If you've been researching lignans for your dog's Cushing's syndrome or Cushing's disease, you've probably run into two terms: SDG lignans and HMR lignans. Both are used to manage Cushing's naturally. Both have a track record. And they're not the same thing.
Here's a plain-English breakdown of how they differ, when one has an advantage over the other, and what most owners end up choosing.
What SDG Lignans Are
SDG (secoisolariciresinol diglucoside) lignans are found in many plants, but are found in particularly high concentrations in flaxseed hulls — the outer layer that's removed when flaxseed is processed. Thus, most available SDG products are extracted from this source. They're the most widely studied form of lignans for canine Cushing's syndrome (including Cushing’s disease), and the dosing protocol most commonly cited by integrative veterinarians is built specifically around them: one (1) to two (2) mg of SDG lignans per pound of body weight, once daily.
SDG also has much the longer history. Flaxseed lignans have been in use, and under research, for considerably longer than the relatively newer HMR extracts, and that accumulated record matters in a practical way: researchers and practitioners simply know more about how they work, what to expect from them, and what their safety profile looks like over time. A newer ingredient is not a worse ingredient, but it is a less documented one.
When a dog digests SDG lignans, gut bacteria convert the lignans into active compounds (enterolactone (ENL or EL) and enterodiol (END or ED)) that interact with the hormonal system. This conversion step is one reason probiotics can enhance (and taking antibiotics can diminish) lignan effectiveness — healthy gut bacteria do more of the converting work. Conversion by gut bacteria (and absorption by the body) can be boosted even further by the addition of the pre-biotic inulin.
What HMR Lignans Are
HMR (7-hydroxymatairesinol) lignans come from the knots of Norwegian spruce trees. Dosing for HMR is the same as for SDG lignans: one (1) to two (2) mg of HMR lignans per pound of body weight, once daily.
How They're Similar
Both HMR and SDG lignans are converted into enterolignans in the gut of the animal. These enterolignans then influence the body's steroid hormone production by blocking enzymes that are essential for that production, thereby helping moderate cortisol overproduction as well as the production of other adrenal hormones involved in Cushing's syndrome.
Both are safe for long-term use and well tolerated, and the dosing protocol is the same for each.
Importantly, the University of Tennessee treatment guidelines — the reference point most integrative veterinarians work from — draw no distinction between HMR and SDG. They do not recommend one over the other.
And in our own experience across a great many customers, people achieve similar results whichever they choose. That is the honest headline of this whole comparison, and it is worth saying before the differences below, because the differences are real but they are not large.
How They're Different
The main practical differences:
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While both require gut bacteria to convert lignans into the biologically active enterolignans, SDG conversion (metabolism) requires a 3-step (for ED) or 4-step (for EL) process, while HMR metabolism involves only a 2-step process. Thus, the conversion of SDG can take longer and involve more types of bacteria (although the first step happens rapidly and is accomplished by a wide array of bacteria). This may be better or worse in any given case, as different animals may respond differently to this time-release effect of enterolignans in the case of SDG lignans
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SDG metabolism results in both types of enterolignans — ED and EL — whereas HMR metabolism can only produce EL. Thus, SDG lignans can provide a more broad-spectrum enterolignan approach. This is not a theoretical distinction: the University of Tennessee study that forms the primary scientific basis for using lignans in Cushing's used SDG enterolignans, meaning both ED and EL rather than EL alone. The data from that study showed differences in response between the two enterolignans, although those differences did not reach statistical significance. The reasonable conclusion is a modest one — it is possible that some individual animals respond better to one enterolignan than the other, which is an argument for having both available rather than only one.
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HMR is more directly bioavailable. However, an unfortunate consequence of this is that the lignans can be absorbed directly into the bloodstream before they have had time to convert into enterolignans, and unconverted lignans are of no effect in blocking cortisol production. Some of that circulating HMR may or may not be metabolized later on; this has not been well studied. What that uncertainty means in practice is twofold. It makes dosing harder to judge, because an unknown fraction of each dose may never become active. And in some animals it may mean a physiologically sufficient level of EL is not reached at all, even though the label dose was given faithfully. By contrast, researchers have found no unconverted SDG (or its first metabolite, SECO) in animal serum.
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SDG carries a higher antioxidant load overall
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HMR comes from a different plant source, which matters if your dog has flaxseed sensitivities
One Practical Note on Flax Hulls
There is a difference between a flaxseed lignan extract and whole flax hulls used as the SDG source.
The University of Tennessee guidelines note that where flax hulls are used rather than an extract, some dogs may have additional bowel movements, simply because of the fiber that comes with the hulls. It is not a safety concern and it is not a reason to avoid SDG. But if you have switched to a hull-based product and noticed your dog going out more often, that is the likely explanation, and moving to an extract usually settles it.
Which Should You Choose?
For most dogs, either works just as well. However, if your dog doesn’t seem to be improving despite varying the dosage, adding probiotics (and the prebiotic inulin) and not being on antibiotics, then try the alternative source. If your dog is on antibiotics, HMR (along with probiotics and inulin) may have the advantage, as less bacteria and less time are involved in its metabolism.
Some owners use both. There's no known interaction, and the protocols complement each other, although you should keep the total dosing between the two to 1-2 mg/pound of body weight/day.
A Note on Atypical Cushing's
If your dog has been diagnosed with atypical Cushing's — the sex hormone-driven form rather than cortisol-driven — SDG lignans may have a specific advantage due to their broader-spectrum approach.
For a list of all our lignans products, click here.
If you're not sure what a proper dose looks like for your dog's weight, our recommended dosages for dogs page walks through it.
The Bottom Line
The guidelines don't pick a winner, and neither do our customers' results. SDG has the longer research record, produces both enterolignans, and carries the higher antioxidant load. HMR converts in fewer steps, which helps dogs on antibiotics, and comes from a non-flax source for dogs with flaxseed sensitivities — with the trade-off that some portion of each dose may be absorbed before it can become active. Choose on your dog's particular circumstances, give it a fair trial at a proper dose, and switch if it isn't working.
Sources:
• University of Tennessee College of Veterinary Medicine — Treatment Considerations