Tukuk-OSFood is not fuel until chemistry breaks it to its parts, and the tract is a seven-to-nine-metre production line in three acts. The mouth mechanically preps and enzymatically starts starch; the stomach is the acid bath — hydrochloric acid at pH 1.5–3.5, strong enough to dissolve meat and kill most pathogens (a genuine immune front line), with a mucus-lined wall protecting the organ from digesting itself, and pepsin snipping proteins into fragments. Then the small intestine does the absorption: its lining folded into villi and microvilli giving a surface area estimated at tens of square metres, where digested sugars, amino acids and fats cross into blood or lymph, with bile — a detergent made by the liver, stored in the gallbladder — emulsifying fat so enzymes can act on it.
The large intestine is water recovery plus a city: roughly 38 trillion bacteria — the microbiome, of the same order as our own cell count — ferment what you could not digest, manufacture vitamins (including a meaningful share of vitamin K), train and modulate immunity, and clearly communicate with the brain via vagus nerve and metabolites. Honest science on this frontier says “influences strongly”, not “controls mood”, and probiotic claims outpace evidence — but that antibiotics can reshape the flora, and that constipation is largely colon-water handling, is settled.
Metabolism is the ledger after absorption: glucose for quick energy, fatty acids for storage and reserve, amino acids rebuilt into your own proteins. You continuously dismantle and rebuild most of your body — liver cells in weeks, red cells in 120 days, bone remodelled over years. “You are what you eat” is literally true in slow motion; the machinery re-assembles you from whatever you supply.