Beynen AC, 2017. Flax in dog food

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Creature Companion 2017; December: 32, 34.
Anton C. Beynen
Flax in dog food
Flax is a plant grown mainly for its oil and occasionally as linen-fiber crop. Flaxseeds, also known
as linseeds, are small, flat and oval-shaped. Flaxseed or its oil component is found in the ingredient
lists of many commercial dog foods. Dry foods may contain up to 10% flaxseed or 4% flaxseed oil.
Flaxseed meal (defatted seeds) is scarcely used.
Alpha-linolenic acid (ALA), an omega-3 fatty acid, is an essential nutrient that must be present in
the diet. Various ingredients provide ALA, but flaxseed (oil) has a uniquely rich content. The dog
can convert ALA into EPA (eicosapentaenoic acid), which elicits biological activity. The conversion
forms the basis for claims that flaxseed (oil) supports healthy skin, hair coat, joints and heart, but
these enhancements are unproven for flaxseed incorporated into an ALA-sufficient diet.
Supplemental flaxseed oil may ameliorate inflammatory skin disease.
Some internet discussions convey the impression that flaxseed causes food allergy. A manufacturer
offers a dog food line with an explicit flax-free claim. In fact, canine food allergy is rare (1),
rendering flaxseed allergy exceptionally rare. Another concern is that dietary flax releases
potentially toxic cyanide. No adverse cyanide-related effects have been reported for practical
flaxseed-containing dog diets, but this status quo awaits controlled study for confirmation (Note
1).
Flaxseed is high in lignans, compounds that can imitate or oppose female sex hormone activity.
Some dog breeders use flaxseed supplements for reproductive health (2). In female rats, dietary
flaxseed altered reproductive development and reduced the risk of chemical-induced breast
cancer. These topics are still unaddressed in dogs. Flaxseed-containing dog foods might affect
long-term health positively or negatively. For now, flaxseed (oil) is a suitable, but dispensable
ingredient.
Composition
The approximate composition of whole flaxseed is 36% crude fat, 22% crude protein, 8% crude fiber,
4% ash and 8% moisture. The specialities, ALA and secoisolariciresinol diglucoside (SDG), constitute
about 20 and 1%. Total SDG (3) consists of various oligomers (4). Measurable amounts of undesired
cyanogenic glucosides, tannins and seed-coat mucilage fall markedly upon heat treatment (5, 6) such
as applied in the production of kibbled or canned food.
Flaxseeds have a tough hull. Human subjects consuming muffins with identical amounts of ALA as
whole flaxseed, milled seed or flax oil, had increasing plasma ALA concentrations, in this order (7).
Grinding seeds or isolating oil enhances ALA availability, but also its oxidation susceptibility. In stored
oil, oxidation (8) and bitter products (9) appear.
Total-tract digestibility

Incorporating flaxseed, presumably ground and at 5.6%, into an extruded food, without changing
macronutrient composition, lowered apparent protein and fat digestibility in dogs by about one
percent unit, while fecal moisture was unaffected (10). Flaxseed fermentation by canine fecal flora
gave uninterpretable results (11).
Solvent-extracted flaxseed meal, as component of extruded kibbles, had an apparent protein
digestibility of 79% (12). It was 77% for pressed flaxseed cake, soaked in hot water and then mixed
with wetted kibbles (13). Intake of dietary dry matter with 1% flaxseed mucilage, which matches
about 9% whole flaxseed (6, 14, 15), resulted in lower fat digestibility and more moist stools (16).
Alpha-linolenic acid
Feeding ground flaxseed or flax oil to dogs raises ALA, EPA and docosapentaenoic acid (DPA), but not
docosahexaenoic acid (DHA), in blood total lipids, phospholipids, triglycerides, erythrocytes,
neutrophils, and heart tissue (17-22). ALA intakes were increased up to 6.5 g per MJ metabolizable
energy for at least 4 weeks.
The fatty acid changes endorse that the dog is capable of converting ALA. The liver desaturates both
linoleic acid (LA) and ALA (23), releases arachidonic acid (24) and likely EPA and DPA also. Canine
retina and brain functionality requires DHA, plasma DPA serving as (co-)precursor (25). Milk fat from
dogs fed flax oil during gestation and lactation was enriched in ALA, but not in EPA, DPA and DHA
(26).
Eicosapentaenoic acid
EPA gives rise to eicosanoids with wide-ranging biological properties. Dogs gather bodily EPA more
efficiently from dietary EPA than from ALA (22) or ALA’s delta-6-desaturase product, stearidonic acid
(27). Using the observed (22) build-up of plasma phospholipid EPA as criterion, it follows (Note 2)
that 1.5 g of flaxseed oil (55% ALA) is as effective as 1 g of fish oil (10% EPA).
Skin health
Increasing the dietary ALA level from 0.08 g/MJ, which met the dog’s requirement (28, Note 3), to
0.56 g/MJ by inclusion of ground flaxseed, did not affect skin and hair coat condition in healthy dogs
(29, 30). Among four canine diets, the diet containing flax was not associated with healthier skin and
hair (31). Supplemental flaxseed oil (0.44 g ALA/MJ diet) improved canine atopic dermatitis
somewhat less than pure EPA (0.20 g/MJ diet) (32), while EPA amounted to twice the maximum
effective dose (33).
Heart and joint health
Fish oil (0.13 g EPA/MJ diet), but not flax oil (0.17 g ALA/MJ diet) supplementation, distinctly
elevated group-mean plasma EPA and reduced ventricular arrhythmia in Boxers (34). Fish oil (0.2-0.4
g EPA/MJ diet) has a small beneficial impact on canine osteoarthritis (35), but flax oil’s position
remains unknown.
Cancer

In specific rat studies (36, 37), flaxseed mitigated cancer risk. Exposure to a diet with 10% ground
flaxseed or 0.02% SDG during suckling, reduced the incidence of dimetylbenzanthracene-induced
palpable mammary tumors later in life (37).
Reproductive development
Dietary flaxseed activated oestrogen-receptor signaling in murine mammary gland (38). Diets
containing 5 or 10% flaxseed delayed or accelerated puberty onset in female rats (39, 40).
Note 1
Cyanogenic glycosides (CG) are plant chemicals consisting of a nitrile aglycone and a carbohydrate
moiety (glucose or gentobiose). Upon destruction of the plant cell structure, CG’s are hydrolysed by
endogenous enzymes, yielding hydroxypropanenitrile, which decomposes in aqueous solution to
hydrogen cyanide. In flaxseed, the main CG’s are linamarin, linustatin and neolinustatin. The
contents differ among flaxseed varieties (41).
The releasable amount of cyanide in flaxseed, as determined by alkaline titration, is about 200
mg/kg (5, 6). This amount translates to 10 mg/kg dry petfood at 10% flaxseed inclusion level and
50% loss (5, 6) during processing. In young dogs fed a diet based on cooked rice and pork, the
addition of sodium cyanide to a level of 9 mg cyanide/kg dry matter markedly reduced body-weight
gain (42). In contrast, a diet containing cooked cassava instead of rice and providing 11 mg
cyanide/kg dry matter, had no effect on growth.
Note 2
Dogs were fed diets (1.9 MJ/100 g) containing either beef tallow (0.09 g ALA and 0 g EPA/MJ)
flaxseed oil (5.42 g ALA and 0 g EPA/MJ) or menhaden oil (0.12 g ALA and 1.62 g EPA/MJ) (22). After
feeding the diets for 28 days, the EPA contents of plasma phospholipids were 0.53, 4.42 and 10.17
mol% of total fatty acids. The diets with flaxseed and menhaden oil had increased phospholipid EPA
by 3.89 and 9.64 mol%, which was caused by the additional intakes of 5.33 g ALA and 1.62 g EPA/MJ,
respectively. Thus, 1 g ALA and 1 g EPA would raise phospholipid EPA by 0.73 and 5.95 mol%, while 1
g flaxseed oil (0.55 g ALA) and 1 g fish oil (0.10 g EPA) would induce increments of 0.40 and 0.60
mol%. In other words, 1.5 g flaxseed oil is equivalent to 1.0 g fish oil. Some of the flaxseed
inefficiency is likely due to its LA which competes with ALA for the delta-6-desaturase. The flaxseed
and menhaden-oil diets contained 2.11 and 0.69 g LA/MJ (22).
Note 3
The recommended LA and ALA allowances for adult dogs at maintenance have been set at 0.66 and
0.026 g/MJ while the LA:ALA ratio should be between 2.6 and 26 (28). Thus, the 0.026 g ALA/MJ
value is the minimum requirement at 0.66 g LA/MJ as the LA:ALA ratio results in 25.4. The role of the
ratio relates to the competition between LA and ALA for the delta-6-desaturase.
With regard to skin and hair coat condition in healthy dogs, diets containing ground sunflowerseed
(2.51 g LA and 0.12 g ALA/MJ, ratio 20.9) or ground flaxseed (1.97 g LA and 0.68 g ALA/MJ, ratio 2.9)
have been compared (29). Diets with either beef tallow (0.66 g LA and 0.05 g ALA/MJ, ratio 13.2),
sunflower oil (2.37 g LA and 0.07 g ALA, ratio 33.9) or ground flaxseed (2.37 g LA and 0.61 g ALA/MJ,

ratio 3.9) have also been compared (30). Two control diets within the comparisons can be
considered ALA sufficient. The sunflowerseed and tallow diets contained more than 0.026 g ALA/MJ
and had a LA:ALA ratio lower than 26. The sunflower-oil diet had a LA:ALA ratio of 33.9 so that the
ALA content of 0.07 g/MJ may be considered deficient. On the other hand, the 0.07 g ALA/MJ is 2.7
times higher than the minimum requirement at the LA:ALA ratio of 26.
The situation is further complicated by the fact that the diets differed as to contents of EPA, which
could be ALA’s mediator in potentially affecting skin and hair condition. The diets with
sunflowerseed and flaxseed contained 7 and 19 mg EPA/MJ (18, 29). The diets with beef tallow,
sunflower oil and flaxseed contained 11, 7 and 34 mg EPA/MJ (30). Thus, the flaxseed diets
contained more EPA than their control diets. The proposed requirement for EPA is 14 mg/MJ (28).
Even though the flaxseed versus sunflower-oil-diet increased both ALA and EPA intake above their
requirements, there was no improvement of skin and hair coat condition (30).
A third study was published in abstract form (31). The data show that dietary flax was not associated
with changes in skin and hair coat condition in healthy dogs. A diet containing flax as ALA source was
compared with three traditional canine diets. The ingredient and analysed compositions of the four
diets, including the fatty acid profiles, are not reported.
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