Hey kiddos! I’m working on a series of posts on gut bacteria and I thought I’d start off with a post about the gut bacterial metabolism of plant lignans and its role in health and disease, primarily because it is something I know a great deal about. Or should I say “it is something about which I know a great deal”? Probably the latter. Proper use of prepositional phrases confounds me sometimes. Wait, this isn’t a blog about grammar; it’s about nutrition! Moving on…
What are lignans?
Lignans are polyphenolic compounds found in many plants that play a role in plant defense. It’s really quite extraordinary what lignans do for the plant. They have quite an array of defensive properties, protecting the plant from harmful pests and pathogens.1 For example, lignans have been shown to have insecticidal properties comparable to that of pyrethrins.2 If you have ever used that to kill aphids in your home garden then you know how powerful that is. They also have other properties that protect plants such as antifungal properties and somewhat paradoxically antimicrobial properties.3,4 I say “paradoxically” because I am about to discuss the fact that some species of bacteria that can live in the gut go nuts for these lignans.
Lignans are not to be confused with their homophone lignins, which are kinda similar in that they are also found in plants and are chemically related. However, lignins are much larger polymers that intercalate with cellulose and hemicellulose within the cell wall to provide structure and support. Interestingly though, since there are lignan structures within the larger lignin molecule, gut bacteria are able to metabolize lignins to some degree and “release” lignans for further metabolism.5,6
What Foods are Lignans Found In?
Or maybe I should say “In What Foods are Lignans Found?” Damn those prepositional phrases! So they are found in a variety of foods. You can find a fair amount in cereal grains (corn, oats, wheat, rye), cruciferous vegetables, fruits (like apricots, oranges, kiwi, strawberries), and you can even find small amounts in beverages like coffee, tea, beer, and wine.7–31 But by far the largest concentration of lignans can be found in seeds, particularly flaxseeds. Seriously. A handful of flaxseeds contain about ten thousand times more lignans than an equivalent amount of broccoli, and about a hundred thousand times the lignans of, say, an orange.
So Where do Gut Bacteria Come In?
Or should I say “In Where do Gut –“ ah, forget it. So it turns out that plant lignans can be converted to what are sometimes called mammalian lignans or enterolignans by bacteria found in the gut.8,32–68 There are several steps involved when converting a plant lignan to an enterolignan, however, and as far as we know there is not one bacterium that can catalyze all the reactions. Rather, a consortium of bacteria is needed to complete the conversion to the enterolignans enterodiol and/or enterolactone. These more physiologically active enterolignans then get absorbed via colonic epithelial cells.69
But the thing is that not everyone possesses the bacterial community necessary to complete this transformation. According to research by Possemiers and others maybe about 2/3rds of the population has the appropriate species in their gut to convert lignans to enterodiol and far fewer are able to convert lignans to enterolactone.70
Here’s a little diagram I made of common food lignans and the bacteria that convert them. Or at least some of them.
I made a diagram of sesaminol if you’d like to see that, too.
Why Should I Care About Lignans Anyway?
There is quite a bit of evidence that lignans have a variety of beneficial health effects.71–90 Let’s look at all these bennies in slightly more detail.
In Vitro Evidence
- Lignans inhibit the proliferation of cancer cells.91–107
- Lignans suppress the flu virus.108
- Lignans have antimicrobial activity.109
- Provide therapeutic effects to cardiovascular tissue by promoting vasorelaxation and reducing fibrosis, inflammation, apoptosis, and oxidative stress.110,111
- Have neuroprotective effects.112,113
- General antioxidant and anti-inflammatory effects.114
- Prevents angiogenesis.115
Evidence from Animal Studies
- A topical cream made with flax lignans aid in wound healing by their antioxidant activity and stimulating collagen synthesis.116
- Reduced breast tumors.117–121
- Protects bone tissue.122
- Can reduce pain and inflammation.123
- Improves vascular biomarkers.124–126
- Reduces radiation damage.127
- Reduced colon cancer biomarkers.128
- Reduced biomarkers of liver cancer.129
- Lignan intake is negatively associated with esophageal cancer.130
- Enterolignans are associated with a reduced risk of type 2 diabetes.131
- Enterolactone levels are negatively associated with asthma.132
- Lignan intake is negatively associated with bladder cancer, especially urothelial cell carcinoma.133
- Reduced risk of breast cancer.117,134–144
- Associated with reduced risk of colon cancer.140,145–148
- Associated with a decreased risk of prostate cancer.140,148–150
- Lignans are associated with a reduction in cardiovascular disease risk factors.151–153
- Inversely associated with obesity and overweight.154
- Flaxseed intake improves lipid profiles and reduces CVD risk factors.155–157
- Might reduce breast tumor growth.117,158
- Small reductions in prostate cancer biomarkers.159
- Lignans attenuate blood glucose levels.160
There is some evidence that lignans might not be so beneficial, particularly in men. This may be due to the fact that plant lignans and enterolignans are considered to be phytoestrogens with weak estrogenic and antiestrogenic properties.56,161–163
- Associated with male infertility.164
- Associated with an increase in prostate cancer.165,166
Despite the bit of evidence that dietary lignans may not be so good for men, I would say that the benefits outweigh the risks, especially if you are one of the lucky people to have the gut bacterial community that makes for efficient lignan conversion.
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