Let your food be your medicine, and your medicine be your food. ...Hippocrates
Flax seed provides all of the nutrients from this small brown or golden hard-coated seed. It is an excellent source of dietary fiber, omega 3 fat (as alpha-linolenic acid), and lignans. The lignans in flax seed are metabolized in the digestive tract to enterodiol and enterolactone, which have estrogenic activity. In fact, flax seed is a more potent source of phytoestrogens than soy products, as flax seed intake caused a bigger change in the excretion of 2-hydroxyestrone compared to soy protein .
Ground flax seeds have been studied for its effect on cancer, including several excellent studies by Lilian Thompson's research group at the University of Toronto. In one study the flax seed, its lignan fraction, or the oil were added to the diet of mice who had previously been administered a chemical carcinogen to induce cancer. All three treatments reduced the established tumor load; the lignan fraction containing secoisolariciresinol diglycoside (SDG) and the flax seed also reduced metastasis . In another study the flax lignan SDG was fed to mice starting 1 week after treatment with the carcinogen dimethylbenzanthracene. The number of tumors per rat was reduced by 46% compared to the control in this study . Flax or its lignan (SDG) were tested to see if they would prevent melanoma metastasis. The flax or lignan fraction were fed to mice two weeks before and after injection of melanoma cells. The flax treatment (at 2.5, 5, or 10% of diet intake) resulted in a 32, 54, and 63 percent reduction in the number of tumors, compared to the control . The SDG, fed at amounts equivalent to the amount in 2.5, 5, or 10% flax seed, also reduced the tumor number, from a median number of 62 in the control group to 38, 36, and 29 tumors per mouse in the SDG groups, respectively .
More recently Thompson's research group studied mice that were injected with human breast cancer cells. After the injection the mice were fed a basal diet (lab mouse chow) for 8 weeks while the tumors grew. Then one group continued the basal diet and another was fed a 10% flax seed diet. The flax seed reduced the tumor growth rate and reduced metastasis by 45% .
Flax seed has been shown to enhance mammary gland morphogenesis or differentiation in mice. Nursing dams were fed the 10% flax seed diet (or an equivalent amount of SDG). After weaning the offspring mice were fed a regular mouse chow diet. Researchers then examined the female offspring and found an increased number of terminal end buds and terminal ducts in their mammary glands with more epithelial cell proliferation, all demonstrating that mammary gland differentiation was enhanced . When these female offspring were challenged with a carcinogen to induce mammary gland tumors there were significantly lower incidence of tumors (31% and 42% lower in the flax seed and SDG groups, respectively), significantly lower tumor load (51% and 62% lower in the flax seed and SDG groups, respectively), significantly lower mean tumor size (44% and 68% lower in the flax seed and SDG groups, respectively), and significantly lower tumor number (47% and 45% lower in the flax seed and SDG groups, respectively) . So, flax seed and its lignan were able to reduce tumor growth (both in number and size of tumors), prevent metastasis, and even cause increased differentiation of mouse mammary tissue in suckling mice, making the offspring less susceptible to carcinogenesis even when not consuming any flax products.
Other researchers have tested flax seed and prostate cancer. In an animal model using mice, Lin et al  found that a diet supplemented with 5% flax inhibited the growth and development of prostate cancer in their experimental mouse model. A pilot study of 25 men who were scheduled for prostatectomy surgery were instructed to eat a low-fat diet (20% or less of energy intake) and to supplement with 30 g of ground flaxseed per day. During the follow-up of an average of 34 days there were significant changes in serum cholesterol, total testosterone, and the free androgen index . The mean proliferation index of the experimental group was significantly lower and apoptotic indexes higher compared to historical matched controls. Ground flax seed may be a very beneficial food for men battling prostate cancer. However, a meta-analysis of nine cohort and case-control studies revealed an association between flax seed oil intake or high blood levels of alpha-linolenic acid and prostate cancer risk . It is quite likely that the lignans in flax seed are a major component of flax's anti-cancer effects so that flax oil without the lignans is not very beneficial. Some brands of flax seed oil retain some of the seed particulate because of the beneficial properties of the lignans.