Scientific base
What Letz reads before it recommends
70 rules and 138 sources, 137 of them verified on PubMed. Each rule shows what the studies found, who it applies to, where the app uses it and what it does not prove. When Letz builds a training plan or a nutrition target, the link comes back here.
Guides: how Letz uses all of this
Longer reads that tie several rules into one product decision.
How Letz builds your workout plan
From frequency per muscle to rest between sets: every number in the workout generator and the rule behind it.
Read the guideHow Letz sets your nutrition target
Calories, protein, fat, and fiber: the math behind the daily target and what the literature says about each part of it.
Read the guideHow Letz reads your recovery
Sleep, steps, and daily signals feed into a range-based reading, with stated confidence and no injury predictions.
Read the guideHow Letz decides when it is time to increase the weight
Progression, autoregulation, and deload weeks: when the app tells you to advance, when it holds you back, and why total failure is not the goal.
Read the guideHow Much Exercise Is Enough
The guidelines for adults, what changes when time is short, and why ten-minute blocks count.
Read the guideCardio and Resistance Training in the Same Week
Does running interfere with resistance training? What the evidence says about combining both and the real role of exercise in fat loss.
Read the guideHow Letz Guides Your Running, Walking, and Sports
Why cardio sessions are structured in blocks, measure effort using the talk test, and only advance when the previous stage was just right—and how swimming, classes, and sports fit into your week.
Read the guideSafe Training: Getting Started, Injury, and Age
Warm-ups, stretching, machines versus free weights, training after 60, and what the menstrual cycle actually changes.
Read the guideWhat Really Matters in Your Diet
Ultra-processed foods, satiety, meal frequency, eating windows, carbohydrates, alcohol, and water—separating what moves the needle from minor details.
Read the guideSupplements: What Has Evidence and What Does Not
Creatine, caffeine, BCAAs, and protein after 60 — four short answers on what the evidence supports for each.
Read the guideBuilding muscle: how much to eat and what to expect
The surplus that sustains hypertrophy without turning into fat, and a realistic rate of gain per week.
Read the guideHow Letz Helps You Stick to Your Routine
Logging, well-timed reminders, and personalization: the three behavior change techniques with the best long-term sustainability.
Read the guideTraining and nutrition during GLP-1 treatment
Why protein and resistance training matter even more during this period, and what the evidence says about regain after discontinuation.
Read the guideWhat the app cannot measure
The honest limits of meal photos, wrist wearables, and any injury prediction.
Read the guideRules and studies
- TrainingWeekly Volume: Sets per Muscle GroupMore weekly sets per muscle produce greater hypertrophy, with diminishing returns; for strength, moderate volumes already capture most of the gains.5 sources · 2 min
- HealthNutritional Support During GLP-1 TreatmentMonitoring should prioritize symptoms, nutritional adequacy, protein, resistance training, and preserving lean mass and function.3 sources · 2 min
- TrainingMinimum Effective Dose and Time-Efficient TrainingA few well-executed sets, two to three times a week, already yield meaningful gains; supersets and combined sets shorten the session without losing volume.3 sources · 2 min
- BehaviourAdaptive support at the right momentContext-tailored digital interventions can be feasible and helpful, but their effect varies and must be measured per person and per decision.2 sources · 2 min
- TrainingProximity to Muscular FailureTraining to failure is not mandatory; sets finished with a few reps in reserve already produce gains, and hypertrophy improves as sets get closer to failure.3 sources · 2 min
- HealthGLP-1: Lean Mass and Regain After DiscontinuationThese medications produce substantial weight loss, part of which is lean mass; upon discontinuation, much of the weight returns. Exercise combined with treatment better preserved body composition and weight maintenance.5 sources · 2 min
- TrainingResistance Training, Strength, and HypertrophyVarious training prescriptions improve strength and hypertrophy; heavier loads favor strength, and multiple sets favor hypertrophy.1 source · 2 min
- TrainingLoad and Volume AutoregulationRIR/RPE and objective measures can help adapt resistance training to daily fluctuations within a structured plan.3 sources · 2 min
- TrainingDeload: reduced-load weekCoaches agree that short periods of reduced stress help manage fatigue and prepare for the next block; experimental evidence remains scarce.1 source · 2 min
- TrainingFree weights, machines, and bodyweightFree weights and machines produce similar strength and hypertrophy gains when directly compared; strength gains are specific to the trained modality.1 source · 2 min
- RecoverySleep, recovery, and performanceSleep interventions may be accompanied by improvements in recovery and certain performance outcomes.2 sources · 2 min
- HealthExercise and Blood PressureMultiple exercise modalities reduce resting blood pressure; isometric and combined training were the most effective in the network comparison.1 source · 2 min
- HealthExercise, Depression, and AnxietyPhysical activity reduces symptoms of depression, anxiety, and psychological distress in adults, showing a moderate effect that is larger at higher intensities and in shorter interventions.1 source · 2 min
- MeasurementLimitations of Image-Based Dietary AssessmentPhotos can make logging easier, but energy and portion sizes carry significant error; user review remains necessary.2 sources · 2 min
- TrainingProgressive Overload and PeriodizationProgressing load or repetitions yields similar gains; varying the stimulus over weeks supports maximal strength more than keeping everything fixed.3 sources · 2 min
- TrainingCombining Aerobic and Resistance TrainingPerforming aerobic and resistance training in the same program does not compromise hypertrophy or maximal strength; only explosive strength may be attenuated, particularly within the same session.1 source · 2 min
- TrainingAccumulated Activity in Short BoutsAccumulating activity across multiple short bouts throughout the day provides benefits similar to a single continuous session.2 sources · 2 min
- NutritionMeal Frequency and Eating WindowEating more or fewer times per day does not change weight loss when calories are equated; restricting the eating window helps only to the extent that it reduces intake.3 sources · 2 min
- NutritionCarbohydrates and Workout PerformanceCarbohydrates sustain long, high-volume sessions; for short resistance training sessions, intake from the previous day is usually sufficient, and daily totals should match the training load.2 sources · 2 min
- RecoverySleep and Fat LossSleeping too little during a deficit shifts fat loss toward lean mass loss and increases intake; extending sleep reduced intake by about 270 kcal per day.3 sources · 2 min
- HealthDaily StepsMore steps per day are associated with lower mortality up to an age-dependent plateau; in weight loss programs, accumulating around 10,000 steps with a portion at a moderate pace was associated with greater weight loss.3 sources · 2 min
- HealthStrength, Conditioning, and LongevityMuscle-strengthening activities for 30 to 60 minutes per week and higher cardiorespiratory fitness are associated with lower mortality and fewer chronic diseases.2 sources · 2 min
- MeasurementLimitations of Smartwatches and WearablesSmartwatches help monitor trends, but energy expenditure and other metrics can carry error and vary by device and activity.1 source · 2 min
- TrainingLoad and rep rangeLight and heavy loads produce similar hypertrophy when sets are taken close to failure; heavy loads are superior for maximizing strength gains.3 sources · 2 min
- TrainingRepetition Tempo, Range of Motion, and Exercise OrderFull range of motion outperforms partial range for lower-body strength and hypertrophy; repetition tempo matters little within broad boundaries; starting with compound exercises maximizes strength gains in them.3 sources · 2 min
- TrainingRecent Workload as a Signal, Not a BlockerThe acute-to-chronic workload ratio has statistical flaws and does not predict injury reliably enough to halt training; it serves as one signal among others.2 sources · 2 min
- TrainingThe Role of Exercise in Fat LossExercise alone produces modest weight loss; combined with diet, it preserves lean mass and improves weight maintenance. Resistance training prevents nearly all muscle loss caused by a calorie deficit.3 sources · 2 min
- NutritionCaffeine and PerformanceCaffeine at doses of 3 to 6 mg/kg improves aerobic performance, strength, and muscular endurance; higher doses do not help and increase adverse effects.1 source · 2 min
- NutritionPlanned Diet BreaksInterspersing periods of calorie deficit with maintenance weeks can improve fat loss efficiency and reduce hunger, yielding a similar final outcome.2 sources · 2 min
- BehaviourDigital Personalization and BehaviorPersonalized mobile interventions show potential benefits for health behaviors, although outcomes vary across programs.1 source · 2 min
- TrainingExercise Intensity via the Talk TestThe point where speaking in full sentences becomes difficult coincides with the first ventilatory threshold; conversing comfortably marks light effort, without needing a watch or chest strap.1 source · 2 min
- TrainingResistance training in older adultsAdults aged 50 and older, including the oldest-old, gain strength and muscle mass with progressive resistance training.2 sources · 2 min
- TrainingMenstrual Cycle and PerformancePerformance may be slightly lower during the early follicular phase, but the effect is trivial and highly variable; the recommended approach is individualized, not a rigid phase-based rule.1 source · 2 min
- TrainingPhysical Activity Guidelines for AdultsAdults should accumulate 150 to 300 minutes of moderate-intensity activity per week, or 75 to 150 minutes of vigorous-intensity activity, along with muscle-strengthening activities on two or more days.2 sources · 2 min
- NutritionRate of weight lossLosing weight slowly, at around 0.5 to 1% of body weight per week, better preserves lean mass, strength, and metabolic rate compared to rapid weight loss.2 sources · 2 min
- RecoveryTraining load does not predict individual injuryRecent training load can provide context for workout decisions, but simple load ratios do not support reliable individual injury prediction.1 source · 2 min
- NutritionUltra-Processed Foods Lead to Eating MoreWhen meals were matched for macronutrients, an ultra-processed diet led participants to eat roughly 500 kcal more per day and gain weight.1 source · 2 min
- NutritionFiber and Carbohydrate QualityConsuming 25 to 29 g of fiber per day, alongside whole grains, is associated with lower mortality and fewer cardiometabolic diseases.1 source · 2 min
- NutritionCalorie surplus for muscle gainA modest surplus supports hypertrophy; large surpluses primarily increase fat gain. Beginners and individuals with higher body fat can build muscle even without a surplus.2 sources · 2 min
- RecoveryHow Much to SleepAdults should sleep 7 to 9 hours per night; sleeping less was associated with more injuries in young athletes.2 sources · 2 min
- HealthSedentary TimeAny intensity of physical activity, including light intensity, is associated with lower mortality, while more sedentary time is linked to higher risk, following a non-linear dose-response relationship.1 source · 2 min
- TrainingFrequency: training each muscle at least twice per weekTraining a muscle group twice a week is superior to once for hypertrophy; for strength, higher frequency helps primarily because it distributes volume more effectively.2 sources · 2 min
- TrainingRest Between SetsRest intervals of two minutes or more favor strength and hypertrophy in compound exercises; short rest intervals work well when time is constrained.2 sources · 2 min
- NutritionProtein and Training AdaptationSufficient protein can support gains in lean mass and strength alongside training; the response varies and does not increase indefinitely with higher intakes.2 sources · 2 min
- NutritionHigher Protein Intake During a Calorie DeficitIn a deficit, higher protein protects lean mass and can even allow muscle gain alongside intense training; the effective range rises to approximately 1.6 to 2.4 g/kg.4 sources · 2 min
- NutritionDistribute protein throughout the dayDividing protein into servings of around 0.4 g/kg across four or more meals stimulates protein synthesis more than concentrating everything into one or two.3 sources · 2 min
- NutritionDiet Type Matters Less Than AdherencePopular named diets lead to similar weight loss; adherence to the plan predicts outcomes far more than the ratio of carbohydrates to fat.3 sources · 2 min
- NutritionWeight Loss Maintenance and Metabolic AdaptationEnergy expenditure drops more than expected after weight loss and appetite increases; maintaining weight requires continuous monitoring, high physical activity levels, and a consistent eating pattern.3 sources · 2 min
- RecoveryMonitoring recoveryRecovery should be tracked using simple, frequent measures such as perceived exertion, sleep, and mood; heart rate variability works better as a weekly trend than as a single daily figure.2 sources · 2 min
- TrainingInterval vs. Continuous TrainingHIIT and moderate continuous training produce similar body composition improvements; interval training improves VO2max more and requires less time.3 sources · 2 min
- NutritionNutrient timing and the anabolic windowTotal daily protein intake matters more than exact timing; the so-called anabolic window is broad, and consuming protein within a few hours around training is sufficient.3 sources · 2 min
- NutritionCreatine monohydrateCreatine monohydrate is the supplement with the strongest evidence for improving performance in high-intensity efforts and supporting lean mass gains with training, with a favorable safety profile.1 source · 2 min
- NutritionIsolated BCAAs Do Not Build MuscleBranched-chain amino acids on their own do not sustain muscle protein synthesis; whole dietary protein fills this role.1 source · 2 min
- TrainingRun-Walk MethodAlternating running and walking yields the same results with less muscle soreness and fatigue in recreational runners.1 source · 2 min
- TrainingWarm-up and stretchingWarming up improves performance in most cases; prolonged static stretching before training can reduce strength and power, whereas dynamic stretching does not impair performance.2 sources · 2 min
- TrainingTraining response varies widely between individualsFollowing the same program, gains in strength and muscle mass range from negligible to substantial; study averages do not predict an individual's response.1 source · 2 min
- NutritionProtein in older adultsOlder adults need more protein per meal to stimulate muscle synthesis; consensus guidelines recommend 1.0 to 1.2 g/kg/day or more for those who train.2 sources · 2 min
- BehaviourBuilding a workout habitIn new gym members, working out at least four times per week for six weeks was the minimum threshold to form a habit; consistency and simplicity predicted habit formation.1 source · 2 min
- TrainingHow Much to Increase Running MileageThe 10% weekly rule did not hold up; volume-related injury risk rose in runners who increased weekly distance by more than 30%.1 source · 2 min
- TrainingResistance training for injury preventionStrengthening programs substantially reduced sports injuries; stretching alone showed no such effect.1 source · 2 min
- NutritionSatiety: Protein and Eating SpeedHigher-protein diets reduce appetite and spontaneous intake; eating more slowly reduces the amount consumed in a meal.2 sources · 2 min
- NutritionAlcohol and muscle recoveryAlcohol after training reduced muscle protein synthesis even when protein was ingested alongside it.1 source · 2 min
- TrainingInterval walkingAlternating 3 minutes of brisk walking with 3 minutes at an easy pace improves aerobic fitness and blood pressure more than continuous walking for the same total duration.2 sources · 2 min
- TrainingBreaks and detrainingStopping training for a few weeks progressively reduces strength and power; short breaks carry little cost, and what is lost returns once training resumes.1 source · 2 min
- NutritionWeight change is dynamicThe body's weight response to sustained changes in energy intake is gradual and involves adaptations; projections should use curves and ranges rather than a rigid linear rule.3 sources · 2 min
- BehaviourTracking is the most effective techniqueSelf-monitoring of diet, weight, and activity is consistently associated with greater weight loss; the behavior change technique taxonomy ranks it among the most effective.2 sources · 2 min
- TrainingShorter, quicker stepsIncreasing steps per minute by 5 to 10% reduces load on the knee and hip; there is no universal optimal cadence.1 source · 2 min
- NutritionHydration and ExerciseDehydration exceeding approximately 2% of body weight impairs performance; fluid requirements are individual and depend on sweat rate, climate, and duration.1 source · 2 min
- NutritionEstimating energy expenditureThe Mifflin-St Jeor equation is the most accurate formula for estimating resting energy expenditure, but it still errs in a significant portion of individuals.2 sources · 2 min
- TrainingSession Training Load from Perceived ExertionDuration multiplied by the effort reported at the end of a session closely tracks training load measured by heart rate across any modality.1 source · 2 min