Flaxseed, also known as linseed, consists of the seeds of the flax plant (Linum usitatissimum). Flaxseed contains phosphorus, manganese, magnesium, cadmium, copper and zinc, with levels depending on the soil in which it is grown. Flaxseed is the richest known source of plant lignans, mainly secoisolariciresinol, which are converted by intestinal microbiota to the mammalian lignans enterodiol and enterolactone. Flaxseed also contains high levels of the omega-3 fatty acid alpha-linolenic acid which has been shown to confer protection against breast cancer. Flaxseed has been shown to have radioprotective, antioxidant and cholesterol-reducing properties. Flaxseed has been reported to inhibit lung and colorectal cancer in mice.
Breast cancer-related effects of consuming flaxseed
Flaxseed and its component have been shown to reduce breast cancer cell growth and enhance breast cancer cell apoptosis. It has been demonstrated that flaxseed can reduce the growth and proliferation of both ER- and ER+ breast cancer cells.
A meta-analysis combining the results of seven studies found that plant lignan consumption was associated with reduced risk of breast cancer in postmenopausal women. A major Swedish prospective study found a reduction in risk of breast cancer in postmenopausal women for those in the highest quartile of plant lignan intake, especially among hormone replacement therapy users. The reduction in risk did not vary by ER or PR status of the tumors. Another study reported that high circulating levels of enterolactone reduce the risk of breast cancer recurrence and death among postmenopausal women, especially those with estrogen receptor negative (ER-) breast cancer.
Enterolactone has also been found to increase the sensitivity of breast cancer cells to radiation, thereby potentially enhancing the treatment effects of radiotherapy. In fact, flaxseed itself has been shown to protect the lungs during radiation for lung cancer without reducing the efficacy of the treatment. Flaxseed has also been shown to enhance the effectiveness of both tamoxifen and Herceptin. Flaxseed has been shown to counteract the breast cancer promoting effects of the soy isoflavone genistein on ER+ breast cancer in mice.
Which is better, flaxseed or flaxseed oil?
One study comparing flax seeds with sesame seeds (also high in lignans) came to the conclusion that the breast cancer protective qualities of flaxseed were due to its high α-linolenic acid content as much as its lignan content. Another study concluded that the lignan component of flaxseed appears to be most beneficial throughout the promotional phase of carcinogenesis whereas the oil component is more effective at the stage when tumors have already been established. Both flaxseed and flaxseed oil have been shown to be associated with reduced risk of breast cancer. However, flaxseed oil has a far lower heavy metal content than flaxseed. Since flaxseed contains both the lignans and the oil, it appears preferable, on balance, to flaxseed oil, but only if cadmium-free.
Because of the high nutrient content of flaxseed, it is subject to spoilage and must be stored and handled properly. Make sure the market where you purchase flaxseed (whether prepackaged or from bulk bin) has frequent product turnover. Check for moisture before buying flaxseed and discard any flaxseed stored at home if there is any evidence of moisture or a rancid smell. Whole flaxseed can safely be stored in an airtight container in a dry, dark, and cool place for several months. Ground flaxseed is also available; the nutrients in ground seeds are more easily absorbed. Ground flaxseed is more subject to oxidation and spoilage than whole flaxseed and should be refrigerated. Flaxseed oil is commonly produced by cold pressing whole flaxseed. It should be stored in opaque bottles and always kept refrigerated.
Flaxseed oil is used in stir-frying in parts of Asia, but we advise against this. Flaxseed oil should be added to foods after they have been heated, if desired, or used in salad dressings or cold sauces prepared without heating. One study that examined the effects of using flaxseed oil in stir-frying found that pan-heating caused a significant loss of important nutrients, as well as increases in undesirable oxidation products.
Linseed oil is also made from flaxseed, but the term refers to a highly processed oil that is used in industrial applications and it is not suitable for human consumption. Fiber cultivars of the flax plant are also used to make linen and related fabrics.
Flaxseed can have relatively high levels of cadmium and copper
Flaxseed can absorb unacceptable levels of cadmium, a heavy metal that has been shown to increase the risk of breast cancer. Mouse studies have demonstrated that significant levels of cadmium can be absorbed from dietary flaxseed. In addition, the female pups of pregnant or lactating rats fed flaxseed have increased susceptibility to developing mammary tumors.
Flaxseed cadmium content is not as unlikely a problem as it may first appear. In addition to the U.S. and Canada, China and India are major flaxseed producers. Both China and India are known for heavy metal pollution in some regions. Farmers do not have to be concerned about heavy metal soil contamination when growing flax for linseed oil or linen production. The market for linseed oil and linen flax have been declining whereas the market for flaxseed meant for human consumption has been increasing and is more lucrative. Therefore, it is possible that flaxseed from high-cadmium areas are reaching the U.S. consumer.
Some areas of Canada, which is the world's largest producer of flaxseed, have naturally high levels of cadmium in the soil. North and South Dakota, the two largest producing U.S. states, also are known to have soil containing cadmium. The "organic" label does not necessarily mean a low-cadmium product. Organic, cadmium-free flaxseed sourced from Northern Canada appears to be the best choice.
In addition, flaxseed contains relatively high levels of copper, which has been shown to increase angiogenesis and metastasis of breast cancer. The cadmium and copper content of flaxseed are the reason we recommend consuming flaxseed in moderation.
Below are links to recent studies concerning this food. For a more complete list, including less recent studies, please click on flaxseed.