The digestive system breaks food down through a genuinely long sequence of mechanical and chemical steps, chewing, acid and enzyme action in the stomach, further enzyme action and absorption in the small intestine, before any actual nutrient reaches the bloodstream and becomes usable by the rest of the body. Each specific class of nutrient, carbohydrate, protein and fat, needs its own distinct set of digestive enzymes to be broken down into a molecule small enough to actually be absorbed, meaning digestion isn't one single process but several running in parallel on different parts of the same meal.
Mechanical and chemical breakdown happen together through the whole tract
Chewing mechanically breaks food into smaller pieces and mixes it with saliva, which already contains an enzyme starting to break down starch, and the stomach continues both processes at once, churning food mechanically while acid and digestive enzymes chemically break it down further. The small intestine is where both final digestion and the bulk of actual nutrient absorption happen, its inner lining is folded into finger-like projections, villi, further covered in even smaller microvilli, creating an enormous internal surface area specifically to maximise how much nutrient the body can actually absorb.
Each nutrient class needs its own distinct enzyme pathway to be broken down
Carbohydrates need amylase enzymes to break them down into simple sugars, proteins need protease enzymes to break them down into amino acids, and fats need lipase enzymes, plus bile to first emulsify the fat into smaller droplets since fat doesn't naturally mix with the watery environment inside the gut. Because these pathways are genuinely separate, a missing or deficient enzyme in just one pathway, lactase deficiency being the clearest common example, blocks digestion of that one specific nutrient class while leaving the digestion of everything else completely unaffected.
What we're still unsure about
That digestion involves both mechanical and chemical breakdown, and that each nutrient class needs its own distinct enzyme pathway, is well established, uncontroversial human physiology. What's more genuinely an active, rapidly evolving area of research is how much additional digestive and metabolic work the gut microbiome does beyond what the body's own enzymes accomplish, particularly for dietary fibre the human body has no enzymes of its own to break down at all, and exactly how much of a given person's overall nutrient extraction and metabolic health actually depends on their specific gut microbial community remains a genuinely open, actively studied question rather than a settled picture.
This sits inside Digestive System & Nutrition, one of eight topics in Human Biology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.