Registered Dietitian (RD) Exam

This chapter covers the biochemistry behind every later clinical decision: how carbohydrates, proteins and lipids are classified and judged for quality, where each nutrient is digested and absorbed, how hormones coordinate the process, which pathways run in the fed and fasting states, and how energy expenditure is measured. Expect questions that ask you to predict the nutritional consequence of a resected or diseased GI segment, interpret a respiratory quotient, or identify the limiting amino acid in a food.

Carbohydrates provide 4 kcal/g and are the preferred fuel for the brain, red blood cells and exercising muscle. The RDA of 130 g/day (adults) is based on the average minimum glucose use of the brain; the AMDR is 45–65% of energy. Glycogen stores are limited: roughly 100 g in the liver (maintains blood glucose) and 400 g in skeletal muscle (used only by that muscle, because muscle lacks glucose-6-phosphatase).

Protein provides 4 kcal/g and is about 16% nitrogen (so grams protein = grams nitrogen × 6.25). Of the 20 amino acids, nine are indispensable: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Conditionally indispensable amino acids (arginine, cysteine, glutamine, glycine, proline, tyrosine) become essential in prematurity, critical illness or metabolic disease; tyrosine, for example, is essential in phenylketonuria. The adult RDA is 0.8 g/kg/day (AMDR 10–35% of energy); needs rise in pregnancy and lactation (1.1 and 1.3 g/kg), growth, illness, wound healing and training.

Complementary proteins pair foods whose limiting amino acids differ (grains + legumes, such as rice and beans or hummus and pita). Complementation does not have to occur at the same meal; adequate variety across the day meets needs.

Lipids provide 9 kcal/g; the adult AMDR is 20–35% of energy, with saturated fat <10% and trans fat as low as possible. About 95% of dietary fat is triglyceride (glycerol + three fatty acids). Linoleic acid (omega-6) and α-linolenic acid (ALA, omega-3) are essential; AMDRs are 5–10% and 0.6–1.2% of energy. Conversion of ALA to EPA and DHA is limited (single-digit percentages), so preformed EPA/DHA come from fatty fish, algae oil or supplements. Essential fatty acid deficiency (scaly dermatitis, hair loss, poor wound healing, thrombocytopenia) can develop within weeks on fat-free parenteral nutrition.

Phospholipids (lecithin = phosphatidylcholine) form membranes and act as emulsifiers. Cholesterol builds membranes and is the precursor of bile acids, steroid hormones and vitamin D; the liver synthesizes most body cholesterol, and dietary cholesterol raises blood cholesterol less than saturated fat does.

The small intestine is the main site of digestion and absorption; folds, villi and microvilli expand its surface area roughly 600-fold. Knowing where each nutrient is absorbed lets you predict deficiencies after resection, bariatric surgery or disease: duodenal bypass compromises iron and calcium, ileal resection compromises vitamin B12 and bile salts, and colectomy compromises water and sodium.

Gastric emptying is slowed most by fat, then protein, then carbohydrate, and by high osmolality, large volume and acid or fat in the duodenum. Liquids leave faster than solids.

Amino acids are glucogenic (most), ketogenic (leucine and lysine only) or both (isoleucine, phenylalanine, tyrosine, tryptophan, threonine). In the fed state, insulin promotes glucose uptake, glycogenesis, lipogenesis and protein synthesis; in fasting and stress, glucagon, cortisol and catecholamines drive glycogenolysis, gluconeogenesis and lipolysis. During starvation, protein breakdown falls as ketones replace glucose for the brain; in metabolic stress (sepsis, trauma), catabolism persists despite feeding.

Physiologic fuel values (Atwater): carbohydrate 4, protein 4, fat 9 and alcohol 7 kcal/g. Resting energy expenditure rises with fat-free mass, fever (≈7% per °F, 13% per °C), burns, sepsis, hyperthyroidism and growth; it falls with age, starvation and hypothyroidism. Direct calorimetry measures heat output; indirect calorimetry (the clinical reference method) measures oxygen consumption (VO₂) and carbon dioxide production (VCO₂).

Normal arterial values: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L. The bicarbonate buffer (CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻) is the main extracellular buffer; the lungs adjust CO₂ within minutes and the kidneys adjust bicarbonate over days. Read pH first, then decide whether PaCO₂ or HCO₃⁻ explains it.

Nutrigenomics studies how nutrients and bioactive food components affect gene expression; nutrigenetics studies how inherited genetic variation alters nutrient requirements and responses. Epigenetics changes gene expression without changing DNA sequence (DNA methylation, histone modification); methyl donors such as folate, B12, choline and methionine participate. Most single gene–diet associations have small effects, so direct-to-consumer genetic diet advice should be interpreted cautiously and integrated with standard assessment.