have that MCP can slow or prevent spread comes from laboratory studies. I've found no evidence suggesting that taking MCP prior to breast cancer surgery will prevent cancer cells from adhering to blood vessel walls.
If you are being treated for breast cancer, I suggest discussing the pros and cons of using MCP with your oncologist.
Andrew Weil, M.D.