Nutrition and fitness content is full of abbreviations and jargon that often go unexplained. This glossary defines the terms you'll run into across our calculators and articles, in plain language, with links to the relevant tool wherever one exists.
- Amino Acids
- The building blocks of protein. Your body uses 20 different amino acids to build muscle, enzymes, and hormones — some it can produce itself, others (essential amino acids) must come from food.
- BCAA (Branched-Chain Amino Acids)
- A group of three essential amino acids (leucine, isoleucine, valine) often sold as a standalone supplement, though a complete protein source already provides all three in useful amounts.
- BMI (Body Mass Index)
- A simple ratio of weight to height, used as a population-level screening tool for weight categories. It doesn't distinguish muscle from fat, so it can misclassify very muscular or very lean individuals. Calculate your BMI →
- BMR (Basal Metabolic Rate)
- The number of calories your body burns at complete rest just to keep basic functions running — breathing, circulation, cell repair — before any movement or digestion is factored in. Calculate your BMR →
- Body Fat Percentage
- The proportion of your total body weight that is fat tissue, as opposed to muscle, bone, water, and organs. A more precise measure of body composition than weight or BMI alone. Estimate your body fat →
- Bulking
- A phase of eating above maintenance calories (a surplus) with the goal of building muscle, usually alongside resistance training. See our Bulking vs Cutting comparison →
- Calorie Deficit
- Eating fewer calories than your body burns in a day, forcing it to draw on stored energy (mostly fat) to make up the difference — the fundamental mechanism behind weight loss. Read the deficit guide →
- Calorie Surplus
- Eating more calories than your body burns in a day, providing the extra energy needed to build new tissue — typically used intentionally during a muscle-building phase.
- Casein Protein
- A slow-digesting milk protein that releases amino acids gradually over several hours, often taken before bed. See Whey vs Casein →
- Complex Carbohydrates
- Carbohydrates made of longer chains of sugar molecules (found in whole grains, legumes, vegetables) that digest more slowly than simple carbohydrates, generally providing steadier energy.
- Creatine
- A naturally occurring compound stored in muscle that helps regenerate energy for short, intense efforts. One of the most well-researched sports supplements. Calculate your dose →
- Cutting
- A phase of eating below maintenance calories (a deficit) with the goal of losing fat while preserving as much muscle as possible. See our Bulking vs Cutting comparison →
- Electrolytes
- Minerals like sodium, potassium, and magnesium that carry an electric charge and are essential for muscle function, nerve signaling, and fluid balance — lost through sweat during exercise. Calculate your water needs →
- Essential Amino Acids
- The 9 amino acids your body cannot produce on its own and must obtain from food. A "complete protein" contains all 9 in meaningful amounts.
- Fiber
- A type of carbohydrate your body can't fully digest, which supports digestive health, feeds gut bacteria, and slows the absorption of sugar into the bloodstream.
- Glycemic Index
- A ranking of how quickly a carbohydrate-containing food raises blood sugar compared to pure glucose. Higher GI foods spike blood sugar faster than lower GI foods. See the GI table →
- Hypertrophy
- The technical term for muscle growth — an increase in the size of muscle fibers, typically driven by resistance training combined with adequate protein and calorie intake.
- Insulin Sensitivity
- How effectively your cells respond to insulin, the hormone that moves sugar from your blood into cells for energy. Higher sensitivity generally supports better blood sugar control.
- Intermittent Fasting
- An eating pattern that cycles between periods of eating and fasting (such as 16:8, eating within an 8-hour window), rather than restricting specific foods or nutrients. Try the fasting timer →
- Ketosis
- A metabolic state in which the body, deprived of sufficient carbohydrates, shifts to burning fat and producing ketones as its primary fuel source — the basis of the ketogenic diet. Read the keto guide →
- Lean Body Mass
- Your total body weight minus fat mass — includes muscle, bone, organs, and water. Often used as a reference point for setting protein targets.
- Macronutrients (Macros)
- The three nutrients that provide calories — protein, carbohydrates, and fat — each needed in relatively large amounts and each serving different roles in the body. Calculate your macros →
- Maintenance Calories
- The number of calories that keeps your body weight stable — matching total daily energy expenditure exactly, with neither a deficit nor a surplus. Read more →
- Micronutrients
- Vitamins and minerals needed in small amounts for normal body function — unlike macronutrients, they don't provide calories but are still essential. Check your vitamin needs →
- Net Carbs
- Total carbohydrates minus fiber (and sometimes sugar alcohols), commonly used on low-carb and keto diets since fiber has minimal impact on blood sugar. Calculate net carbs →
- One Rep Max (1RM)
- The maximum weight you can lift for a single repetition of an exercise with proper form — commonly used to set training percentages for strength programs. Estimate your 1RM →
- Progressive Overload
- The principle of gradually increasing the demands placed on your muscles over time — more weight, reps, or volume — as the primary driver of continued strength and muscle gains.
- RDA (Recommended Daily Allowance)
- The average daily intake of a nutrient considered sufficient to meet the needs of nearly all healthy people in a specific group, used as a reference for vitamin and mineral targets.
- Recomposition (Body Recomp)
- Building muscle and losing fat at roughly the same time, rather than in separate bulking and cutting phases — most achievable for beginners or people returning to training.
- REM Sleep
- A stage of sleep associated with dreaming and memory consolidation, occurring in cycles throughout the night alongside deep and light sleep stages. Calculate your sleep cycles →
- Sleep Cycle
- A roughly 90-minute pattern the brain repeats throughout the night, moving through light sleep, deep sleep, and REM sleep — waking at the end of a cycle tends to feel less groggy. Find your best bedtime →
- TDEE (Total Daily Energy Expenditure)
- The total number of calories your body burns in a day, including your BMR plus calories burned through activity, exercise, and digestion — the starting point for any calorie target. Calculate your TDEE →
- Visceral Fat
- Fat stored deep around abdominal organs, more metabolically active and more strongly linked to health risk than fat stored just under the skin. Check your waist-to-hip ratio →
- VO2 Max
- A measure of the maximum amount of oxygen your body can use during intense exercise, commonly used as an indicator of cardiovascular fitness.
- Waist-to-Hip Ratio
- The ratio of your waist circumference to your hip circumference, used to assess fat distribution and associated health risk. Calculate your ratio →
- Whey Protein
- A fast-digesting milk protein, popular post-workout for its quick amino acid delivery. See Whey vs Casein → and calculate your protein needs →