Protein

Protein is the macronutrient built of amino acids. Dietary protein supplies the amino-acid substrate for every protein-based structure in the body — muscle, tendon, enzyme, hormone, antibody, signalling peptide. After digestion, protein is either used for synthesis, oxidised for energy, or (under surplus) converted to glucose via gluconeogenesis.

Caloric density and effective yield

Protein supplies 4 kcal/gram by Atwater factor. Two corrections matter:

• Thermic effect of food (TEF). 20–30% of protein calories are spent on digestion, amino-acid deamination, urea-cycle processing, and peptide-bond synthesis. The "absorbed and available" calorie count from protein is closer to 3 kcal/g.
• Quality and digestibility. Not all protein is equally bioavailable. DIAAS and PDCAAS scores normalise for amino-acid profile and digestibility; isolated whey and egg score highest, most plant sources lower (though complementary combinations close the gap).

Intake recommendations

• Sedentary adults (US RDA): 0.8 g/kg bodyweight.
• Active adults: 1.2–1.6 g/kg.
• Resistance-trained adults in bulk: 1.6–2.2 g/kg.
• Cutting, to preserve lean mass: 1.8–2.4 g/kg.
• Older adults (sarcopenia prevention): 1.2–1.6 g/kg, frequently cited as an update beyond the 0.8 RDA which was set in sedentary-population contexts.

Why protein matters for calorie tracking specifically

Three reasons protein is worth prioritising over total-calorie micromanagement:

• Satiety. Per calorie, protein is the most filling macronutrient. This is one of the most replicated findings in appetite research.
• Muscle preservation during deficit. Higher protein intake during a cut produces a better lean-mass-to-fat-mass loss ratio.
• TEF boost. Shifting a diet from 15% to 30% protein raises effective TDEE by roughly 50–100 kcal/day at typical intakes.

Timing and distribution

Current evidence supports spreading protein across 3–5 meals of 20–40 g each for maximising muscle protein synthesis in trained individuals. The "anabolic window" of 30 minutes post-training is largely discredited as a critical variable; total daily intake and adequate per-meal distribution matter more. Pre-sleep protein (20–40 g casein) has modest evidence for overnight muscle protein synthesis support.

Practical tracking notes

Consumer databases vary in protein-estimation accuracy. Whole-food entries (chicken breast, lentils, whey powder) are typically within a few percent of reference values in USDA FoodData Central. Mixed-dish and restaurant-meal entries carry substantially more uncertainty. A user aiming for 150 g/day should treat anything within 140–160 g as "on target" given database noise.

Protein quality in practice

For most calorie-tracking consumers eating a varied diet with animal-protein sources, protein quality is not a tracking variable worth worrying about. For tracking consumers on restrictive patterns — vegan, vegetarian, very-low-calorie, or elderly with reduced intake — quality matters more. In those contexts, deliberate attention to complete-protein combining (soy, quinoa, legume-grain combinations), leucine-content per meal (~2.5 g leucine per serving is a commonly-cited threshold for muscle-protein-synthesis stimulation), and supplementation (whey isolate, pea-rice blends) becomes meaningful.