Fitness & Nutrition Glossary

Plain-English definitions for 46+ fitness, nutrition, and body composition terms — from macros to metabolic adaptation.

Macronutrients & Nutrition Energy & Metabolism Body Composition Training & Performance Diet Strategies Protein Supplements

Macronutrients & Nutrition

Macronutrients (Macros): The three primary nutrients that provide calories: protein (4 kcal/g), carbohydrates (4 kcal/g), and fat (9 kcal/g). Fibre is sometimes counted separately. Tracking macros means monitoring the grams of each consumed daily rather than just total calories.
Protein: A macronutrient made of amino acid chains. Essential for muscle repair, enzyme production, immune function, and satiety. Has the highest thermic effect of any macronutrient (~25–30% of calories consumed are used in digestion).
Complete Protein: A protein source containing all nine essential amino acids in sufficient quantities. Animal proteins (meat, eggs, dairy) are typically complete. Some plant sources (soy, quinoa) are also complete.
Essential Amino Acids (EAAs): Nine amino acids the body cannot synthesise and must obtain from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Leucine is the most critical for triggering muscle protein synthesis.
BCAAs (Branched-Chain Amino Acids): Three of the nine EAAs — leucine, isoleucine, and valine — that are metabolised directly in muscle tissue rather than the liver. Leucine is the primary driver of muscle protein synthesis. BCAA supplements are largely redundant if protein intake from whole foods is adequate.
Leucine Threshold: The minimum leucine intake per meal (~2–3g) needed to maximally trigger muscle protein synthesis. A key reason why protein quality (leucine content) matters as much as protein quantity.
Net Carbs: Total carbohydrates minus fibre (and sometimes sugar alcohols). Used in low-carb and ketogenic diets because fibre is not digested and does not raise blood glucose. Formula: Net Carbs = Total Carbs − Fibre.
Thermic Effect of Food (TEF): The energy expended digesting, absorbing, and metabolising food. Protein: 20–30%, Carbohydrates: 5–10%, Fat: 0–3%. Higher protein diets have a meaningfully higher total TEF, contributing to a modest metabolic advantage.
Micronutrients: Vitamins and minerals required in small amounts for physiological function. Unlike macronutrients, they provide no calories but are essential for energy metabolism, immune function, bone health, and countless enzymatic reactions.
Caloric Density: Calories per gram (or per 100g) of a food. Fat has the highest caloric density (9 kcal/g); vegetables have the lowest (~0.2–0.5 kcal/g). High-volume, low-caloric-density foods improve satiety during a caloric deficit.
Protein Quality / DIAAS: Digestible Indispensable Amino Acid Score — the current gold standard for measuring protein quality. Scores above 100 indicate the protein meets or exceeds all essential amino acid requirements. Whey, eggs, and beef score >100; most plant proteins score lower.

Energy & Metabolism

BMR (Basal Metabolic Rate): The calories your body burns at complete rest — the energy required for basic functions (breathing, circulation, cell repair). Calculated using equations like Mifflin-St Jeor. Everything you do above lying still burns additional calories on top of BMR.
TDEE (Total Daily Energy Expenditure): Total calories burned in a day, including BMR plus the energy cost of physical activity and food digestion. The most important number for nutrition planning — caloric intake relative to TDEE determines whether you gain, maintain, or lose weight.
Caloric Deficit: Consuming fewer calories than your TDEE. The fundamental requirement for fat loss. A 500 kcal/day deficit produces roughly 0.5 kg/week of fat loss. Larger deficits accelerate weight loss but increase muscle loss risk without sufficient protein.
Caloric Surplus: Consuming more calories than your TDEE. Required for muscle gain. A modest surplus (250–500 kcal/day) minimises fat gain while supporting muscle growth. Excessive surpluses ('dirty bulk') primarily increase fat mass.
Maintenance Calories: Caloric intake equal to TDEE — neither gaining nor losing weight. Used during diet breaks, phases between bulk and cut, or long-term weight maintenance after a diet.
Mifflin-St Jeor Equation: The most validated BMR equation for most populations. Men: (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5. Women: same formula but −161 instead of +5. More accurate than the older Harris-Benedict equation for the general population.
Metabolic Adaptation: The body's downward adjustment of TDEE in response to prolonged caloric restriction. Includes reduced BMR, reduced NEAT, and hormonal changes (lower leptin, thyroid, testosterone). A key reason why diet breaks are recommended every 8–12 weeks.
NEAT (Non-Exercise Activity Thermogenesis): Calories burned through all movement that is not formal exercise — walking, fidgeting, standing, posture. NEAT varies enormously between individuals (up to 2,000 kcal/day difference) and is one of the largest sources of individual variation in TDEE.

Body Composition

Body Composition: The proportion of fat mass, lean mass (muscle, bone, organs, water) in the body. A better measure of fitness than body weight alone — two people at the same weight can have vastly different body compositions and health profiles.
Lean Body Mass (LBM): Total body weight minus fat mass. Includes muscle, bone, organs, water, and connective tissue. Used in many protein and nutrition calculations because protein needs scale more accurately to LBM than to total body weight — particularly for people with high body fat.
Body Fat Percentage: Fat mass as a proportion of total body weight. Average healthy ranges: men 10–20%, women 20–30%. Essential fat: 2–5% (men), 10–13% (women). Athletes typically carry 6–13% (men) and 14–20% (women).
BMI (Body Mass Index): Weight (kg) ÷ height (m²). A population-level screening tool with well-known limitations — it does not distinguish fat from muscle, leading to misclassification of muscular individuals. Useful as a rough indicator; not a diagnostic tool.
Body Recomposition: Simultaneously losing fat and gaining muscle — typically achieved by eating at or near maintenance calories with high protein and a consistent resistance training programme. Slower than a dedicated bulk or cut but avoids the tradeoffs of either.
Bulking: A deliberate caloric surplus phase aimed at maximising muscle growth. A 'clean bulk' uses a moderate surplus (250–500 kcal/day); a 'dirty bulk' accepts significant fat gain for faster mass gain. High protein (1.6–2.2g/kg) is essential to direct surplus calories toward muscle.
Cutting: A caloric deficit phase designed to lose fat while preserving muscle. Protein intake is typically kept highest during a cut (2.0–2.4g/kg) to provide the anti-catabolic signal needed to protect lean mass.

Training & Performance

Muscle Protein Synthesis (MPS): The biological process of building new muscle protein. Stimulated by resistance exercise and protein intake (particularly leucine). MPS is elevated for 24–48 hours post-training. Maximised at ~0.4g/kg protein per meal for most individuals.
Muscle Protein Breakdown (MPB): The continuous process of breaking down old or damaged muscle proteins. The balance between MPS and MPB determines net muscle gain or loss. Resistance training increases both, but elevates MPS more — producing a net anabolic effect.
Anabolic: Relating to processes that build up — particularly muscle and tissue synthesis. An anabolic environment (sufficient calories, protein, sleep, and hormonal health) favours muscle growth. The opposite of catabolic.
Catabolic: Relating to processes that break down tissue — muscle, fat, or glycogen — for energy. Prolonged caloric restriction, overtraining, poor sleep, and stress are catabolic. High protein intake is the primary tool for minimising muscle catabolism during a deficit.
Anabolic Window: The widely-cited (and partly overstated) period immediately post-workout during which protein and carbs were believed to be particularly beneficial. Current evidence shows the 'window' is 3–5 hours wide, not 30 minutes. Total daily protein and calorie intake matters more than exact timing.
Progressive Overload: The foundational principle of strength training: gradually increasing the demands placed on muscles over time (weight, reps, volume, or difficulty). Without progressive overload, muscle adaptation plateaus regardless of protein intake.
One Rep Max (1RM): The maximum weight that can be lifted for exactly one repetition with correct form. Used to set training intensities as percentages of 1RM. Can be estimated from sub-maximal lifts using the Epley or Brzycki formula.
Hypertrophy: Muscle growth via increased size of individual muscle fibres. Stimulated by mechanical tension, metabolic stress, and muscle damage from resistance training. Optimal hypertrophy requires both adequate training stimulus and sufficient protein (1.6–2.2g/kg/day).
DOMS (Delayed Onset Muscle Soreness): Muscle soreness appearing 24–72 hours after unaccustomed exercise. Caused by micro-tears in muscle fibres and the inflammatory repair process. Not a reliable indicator of training quality or muscle growth — you can have a highly effective workout with no DOMS.

Diet Strategies

Intermittent Fasting (IF): An eating pattern that cycles between defined fasting and eating windows. Common protocols: 16:8 (fast 16h, eat 8h), 18:6, OMAD (one meal a day), and 5:2 (5 normal days, 2 very-low-calorie days). IF does not provide a metabolic advantage over continuous caloric restriction when calories and protein are matched.
Ketogenic Diet (Keto): A very-low-carbohydrate, high-fat diet (typically <50g net carbs/day) that shifts the body into ketosis — burning ketone bodies derived from fat as the primary fuel instead of glucose. Protein must be moderate (not excessive) to avoid gluconeogenesis disrupting ketosis.
Carb Cycling: Alternating between high-carb, moderate-carb, and low-carb days based on training schedule. High-carb days coincide with heavy training to fuel performance; low-carb days on rest days. Protein intake stays consistent across all days.
Diet Break: A planned period (typically 1–2 weeks) of returning to maintenance calories during a prolonged cut. Partially reverses metabolic adaptation, restores hormones (leptin, testosterone, thyroid), and improves adherence to the subsequent deficit phase.
Refeeding Day: A single day of increased carbohydrate intake (up to or above maintenance) during a cut. Temporarily raises leptin, improves performance, and provides a psychological break. Less powerful than a full diet break but easier to schedule.
GLP-1 Medications: A class of weight-loss drugs (semaglutide/Ozempic, Wegovy; tirzepatide/Mounjaro) that mimic the GLP-1 gut hormone, suppressing appetite and slowing gastric emptying. Typically produce 10–20% body weight loss, with significant muscle loss risk — making high protein intake (1.8–2.2g/kg) critical for users.
Protein-Maxxing: A 2025–2026 viral nutrition trend of deliberately maximising daily protein intake, typically to 2.0–3.0g/kg. Partially evidence-backed (higher protein does benefit most people) but with diminishing returns beyond ~2.2g/kg for muscle growth.

Protein Supplements

Whey Protein: A fast-absorbing complete protein derived from milk during cheese production. High leucine content (~11%) makes it highly effective for stimulating MPS. Comes in concentrate (70–80% protein), isolate (90%+ protein, less lactose), and hydrolysate (pre-digested, fastest absorption) forms.
Casein Protein: A slow-digesting milk protein that forms a gel in the stomach, releasing amino acids over 5–7 hours. Often used before sleep to sustain elevated MPS overnight. Lower leucine content than whey but comparable muscle-building outcomes over 24 hours.
Protein Isolate vs Concentrate: Isolate is more processed — typically 90%+ protein with minimal fat and lactose. Concentrate is less processed — 70–80% protein with more natural cofactors. For lactose intolerance, isolate is preferred. For whole-food nutrition profiles, concentrate is adequate and cheaper.
Creatine: The most research-backed ergogenic supplement. Increases phosphocreatine stores in muscles, enhancing ATP regeneration during high-intensity efforts. Produces 5–15% strength and power increases. Standard dose: 3–5g/day. No cycling required.

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