Cassava.

Heat-moisture treatment and retrogradation convert part of cassava's starch into type 3 resistant starch, which passes the small intestine undigested. That fraction reaches the colon as fermentable substrate. The rest of the starch is rapidly digestible, so the ratio between the two is set entirely by how the root was cooked and cooled.

Bacteria in the large bowel ferment the resistant fraction of cassava starch and produce short-chain fatty acids, of which butyrate is the main energy source for colonocytes. This is why cassava resistant starch is studied as a prebiotic substrate rather than as a nutrient in its own right. How much butyrate any individual generates depends on which species they carry.

Fermentable starch reaching the colon feeds saccharolytic bacteria, including several genera used in probiotic products. Pairing substrate with organisms is the standard synbiotic logic. Whether the pairing outperforms either component has not been settled for cassava starch specifically.

Linamarin in cassava is broken down to cyanide, which the body detoxifies to thiocyanate. Thiocyanate competes with iodide for the sodium-iodide symporter, so a high cassava intake raises the iodine requirement. Where dietary iodine is already low, this competition is the mechanism behind the observed association between heavy poorly-processed cassava intake and goitre. Adequate iodine intake is expected to blunt the competition rather than abolish it.

The rhodanese reaction detoxifies cyanide by transferring sulfur onto it, and that sulfur ultimately comes from sulfur amino acids. Cassava root is very low in methionine and cysteine, so a diet built on it supplies the cyanogen but not the sulfur to handle it. This mismatch, combined with low protein intake overall, is the recognised background to cassava-associated neurological injury in regions where the root is the staple.

Hydroxocobalamin binds cyanide directly to form cyanocobalamin, which is one route by which small cyanide loads are handled. That route consumes B12. In populations where cassava is the staple and B12 intake is low from the same dietary pattern, the two deficits sit together. The direction is mechanistically clear. The size of the contribution in ordinary diets is not.

As a staple, cassava root delivers energy with a thin mineral and protein profile compared with the cereals it often replaces. Where it dominates the diet, zinc intake tends to fall with it. This is a food-matrix observation about dietary displacement, not a chemical interaction inside the gut.

Conventional white cassava has essentially no provitamin A. Yellow-fleshed biofortified varieties accumulate beta-carotene in the root, which is converted to retinol after absorption. The conversion needs some dietary fat present in the same meal, which cassava itself does not supply.

Biofortification programmes target beta-carotene concentration in the cassava root as the deliverable trait. Retention through processing is the limiting step, because drying and prolonged storage degrade carotenoids. How much survives to the plate depends on the processing method more than on the variety.

Carotenoids are fat soluble and need lipid present to form the mixed micelles that carry them across the intestinal wall. Cassava root is close to fat free. Any fat source eaten with it, culinary oil included, raises carotenoid uptake from a biofortified variety.

Cassava starch granules are relatively resistant in the raw state and become far more digestible after gelatinisation. Amylase acts on the gelatinised fraction. Heat-moisture treatment shifts the balance the other way by increasing the resistant fraction, so processing and enzyme access pull in opposite directions.

Salivary and pancreatic alpha-amylase cleave the alpha-1,4 bonds of cassava's amylose and amylopectin. Only the fraction accessible to the enzyme contributes glucose. The retrograded fraction is not a substrate. That is exactly the split that defines resistant starch.