The DCM and diet connection?

What with all of the concern recently on the dogs who have developed heart disease, I've gotten a lot of questions on whether we know the cause and what to do about it.  There are a couple of great summaries that I will refer you to, but the short answer from me is this - the problems seem to be associated solely with small companies or homemade diets.  From what I can tell, none of the larger, very well established companies are involved.  So that's the answer for now - I would try to stick with large, well established companies and not just one of them. I've always said that it's safer to rotate between a few different companies and this is a good example of why I recommend that.

That's not what this is about though - there has recently been some kerfuffle claiming that the reason for this crisis is plant lectins.  I was curious and did some research myself.  This is what I've come up with.

Lectins in Pet foods

Susan G. Wynn, DVM, DACVN

Antinutritional factors appear, as a whole, to benefit plants by deterring predators and microbes. The varying degrees of toxicity to animals, insects and microbes have been exploited in studies that explore medical applications.

Plant defenses include low molecular weight compounds (alkaloids, terpenoids, tannins, and glycosides), and proteins (lectins, ribosome-inactivating proteins (RIPs), protease inhibitors, a-amylase inhibitors, ureases, arcelins, canatoxins, antimicrobial peptides and pore-forming toxins). The proteins tend to accumulate in vulnerable (i.e. edible) parts of the plants - seeds and storage tissues.

The first toxic carbohydrate-binding protein - a lectin called ricin - was discovered in the seeds of castor beans in 1888. Lectins have been reported in bean, tomato, potato, banana and garlic. Lectins are ubiquitous among plants and to date, several hundred have been identified. In addition to plants, animals, insects, viruses, fungi and bacteria also synthesize lectins.

Lectins are globular proteins of non-immune origin that have specific CHO-binding activity and have at least one non-catalytic domain that binds reversibly with specific mono- or oligosaccharides without altering the substrate. Binding is specific but diverse, with some lectins recognizing monosaccharides (such as mannose, glucose, galactose, and fucose). However most plant lectins preferentially bind oligosaccharides like N- and O-linked glycans. Thus far, 12 lectin families have been described, based on sequence similarities and evolutionary relationships.

Toxicity is, in part, determined by resistance to proteolysis by gut enzymes. Lectin toxicity varies widely, from antinutritional activity to lethal effects. Common beans (Phaseolus vulgaris) contain phytohemagglutinin (PHA) which is highly toxic. In the gastrointestinal tract, lectins can act as toxic allergens and hemagglutinins. If lectins bind epithelial cells in the animal digestive tract, the activity at these receptors may have adverse effects on the cellular morphology and metabolism of the stomach and/or the small intestine, and can alter permeability and activate signaling cascades that alter intermediary metabolism (Vasconcelos, 2004).

Most of our knowledge about the anti-nutritional effects of lectin is derived from animal experiments, using purified lectins, with very few clinical studies in humans. Short term experiments in rats have shown that purified lectins from beans or soybeans damaged the epithelial cells. led to enlargement of the small intestine, stimulated hypertrophy and hyperplasia of the pancreas, and impaired the growth of rats in the studies. In addition to disrupting the cell membrane, lectins can inhibit the activity of brush border enzymes. If the activity of lectins is marked enough and not modulated by the intraluminal food matrix, the potential toxicity is primarily  interference with protein and carbohydrate digestion and absorption. It does appear that lectins must bind to the gut epithelium to cause damage, and any that pass through the gut are harmless.

Phytohemagglutinins (PHA) from Phaseolus vulgaris, the common bean including black turtle bean, string bean, flageolet bean, kidney bean, pea bean, pink bean, pinto bean, white bean, yellow bean, cranberry and borlotti bean, and so on, have been studied extensively. PHA is a mitogen and has been used in laboratory studies to stimulate cellular activity. PHA binds strongly to the brush border of the small intestine, and disrupts normal development of intestinal microvilli (in rats). Since lectins remain attached to the epithelial surface, they may also inhibit plasma membrane repair resulting in a net loss of cells.

Even endogenous lectins have been implicated in GI damage - several galectins appear to be involved in the development and progression of inflammatory bowel disease and intestinal tumors. Galectins are a group of mammalian lectins which are involved in cell-to-cell adhesion, growth regulation, signalling and cytokine secretion.

A potential secondary toxic effect of undigested lectin in the small intestine is to lead to overgrowth of coliform bacteria, possibly due to the intraluminal changes in nutrient content induced by increased mucus secretion, epithelial cell loss, serum protein leakage and reduced digestion of dietary protein.

As additional adverse effect is stimulation of an allergic response.  When systemically absorbed intact, PHA can lead to a Th2 response and a Type I (IgE-mediated) allergy, at least in rat studies.

What isn’t understood is the physiologic and nutritional significance of low level, chronic ingestion of lectins and other anti-nutritional factors when taken in as part of the natural diet, cooked or raw, or even whether beans can be considered a natural part of the canine or feline diet. Soy products, for instance, retain 5-20% of the trypsin inhibitory activity originally present in raw soybeans. While some have theorized that the amounts of trypsin and chymotrypsin produced by a human could be completely inhibited by ingestion of raw soybeans or 200 g of other raw legume containing 2 grams of inhibitors, humans do not routinely eat raw beans.

Lectins have been subjected to industrial and experimental processing methods such as steam heating, autoclaving, extrusion and dry roasting in order to study their antinutritional effects.  Although lectins are in general considered heat-resistant, it has been well established that they can be inactivated after cooking (around 100C or 212 farenheit) for short periods of time (up to about 15 minutes), with an improvement of the accessibility of the protein to enzymatic attack(Coffey et al., 1992) (He 2018 p78). He (2018) reports that the navy bean (Phaseolus vulgaris) hemagglutinin (HA) lectin “could be fully eliminated at 121C for 5 minutes after the autoclaving treatment, and white bean (Phaseolus vulgaris) lectin could be fully inactivated after steam treatment of 100C for 15 minutes; extrusion treatment of 145C for 16 minutes resulted in 98% reduction in the HA of small red bean (Phaseolus vulgaris) lectin, as well as 99% reduction in the HA of navy bean (Phaseolus vulgaris) lectin observed with the dry roasting treatment at 200C for minutes (Van der Poel, 1990; Shimelis and Rakshit, 2007)." Purified PHA was subjected to high pressure pasteurization (HPP) and while low pressure (150 mPA) did not change the hemagglutinin activity, high pressure (450 mPA) led to a ‘noticeable decrease in the hemagglutinin activity”.  To summarize, normal cooking temperatures for 15 minutes appear to inactivate most of the undesirable activity, as does high pressure pasteurization.

Lectins may have biomedical applications and are being investigated for potential anti-tumor activity. The antiviral, antifungal and insecticidal properties of some toxic plant proteins has given them potential use in agriculture. Lectins have been used to target malignant cells for specific drug delivery, especially Concanavalin A and BSA. Some PHAs may bind human tumor cells and elicit production of inducible NO synthasae which is anticarcinogenic.  PHA has also been shown to inhibit HIV replication. PHAs have also been shown to enhance the immune response against infections in general as it stimulates lymphocyte replication.

Extracts of beans are marketed for weight loss with the premise that PHA inhibit carbohydrate absorption and metabolism. These effects are seen using isolated, uncooked plant lectin - obviously a plate of cooked beans would not have the same benefit!

Summary

While lectins do have adverse effects on the Gi tract of rats when administered as an isolated treatment, similar damage has not been demonstrated when lectin-containing foods are ingested as a normal part of the diet in humans. Reviewers have concluded that when properly cooked, lectin-containing foods are unlikely to pose any risk to humans. While beans in significant amounts are not a natural component of the feline diet, they may or may not be a natural part of the canine diet. In pet foods, lectin-containing foods are extruded or canned, which should destroy most of the antinutritional activity. In addition, these foods are only one component in a recipe containing other ingredients, which are usually meats. Vegan diets containing no animal protein may have enough residual lectin activity to be of concern, especially if they have been subjected to dry heat only, as wet heat is more effective. The recent crisis in dogs who develop dilated cardiomyopathy is unlikely to be related to the lectin content of bean- or lentil- containing foods because many kinds of foods have been implicated, including some that contain no beans or other  foods high in lectins. Further, if lectins were causing mucosal damage, why would the metabolism or absorption of taurine in particular be targeted? Damage from lectins appears to have a much broader effect, resulting in a leaky gut that should theoretically have much more systemic results. But probably only if you are a rat, eating pure lectin.