In order for dietary lectins to be promoters of atherosclerosis, the following physical and physiological processes must occur: 1) must survive cooking and processing; 2) they must survive dig enzymatic degradation; 3) they must bind gut tissue; 4) they must cross gut tissue barrier; 5) they must resist immunological and hepatic (liver) disposal; 6) they must arrive in peripheral circulation intact in physiological concentrations; 7) they must interact with one or more mechanisms known to influence atherosclerosis. The six of these seven steps are known to transpire, as ingested lectins rapidly appear intact in the bloodstream of humans and animals and cross the intestinal barrier in human cultured tissue.
So, we know that dietary lectins can get into the bloodstream of humans and animals, and we know that chronic, low-level inflammation is essential for all facets of atherosclerosis. Is there any evidence that lectins are involved in the progression or acceleration of atherosclerosis? Further, is there any evidence that dietary lectins may promote chronic, low level inflammation in humans?
Plaques only form in turbulent flow areas along the artery, such as sites where arteries branch (Figure 1, below), but not where blood flow is smooth (laminar flow), such as in small arteries and at non-branching or non-curving sites. Until recently, the mechanisms underlying this phenomenon was poorly understood. Let's again examine the artery cross-section shown in Figure 2 (below) and take a more detailed look at the true structure lining the inside of arteries, the glycocalyx. This wispy, hairy structure is composed of sugars and carbohydrate molecules that form a physical barrier between the red and white blood cells in circulation and the endothelial cell surface, thereby preventing white blood cells from attaching to adhesion molecules. In other words, one of the very first steps in atherosclerosis, the entry of monocytes into the intima, is blocked when the glycocalyx is fully intact17.Only when the glycocalyx mass is reduced can monocytes and T-cells bind adhesion molecules and find their way into the intima. Turbulent flow areas are more susceptible to atherosclerosis because the glycocalyx mass is reduced in these areas18. It is known that high-fat diets (which increase the rate of oxidized LDL formation) cause the glycocalyx size and mass to be reduced 18, as do inflammatory cytokines19. From a physiological perspective, reductions in glycocalyx size and mass in response to injury or inflammation make sense. Shedding of the glycocalyx allows the white blood cells entry to the inflamed tissue from circulation and therefore begins the healing process by first destroying and then taking up the foreign substance.
Common dietary lectins are potent stimulators of inflammatory cytokines in white blood cell cultures20,21. In Figure 3 you can see that lectins from lentils, kidney beans, peas and wheat potently increase the production of inflammatory cytokines (IL-12, IL-2, and INFγ). Wheat lectin (WGA) also stimulates production of two other inflammatory cytokines (TNFα and IL-1β)21 that promote the atherosclerotic process. Consequently, if dietary lectins reach circulation intact, which previous human and animal studies demonstrate1-5, they have a high probability of causing glycocalyx shedding, thereby increasing entry of monocytes into the intima and contributing to the formation of the fatty streak. Because of their potent inflammatory nature, dietary lectins have the potential to promote and accelerate atherosclerosis at all steps of this disease where inflammatory cytokines are operative. You will recall that one of the deadly steps involved in atherosclerosis is the rupturing of the fibrous cap and the formation of a blood clot. Enzymes called matrix metalloproteinases (MMPs), secreted by white blood cells and other cells within the plaque, are known to cause collagen and elastic tissue within the fibrous cap to disintegrate.
Consequently, any dietary or environmental factor which facilitates synthesis of MMPs is not a good thing for cardiovascular disease patients. Well guess what? Lectins from wheat, WGA22, and lectins from kidney beans, PHA23, cause tissue cultures of white blood cells to increase their production of MMP
Integral to the formation of clots are platelet cells, which circulate in the bloodstream.Platelets are normally activated when they contact collagen from a damaged blood vessel. WGA directly causes the activation of platelets and potently increases their aggregation (clumping) 24. Hence, the consumption of whole wheat may be integral in the thinning and destruction of the fibrous cap as well as the formation of the fatal clot.