Flaxseeds are the hard, tiny seeds of Linum usitatissimum , the flax plant, which has been widely used for thousands of years as a source of food and clothing. There are at least three flaxseed components with potential health benefits. The first is fiber, valuable in treating constipation . Flaxseed also contains alpha-linolenic acid, a type of omega-3 fatty acid similar to the omega-3 fatty acids found in fish oil, but significantly different in other ways, and perhaps offering some of the same benefits. Finally, substances called lignans in flaxseed have phytoestrogenic properties making them somewhat similar to the isoflavones in soy.
The fiber in flaxseed binds with water, swelling to form a gel which, like other forms of fiber, helps soften the stool and move it along in the intestines. One study found that flaxseed can help with chronic constipation in irritable bowel disease. 2Germany's Commission E authorizes the use of flaxseed for various digestive problems, such as chronic constipation , irritable bowel syndrome , diverticulitis , and general stomach discomfort . 1
Flaxseed may be slightly helpful for improving cholesterol profile , according to some but not all studies. 3-5,60-62,72 Purified alpha linolenic acid or lignans alone have not consistently shown benefits. 63-64 It may be the generic fiber and not the other specific ingredients in flaxseed that benefit cholesterol levels.
Because it is believed to have soothing properties, flaxseed is sometimes used for symptomatic relief of stomach distress and applied externally for inflammation of the skin. 22 However, research on these potential uses is essentially nonexistent.
Although flaxseed is often advocated for the treatment of symptoms related to menopause , a sizable 12-month study failed to find it more helpful than wheat germ placebo. 61 Besides failing to improve immediate symptoms such as hot flashes, flaxseed did not appear to provide any protection against loss of bone density. A previous, much smaller study by the same researchers found it equally effective for menopausal symptoms as hormone replacement therapy, but due to the absence of a placebo group and the high rate of placebo response in menopausal symptoms, these results cannot be taken as indicating much. 60 Another study tested flaxseed without comparing it to placebo and reported a 50% reduction in hot flashes. 66 The researchers went on to state that this reduction in hot flashes was “greater than what would be expected with placebo,” a rather curious claim since because menopauseal women given placebo typically experience almost exactly a 50% decrease in hot flashes. 67
In a preliminary double-blind trial of 78 older men, flaxseed extract modestly improved the urinary symptoms associated with benign prostatic hyperplasia (prostate enlargement) after four months of treatment. 70
Use of essential fatty acids in the omega-3 family has also shown some promise for the treatment of non-alcoholic fatty liver. 69 Based on a small, randomized study, flaxseed may also decrease levels of certain liver enzymes, possibly reducing the risk of liver disease in men who have high cholesterol. 72
In a double-blind study, 55 people with chronic constipation caused by irritable bowel syndrome received either ground flaxseed or psyllium seed (a well-known treatment for constipation) daily for 3 months. 23 Those taking flaxseed had significantly fewer problems with constipation, abdominal pain, and bloating than those taking psyllium. The flaxseed group had even further improvements in constipation and bloating while continuing their treatment in the 3 months after the double-blind part of the study ended. The researcher concluded that flaxseed relieved constipation more effectively than psyllium.
Some but not all human studies have found that flaxseed improves cholesterol profile . 24-26,58,60,61,68,71,72 However, the benefits, if they do exist, are very modest. For example, in a double-blind study of about 200 postmenopausal women, use of flaxseed at a dose of 40 g daily produced measurable improvements in cholesterol profile, but the improvements were so small that the researchers considered them "clinically insignificant." 61
It has been claimed that flaxseed might also have a direct effect in helping to prevent atherosclerosis based on its lignan ingredients, but the evidence upon which these claims are based is limited to studies in rabbits. 30,31
Some evidence hints that flaxseed or its lignan components might have cancer-preventive properties. Observational studies and other forms of highly preliminary evidence suggest that people who eat more lignan-containing foods have a lower incidence of breast and perhaps colon cancer. 33,65
The lignans in flaxseed are phytoestrogens , plant chemicals mimicking the effects of estrogen in the body: Phytoestrogens hook onto the same spots on cells where estrogen attaches. If there is little estrogen in the body, for example after menopause, lignans may act like weak estrogen. However, when natural estrogen is abundant, lignans may reduce the hormone's effects by displacing it from cells; displacing estrogen in this manner might help prevent those cancers that depend on estrogen, such as breast cancer, from starting and developing. (This is also, in part, how soy is believed to work in breast cancer prevention, although the phytoestrogens in soy are isoflavones .)
Animal studies using flaxseed and its lignans offer supporting evidence for a potential cancer-preventive or even cancer-treatment effect; several found that one or the other inhibited breast and colon cancer in animals 37-39 and reduced metastases from melanoma (a type of skin cancer) in mice. 40Test tube studies have found that flaxseed or one of its lignans inhibited the growth of human breast cancer cells, 41 and that the lignans enterolactone and enterodiol inhibited the growth of human colon tumor cells. 42
This preliminary research is promising, but much more is needed before we can draw any conclusions.
Although much of this anticancer work has focused on the lignans in flaxseed, one study also found that flaxseed oil—which contains no appreciable amounts of lignans—slowed the growth of malignant breast tumors in rats. 43
According to the European Scientific Cooperative on Phytotherapy (ESCOP), the usual dose of flaxseed for constipation is 5 g of whole, cracked, or freshly crushed seeds soaked in water and taken with a glassful of liquid 3 times a day. 44 Expect effects to begin 18 to 24 hours later. Because of this time delay, it's recommended to take flaxseed for a minimum of 2 to 3 days. Children aged 6 to 12 should be given half the adult dose, while children younger than 6 should be treated only under the guidance of a physician. 45
In one study, people received 6 to 24 g per day of flaxseed for 6 months for constipation caused by irritable bowel syndrome. 46
To soothe an upset stomach, soak 5 to 10 g of whole flaxseed in a half cup of water, strain after 20 to 30 minutes, then drink. 47 For painful skin inflammations, the recommended dose is 30 to 50 g of crushed or powdered seed applied externally as a warm poultice or compress. 48
Like other sources of fiber, flaxseed should be taken with plenty of fluids, or it may actually worsen constipation. Also, it's best to start with smaller doses and then increase.
Flaxseed is generally believed to be safe. However, there are some potential risks to consider.
As with many substances, there have been reports of life-threatening allergic reactions to flaxseed.
Because of its potential effects on estrogen, pregnant or breastfeeding women should probably avoid flaxseed. One study found that pregnant rats who ate large amounts of flaxseed (5% or 10% of their diet), or one of its lignans, gave birth to offspring with altered reproductive organs and functions 49 —in humans, eating 25 g of flaxseed per day amounts to about 5% of the diet. 50 Lignans were also found to be transferred to baby rats during nursing. 51 Additionally, a study of postmenopausal women found that use of flaxseed reduced estrogen levels and increased levels of prolactin. 59 This suggests hormonal effects that could be problematic in pregnancy.
Flaxseed may not be safe for women with a history of estrogen-sensitive cancer, such as breast or uterine cancer. A few test tube studies suggest that certain cancer cells can be stimulated by lignans such as those present in flaxseed. 52 Other studies found that lignans inhibit cancer cell growth. 53 As with estrogen, lignans' positive or negative effects on cancer cells may depend on dose, type of cancer cell, and levels of hormones in the body. If you have a history of cancer, particularly breast cancer, talk with your doctor before consuming large amounts of flaxseeds.
If you have diabetes , flaxseed (like other high-fiber foods) may delay glucose absorption. 54 This may lead to better blood sugar control but it also may increase the risk of hypoglycemic reactions. Talk with your doctor about appropriate use.
Finally, flaxseeds contain tiny amounts of cyanide-containing substances, which can be a problem among livestock eating large amounts of flax. 55 While normal cooking and baking of whole flaxseeds or flour eliminates any detectable amounts of cyanide, 56 it is at least theoretically possible that eating huge amounts of raw or unprocessed flaxseeds or flaxseed meal could pose a problem. However, most authorities do not think this presents much of a risk in real life. 57
4. Jenkins DJ, Kendall CW, Vidgen E, et al. Health aspects of partially defatted flaxseed, including effects on serum lipids, oxidative measures, and ex vivo androgen and progestin activity: a controlled crossover trial. Am J Clin Nutr. 1999;69:395-402.
7. Jenkins DJ, Kendall CW, Vidgen E, et al. Health aspects of partially defatted flaxseed, including effects on serum lipids, oxidative measures, and ex vivo androgen and progestin activity: a controlled crossover trial. Am J Clin Nutr. 1999;69:395-402.
21. Ogborn MR, Nitschmann E, Bankovic-Calic N, et al. The effect of dietary flaxseed supplementation on organic anion and osmolyte content and excretion in rat polycystic kidney disease. Biochem Cell Biol. 1998;76:553-559.
25. Jenkins DJ, Kendall CW, Vidgen E, et al. Health aspects of partially defatted flaxseed, including effects on serum lipids, oxidative measures, and ex vivo androgen and progestin activity: a controlled crossover trial. Am J Clin Nutr. 1999;69:395-402.
28. Jenkins DJ, Kendall CW, Vidgen E, et al. Health aspects of partially defatted flaxseed, including effects on serum lipids, oxidative measures, and ex vivo androgen and progestin activity: a controlled crossover trial. Am J Clin Nutr. 1999;69:395-402.
61. Dodin S, Lemay A, Jacques H, et al. The effects of flaxseed dietary supplement on lipid profile, bone mineral density and symptoms in menopausal women: a randomized double-blind wheat germ placebo-controlled clinical trial. J Clin Endocrinol Metab. 2004 Dec 21. [Epub ahead of print]
63. Hallund J, Ravn-Haren G, Bugel S et al. A lignan complex isolated from flaxseed does not affect plasma lipid concentrations or antioxidant capacity in healthy postmenopausal women. J Nutr. 2005;136:112-116.
Last reviewed December 2015 by EBSCO CAM Review Board
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