Flaxseed is available as flaxseed flour, flaxseed meal (which has the texture of cornmeal), flaxseed oil and whole flaxseeds. The whole seeds cannot be digested, so they provide no nutritional or health benefits unless they are ground.
Flaxseed is the best dietary source for substances called lignans. Lignans are classified as phytoestrogens (plant estrogens) because they seem to mimic the action of estrogen in the body. In some studies, this behavior has been linked to reduced cancer risk. Note that flaxseed oil does not naturally contain lignans, although some manufacturers add them during processing.
Flax is also the richest plant source of one kind of omega-3 fatty acid, alphalinolenic acid (ALA). In several large scale studies, this fat has shown promising health benefits, including offering protection from heart disease and some cancers.
In several laboratory studies, flaxseed has inhibited the formation of colon, breast, skin and lung tumors. However, there is some indication that the anti-cancer effect of flaxseed varied widely depending upon the variety of flaxseed and its growing conditions.
A few laboratory studies involving rats have suggested that consumption of flaxseed during pregnancy and lactation produces hormonal changes in offspring that may impact their cancer development. These findings are preliminary and warrant further study.
In some short-term human studies, consumption of flaxseed has altered estrogen metabolism in ways that may indicate a protective effect against breast cancer. To date, the laboratory and clinical work on flaxseed and breast cancer has focused on estrogen-receptor negative breast cancers. The effect of flaxseed on estrogen-positive breast cancers has not been studied.
Currently, the evidence associating consumption of flaxseed with protection against prostate cancer is less consistent; some human studies indicate an increased risk, some a decreased risk, and still others find no association at all.
Note: High amounts of flaxseed and flaxseed oil can reduce blood clotting and promote bleeding, and may interact with drugs that that have a similar effect, such as aspirin.
AICR has funded research on the following topics relating to flaxseed and the cancer-fighting components it contains. Click each topic to search for relevant AICR-funded research studies performed to date.