Basic Nutrition Science: Nutrient Chemistry, Absorption Sites and Energy Arithmetic
Basic Nutrition Science is the mechanistic backbone of the Chinese Registered Dietitian exam. Items here test whether you can move fluently between nutrient chemistry, the exact site and transporter responsible for absorption, and the quantitative energy arithmetic that later domains depend on. Expect calculation stems, enzyme-to-substrate matching, and 'which statement is FALSE' items on intermediary metabolism.
Macronutrient chemistry and energy yield
Atwater factors: carbohydrate 4 kcal/g, protein 4 kcal/g, fat 9 kcal/g, alcohol 7 kcal/g. Dietary fibre is only partly fermented and is usually credited at about 2 kcal/g. Protein quality is limited by the first limiting amino acid; PDCAAS is capped at 1.0, DIAAS is not. The essential fatty acids are linoleic acid (18:2 n-6) and alpha-linolenic acid (18:3 n-3). Nitrogen balance = intake minus urinary, faecal and skin losses.
Digestion and absorption map
- Starch: salivary alpha-amylase starts luminal digestion and is inactivated by gastric acid; pancreatic alpha-amylase completes it in the duodenum; brush-border maltase, sucrase and lactase yield monosaccharides. Glucose and galactose enter by SGLT1 (sodium-dependent), fructose by GLUT5, and all leave basolaterally by GLUT2.
- Protein: pepsin in the stomach, then pancreatic zymogens activated by enteropeptidase in the duodenum; di- and tripeptides are absorbed by PepT1, free amino acids by sodium-dependent carriers.
- Fat: bile-salt emulsification, pancreatic lipase with colipase, mixed micelles, absorption mainly in the jejunum, chylomicrons exported via lymph. Medium-chain fatty acids go directly into the portal vein.
- Site-specific micronutrients: vitamin B12 needs intrinsic factor and is absorbed in the terminal ileum; non-haem iron absorption is enhanced by ascorbic acid and inhibited by phytate, tannins and calcium; folate is absorbed in the jejunum.
Energy expenditure
BMR is 60-70% of total daily energy expenditure; estimate it with Mifflin-St Jeor (10 x kg + 6.25 x cm - 5 x age + 5 for men, -161 for women). The thermic effect of food is about 10% of intake; physical activity is the most variable component. Anaerobic glycolysis yields 2 ATP net; complete oxidation of one glucose yields roughly 30-32 ATP.
Worked example
A 40-year-old man, 175 cm, 70 kg. BMR = 700 + 1093.75 - 200 + 5 = about 1599 kcal; with a physical activity factor of 1.55, TDEE is about 2478 kcal. His intake of 90 g protein, 60 g fat, 250 g carbohydrate and 25 g fibre supplies 360 + 540 + 1000 + 50 = 1950 kcal, so he is in negative balance unless his real activity is lower than assumed.
Traps that cost marks
Kilojoules versus kilocalories (1 kcal = 4.184 kJ) - Chinese labels print kJ first, and forgetting the conversion is the commonest arithmetic error. Giving fibre 4 kcal/g. Confusing the jejunum (most absorption) with the ileum (B12, bile salts). Counting alcohol as a nutrient rather than an energy source.