What are toxic effects of lectins or haemagglutinins?
Well, these can cause growth inhibition in animals and diarrhoea, nausea, bloating and vomiting in case of human beings. When injected lectins agglutinate RBCs, haemolysis and death occurs in extreme cases. Several outbreaks in England after consuming improperly cooked beans have been related to lectins although other antinutritional factors may have played a role. Since lectins are proteins, they are denaturated by heal: however, low temperature or slow cooking may not completely eliminate their toxicity. Toxic effect of lectins also relates to their binding with specific receptor sites on intestinal mucosal cells. This results in lesions, disruption and abnormal development of microvilli. Other effects include reduced activity of brush border enzymes viz. peptidases, disaccharidases, alkaline phosphatase, as well as, pepsin and pancreatic1 salivary amylase.
Secretion of mucin and increased weight/number of intestinal mucosal cells in presence of lectin has also been observed. Since the enzymes secretionlactivity are affected, carbohydrates and proteins are not completely digested. When these substances reach the colon, they act as substrates for the colonic microflora, which subsequently ferment them and produce short chain fatty acids and gases. This is the underlying reason for some of the gastrointestinal symptoms associated with intake of raw beanslpurified lectin. Bacterial overgrowth may also occur and may contribute to lectin toxicity. This has been linked to thinning of the jejunal mucosa and damage to duodenal.microvilli. Increase in bacterial colonization by lectin may be due to its polyvalent nature, which allows lectin to bind to both mucosal cells and bacteria simultaneously. This would 'fix' the bacteria close to the intestinal mucosa. If the mucosa is disrupted, it may allow translocation of the bacteria andfor endotoxin and cause a toxic response. In addition, there are reports that lectins themselves may be internalized, causing systemic effects such as increasing protein catabolism, breakdown of stored glycogen and lipid, as well as, disturbing mineral metabolism. Yet another type of food components which may influence nutrient absorption health include phytates. You must have read about them before especially with respect to iron absorption. We will now discuss some salient features of phytates.