Nutrition Science Foundations and Nutrient Requirements
Energy and macronutrients
Nutrition science questions test whether you can separate established physiology from popular claims. Know the Atwater factors: carbohydrate 4 kcal/g, protein 4 kcal/g, fat 9 kcal/g, alcohol 7 kcal/g. Fiber is often counted at about 2 kcal/g, and labels may vary, so use the convention stated in the question. The Acceptable Macronutrient Distribution Ranges are approximately 45–65% carbohydrate, 20–35% fat, and 10–35% protein for adults. Understand that the Dietary Reference Intakes include the EAR, RDA, AI, and UL: an RDA is a recommended intake, an AI is used when evidence is insufficient for an RDA, and a UL is a ceiling for usual intake, not a target. Essential amino acids, essential fatty acids, water, and micronutrients must be supplied by diet; conditionally essential nutrients become limiting under stress or illness.
Digestion, absorption, and metabolism
Follow macronutrients from mouth to bloodstream: starch digestion begins with salivary amylase, protein with gastric pepsin and pancreatic proteases, and fat requires bile emulsification and lipases. Absorbed glucose, amino acids, and fatty acids enter different transport pathways; fat travels via chylomicrons and lymph before systemic circulation. Glycolysis, the citric acid cycle, and oxidative phosphorylation produce ATP; anaerobic glycolysis is fast but limited. Fiber supports bowel health and glycemic control, though excessive fiber too quickly can cause GI distress.
Traps
Common traps include treating the UL as a goal, assuming all protein supplements are necessary, confusing glycemic index with glycemic load, and forgetting that nutrient density is about nutrients per calorie. Another trap is labeling a food “bad” rather than describing its role in the total diet.
Worked rule
For a day with 240 g carbohydrate, 120 g protein, and 70 g fat: (240×4)+(120×4)+(70×9)=960+480+630=2,070 kcal; percentages are about 46% carbohydrate, 23% protein, and 31% fat. Use 4-4-9 as a quick check.