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 1 —in humans, eating 25 g of flaxseed per day amounts to about 5% of the diet. 2 Lignans were also found to be transferred to baby rats during nursing. 3 Additionally, a study of postmenopausal women found that use of flaxseed reduced estrogen levels and increased levels of prolactin. 4 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. 5 Other studies found that lignans inhibit cancer cell growth. 6 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. 7 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. 8 While normal cooking and baking of whole flaxseeds or flour eliminates any detectable amounts of cyanide, 9 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. 10
For information on dosage, sources and specific safety issues, see the following articles:
Some but not all human studies have found that flaxseed improves cholesterol profile.96 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."94 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.95
Inconsistent evidence hints that flaxseed might reduce LDL cholesterol and, overall, slow down atherosclerosis.95,96,98-100,157,208,284 Flaxseed oil may be helpful as well, although evidence is again inconsistent.209,242
It may be the generic fiber and not the other specific ingredients in flaxseed that benefit cholesterol levels.97,184 Studies of purified lignans (found in flaxseed) have yielded mixed results.210,269
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It has been suggested that flaxseed might help reduce cholesterol and slow the progression of atherosclerosis. There are at least three flaxseed components with potential health benefits:
Some but not all human studies have found that flaxseed improves cholesterol profile.96 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."94 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.95
Inconsistent evidence hints that flaxseed might reduce LDL cholesterol and, overall, slow down atherosclerosis.95,96,98-100,157,208,284 Flaxseed oil may be helpful as well, although evidence is again inconsistent.209,242
It may be the generic fiber and not the other specific ingredients in flaxseed that benefit cholesterol levels.97,184 Studies of purified lignans (found in flaxseed) have yielded mixed results.210,269
Flaxseed and some of its components, flaxseed oil and lignans, have been studied as potential treatments for high cholesterol and atherosclerosis. Research evidence is mixed.
It has been suggested that flaxseed might help reduce cholesterol and slow the progression of atherosclerosis. There are at least three flaxseed components with potential health benefits: