| name | nutrient-metabolism |
| description | Biochemical metabolism of macronutrients and key micronutrients — digestion, absorption, transport, utilization, and excretion — with emphasis on the pathways that matter for dietary-guideline debates (insulin response, lipoprotein metabolism, one-carbon metabolism, iron homeostasis). Use when a question asks what happens biochemically to a food after it is eaten, or when a claim about "metabolic effect" needs to be tested against mechanism. |
| type | skill |
| category | nutrition |
| status | stable |
| origin | tibsfox |
| modified | false |
| first_seen | "2026-04-12T00:00:00.000Z" |
| first_path | examples/skills/nutrition/nutrient-metabolism/SKILL.md |
| superseded_by | null |
Nutrient Metabolism
Dietary claims that sound plausible at the food level sometimes collapse when examined at the metabolic level, and dietary claims that sound implausible at the food level sometimes turn out to have solid biochemical support. This skill grounds the department in the pathways that recur in nutrition debates: how macronutrients are digested and transported, what the lipoprotein system actually does, where insulin and glucagon fit, why iron is unlike other minerals, and what one-carbon metabolism has to do with folate recommendations. It is not a replacement for a biochemistry course; it is a working reference for the questions the department actually sees.
Agent affinity: atwater (macronutrient calorimetry), ancel-keys (lipid metabolism and the saturated-fat question)
Concept IDs: nutrition-biochemistry, nutrition-lipid-metabolism, nutrition-glucose-insulin
Macronutrient digestion and absorption
Carbohydrate
Complex carbohydrates are hydrolyzed stepwise. Salivary amylase begins starch digestion in the mouth; pancreatic amylase continues in the duodenum, producing maltose, maltotriose, and alpha-limit dextrins. Brush-border enzymes (maltase, sucrase, lactase, isomaltase) finish the job, producing monosaccharides: glucose, fructose, and galactose.
- Glucose and galactose are absorbed via SGLT1 (sodium-coupled) and enter the portal circulation.
- Fructose is absorbed via GLUT5 (sodium-independent) and enters the portal circulation. Hepatic fructose metabolism bypasses the phosphofructokinase regulatory step, which is one reason fructose loading differs metabolically from glucose loading.
Dietary fiber resists small-intestine hydrolysis and reaches the colon, where gut microbes ferment it to short-chain fatty acids (acetate, propionate, butyrate). SCFAs are absorbed by the colonocytes and contribute ~2 kcal/g of fiber, which is the basis for the reduced Atwater factor for fiber.
Protein
Protein digestion begins with pepsin in the acidic stomach, continues with pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidases), and is finished by brush-border peptidases. The products — free amino acids and di/tripeptides — are absorbed through multiple transporters and enter the portal circulation.
The liver is the first stop for absorbed amino acids, and it exerts substantial control over systemic amino acid levels. A high-protein meal raises portal amino acids sharply but systemic amino acids more modestly because the liver is actively extracting and metabolizing them. This matters for interpreting "protein pulse" claims.
Fat
Dietary fat is emulsified by bile salts, hydrolyzed by pancreatic lipase (and colipase) to monoglycerides and free fatty acids, packaged into mixed micelles, and absorbed at the enterocyte brush border. Long-chain fatty acids are re-esterified to triglycerides in the enterocyte and packaged into that enter the lymphatic system and bypass the portal circulation. Short- and medium-chain fatty acids are absorbed directly into the portal circulation.
