Carbohydrates are the body's primary energy source at 4 kcal/g, divided into simple sugars that digest quickly for fast energy and complex starches that release glucose more gradually for sustained activity. The glycemic index (GI) ranks how quickly a food raises blood sugar, with low GI (55 or below) like oats preferable for stable energy, while glycemic load (GL) multiplies GI by actual carbohydrate quantity to reflect real-world portions; a watermelon has high GI around 72 but low GL around 5 per typical serving, whereas a bagel has both high. Dietary fiber, an indigestible plant carbohydrate, divides into soluble types (oats, psyllium, beans) that dissolve into a gel to slow gastric emptying, bind bile acids, and lower LDL cholesterol, and insoluble types (wheat bran, vegetables) that add bulk and speed transit to prevent constipation; adults should aim for 25-38 g daily, with attention to fiber's role in promoting short-chain fatty acid production in the colon. Functional fiber, by contrast, is isolated or synthetic (inulin, polydextrose) and added to processed foods to boost fiber content on labels.
Proteins supply 4 kcal/g and are composed of 20 amino acids, nine of which are essential and must come from the diet while others become conditionally essential under stress, illness, or prematurity. A complete protein contains all nine essential amino acids in adequate proportions in a single food (meat, eggs, soy, quinoa), while complementary proteins combine two or more incomplete sources, such as rice and beans, to supply the full set; modern nutrition views daily intake as adequate rather than requiring every meal to be complete. Protein quality is increasingly measured by the Digestible Indispensable Amino Acid Score (DIAAS), which evaluates digestibility at the ileum and can exceed 1.0, replacing the older Protein Digestibility-Corrected Amino Acid Score (PDCAAS) that capped at 1.0. Leucine is the key anabolic signal that activates mTORC1 to trigger muscle protein synthesis, with about 2.5-3 g per meal maximizing this response in younger adults, though older adults often need 30-40 g per meal to overcome anabolic resistance. High-protein diets enhance satiety through GLP-1, CCK, and PYY release while suppressing ghrelin, modestly raising 24-hour energy expenditure via a thermic effect of 20-30%. Gelatin and hydrolyzed collagen peptides, while incomplete proteins lacking tryptophan, may modestly support skin and joint health.
Fats, the most energy-dense macronutrient at 9 kcal/g, include saturated fats (butter, palm oil) that generally raise LDL cholesterol, monounsaturated fats (olive oil, avocado, nuts) that lower LDL while maintaining HDL, and polyunsaturated fats (sunflower oil, fish oil) that lower LDL and supply essential linoleic and alpha-linolenic acids; trans fats should be avoided. Omega-3 fatty acids deserve special mention: ALA from plants must be converted to EPA and DHA, but conversion is less than 10% efficient, so direct sources like fatty fish or algae oil are preferred for the anti-inflammatory and brain-health benefits of EPA and DHA. The liver regulates cholesterol homeostasis by downregulating synthesis when dietary cholesterol rises, though about 25% of people are hyper-responders whose LDL rises more sharply with intake. Plant sterols and stanols, added to margarines and other foods at 2 g/day, can lower LDL by 7-12% by competing with cholesterol for intestinal absorption, and conjugated linoleic acid (CLA) from ruminant fat has modest, isomer-specific effects on body composition.