2.1 Food & Nutrition Sciences

Food and nutrition science gives you the numbers and mechanisms behind everything else on the CDR Registration Examination for Dietitians (the RD exam). The exam is a computer-adaptive test of 125-145 questions delivered over a 2.5-hour testing window, scored on a scaled range of 1-50 with a passing score of 25. Domain 1 (Principles of Dietetics) is 21% of the test — roughly 26 of the 125 scored items — and these foundational sciences feed directly into the assessment and intervention math in the larger Domain 2.

Energy Yields (Atwater Factors)

Every diet calculation starts with how many kilocalories each macronutrient provides. Memorize the Atwater factors:

Worked example: a 2,000 kcal diet at 55% carbohydrate provides 1,100 kcal from carbohydrate, which is 275 g (1,100 / 4). A second pass: if a patient drinks three 12-oz beers (~14 g alcohol each, 42 g total), alcohol alone contributes 42 x 7 = 294 kcal — a number patients routinely omit when reporting intake. Practice converting between percent of energy, kilocalories, and grams in both directions, because the adaptive engine will hand you the calculation from whichever end is least convenient.

Macronutrients

Carbohydrates are the body's preferred energy source. They range from simple sugars (monosaccharides — glucose, fructose, galactose; disaccharides — sucrose, lactose, maltose) to complex polysaccharides (starch, glycogen, fiber). Dietary fiber is not digested by human enzymes: soluble fiber (oats, psyllium, pectin) lowers LDL cholesterol and slows gastric emptying, while insoluble fiber (wheat bran, cellulose) adds stool bulk and speeds transit. The Adequate Intake for total fiber is 14 g per 1,000 kcal (about 25 g/day for women, 38 g/day for men).

Proteins are chains of amino acids. Nine are essential (must come from diet): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (mnemonic: "PVT TIM HaLL"). Protein quality is measured by the Protein Digestibility-Corrected Amino Acid Score (PDCAAS), capped at 1.0, and the newer Digestible Indispensable Amino Acid Score (DIAAS), which is not truncated and can exceed 1.0.

Reference proteins — whole egg, casein, whey — score at or near 1.0; the limiting amino acid in a food (lysine in grains, methionine in legumes) sets its score, which is why complementary proteins (rice + beans) raise overall quality.

Lipids include triglycerides (storage and energy), phospholipids (cell membranes), and sterols (cholesterol, bile, hormones). The two essential fatty acids are linoleic acid (omega-6) and alpha-linolenic acid (omega-3) — the body cannot synthesize the omega-6 or omega-3 double bonds.

Digestion and Absorption

Digestion is both mechanical and chemical, and most absorption happens in the small intestine across roughly 20+ feet of villi-lined mucosa.

• Mouth: salivary amylase (ptyalin) begins starch breakdown; lingual lipase starts fat digestion; chewing is mechanical.
• Stomach: hydrochloric acid (HCl) denatures protein and activates pepsinogen to pepsin; gastric lipase acts on fat; intrinsic factor is secreted by parietal cells for later vitamin B12 absorption. Almost nothing is absorbed here except water, alcohol, and some drugs.
• Small intestine: pancreatic enzymes (amylase, trypsin, chymotrypsin, lipase) plus bile salts complete digestion. The duodenum absorbs iron, calcium, and folate; the jejunum absorbs most carbohydrate, protein, and water-soluble vitamins; the ileum absorbs vitamin B12, bile salts, and fat-soluble vitamins.
• Large intestine: reabsorbs water and electrolytes; resident bacteria ferment soluble fiber into short-chain fatty acids (SCFAs) — acetate, propionate, and butyrate, the latter fueling colonocytes.

Exam trap: a patient who has had the terminal ileum resected loses B12 and bile-salt reabsorption, so memorize "ileum = B12 + bile salts."

Metabolism and Energy Balance

Total Energy Expenditure (TEE) has three components:

1. Basal/Resting Metabolic Rate (BMR/RMR) — 60-75% of TEE; energy for vital functions at rest. Fat-free (lean) mass is the strongest determinant, which is why men and larger people have higher RMR.
2. Thermic Effect of Food (TEF) — about 10% of TEE; the energy cost of digesting and storing nutrients. Protein has the highest TEF (20-30% of its own calories), versus 5-10% for carbohydrate and 0-3% for fat.
3. Physical Activity — the most variable component, from sedentary to athlete.

Energy balance drives weight change: roughly 3,500 kcal equals about 1 pound (0.45 kg) of body fat, so a sustained 500 kcal/day deficit predicts roughly 1 lb/week loss. Worked example: a 250-kcal/day deficit projects about 0.5 lb/week, or ~2 lb/month.

Macronutrient metabolism converges on shared pathways — glycolysis, the citric acid (Krebs) cycle, and oxidative phosphorylation (the electron transport chain, where most ATP is made). Gluconeogenesis synthesizes glucose from lactate, glycerol, and glucogenic amino acids; ketogenesis generates ketone bodies during prolonged fasting or very-low-carbohydrate intake, supplying the brain when glucose is scarce.

Dietary Reference Intakes (DRIs)

The Dietary Reference Intakes (DRIs) are reference values from the National Academies (formerly the Institute of Medicine) used to plan and assess diets for healthy people — not to treat disease.

Key relationship the exam loves: the RDA is derived from the EAR, so an RDA cannot exist without an EAR. When an EAR cannot be determined, an AI is set instead (this is why fiber and vitamin K have AIs, not RDAs). Use the EAR to evaluate the prevalence of inadequacy in a population; use the RDA or AI as a target for an individual. The UL is not a recommended intake — it is the threshold above which toxicity risk rises, a frequent distractor on micronutrient questions.