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Methodology

PK MODEL v2 · UPDATED SEPTEMBER 8, 2026 · EDUCATION, NOT MEDICAL ADVICE

Every number in Bild Health is either something you entered, something a device measured, or something we computed from those two. This page covers the third kind: what we compute, the exact model, the parameters, where they come from, and where they break down.

We publish this for one reason: an estimate you can't audit is a number you shouldn't trust.

Words we use. *Fat-free mass* is everything that is not fat, bone included; it is what a body-composition scale reports as lean and what our charts plot. *Lean soft tissue* is fat-free mass without the bone mineral, and it is what most DEXA reports print as lean. On our screens the single word *lean mass* currently covers both, and the Body composition section below says which quantity each screen shows. *Fat mass* is chemical fat. None of these is a measurement of skeletal muscle.

Estimated medication levels

What we chart. The curve shows the estimated amount of drug in your body, in milligrams, over time. It is not a blood concentration and not a lab value. We chart amount rather than concentration deliberately: concentration would require your apparent volume of distribution — a parameter we cannot know for you — while amount depends only on bioavailability and the well-published half-life and time-to-peak. It's the one form of this number we can state honestly.

The model. One-compartment, first-order absorption and elimination, summed over your logged doses by superposition. For each dose *D* taken *t* hours ago:

amount(t) = Σ F · D · ka/(ka − ke) · (e^(−ke·t) − e^(−ka·t))

where ke = ln2 / half-life is how fast the drug leaves, and ka — how fast it's absorbed — is solved from the published time-to-peak by inverting Tmax = ln(ka/ke) / (ka − ke). Each dose gets its own rise-and-fall curve; the chart is their sum. That's the whole model: no machine learning, and nothing tuned to you beyond the doses, dates, and schedule you entered.

Two details, since we said nothing is hidden. ka is solved numerically and held at least 5% above ke, because the formula divides by the gap between them and would blow up if they met. And where any calculation on this page can produce an absurd number, we clamp it — every clamp is stated in its own section below.

Parameters, by ingredient and route — half-life (t½), representative time-to-peak (Tmax), and bioavailability (F), each read from the FDA label and the peer-reviewed pharmacokinetic literature cited below:

  • Tirzepatide (Mounjaro®, Zepbound®) — t½ 120 h · Tmax 24 h · F 0.80 · weekly
  • Semaglutide, injectable (Ozempic®, Wegovy®) — t½ 160 h · Tmax 48 h · F 0.89 · weekly
  • Dulaglutide (Trulicity®) — t½ 120 h · Tmax 48 h · F 0.55 · weekly
  • Liraglutide (Saxenda®, Victoza®) — t½ 13 h · Tmax 11 h · F 0.55 · daily
  • Retatrutide (investigational; no approved product) — t½ 144 h · Tmax 48 h · F 0.80 · weekly. There is no label to read, so these come from the published phase 2 pharmacokinetics cited below; the app lists it as investigational and models it the same way as the others.
  • Orforglipron (Foundayo®) — t½ 40 h · Tmax 6 h · F 0.30 · daily
  • Semaglutide, oral (Rybelsus®, Wegovy® Pill) — t½ 160 h · Tmax 1 h · F ~0.008 · daily

Oral semaglutide is the one exception to the milligram curve. Its absorption varies so much dose to dose (within-subject variability around 137%) that a milligram number would be false precision. For oral semaglutide only, we show a 0–100% buildup envelope against the steady-state plateau instead — the honest claim is "how built up you are", not "how many milligrams are in you". Every other medication above, including oral orforglipron, gets the milligram curve.

If you were already taking your medication when you joined. We back-fill from the start date and schedule you gave us at signup, in two ways. Your shot log is seeded with up to four weeks of assumed doses — they appear in your history exactly as if you had entered them, and you can edit or delete them like anything else. The level chart additionally draws an assumed lead-in from your stated start date, up to 20 weeks back — long enough to reach steady state for every medication we model — so the curve begins at your accumulated level rather than at zero. Both assume you followed the schedule you reported; if you didn't, correct your log and the curve follows.

Covariates we deliberately don't apply. We do not adjust the curve for your weight, age, or sex. The labels for these drugs state that no dose adjustment is required for those factors, and a population PK analysis of dulaglutide found age, body weight, sex, race and ethnicity did not influence its kinetics to any clinically relevant degree (Geiser et al., 2016). Inventing a personalization the evidence doesn't support would make the curve feel more precise while making it less true.

What the curve is not

  • It is not a measurement of you. It is a population-average model. Real people vary widely around it — two people on the same dose can differ substantially in actual exposure. Your true levels are not knowable from an app.
  • There is no target level to hit. These medications have no established therapeutic concentration range to aim for. A curve that runs higher or lower than someone else's means nothing about whether your protocol is working. Nothing in the app is a threshold, and no number here is a goal.
  • It is not dosing guidance. Bild Health does not calculate doses, recommend doses, suggest dose changes, or tell you when to inject. Never start, stop, delay, split, or change a dose based on this chart. Dose and timing decisions belong to you and your prescriber.
  • It cannot tell you when the drug has cleared. We deliberately do not compute washout, clearance dates, or missed-dose catch-up. This matters most before surgery, anesthesia, or any procedure with sedation: GLP-1 medications carry labeled warnings about pulmonary aspiration under anesthesia, and that decision requires your surgeon and anesthesiologist — who need to know you take this medication regardless of what any chart says. Never use Bild Health to decide it's safe to proceed.
  • It assumes your log is the truth. The curve is built from the doses you recorded, on the dates you recorded them, at the strengths you entered. A missed entry, a typo, or a dose logged on the wrong day produces a confidently wrong chart. Compounded products add further uncertainty: their actual delivered strength is outside anything we can model.
  • It is not a medical device. Bild Health does not diagnose, treat, cure, or prevent any condition, and it is not a substitute for professional medical care. Education, not medical advice — consult your prescriber.

Protein target

Once you adopt a body-composition goal, your protein target comes from your fat-free mass, the tissue the protein exists to protect. That basis comes straight from the muscle-retention literature for people in a caloric deficit: requirements track lean mass, not scale weight, and body-weight formulas over-prescribe for body fat, which needs almost none (Helms et al., 2014: 2.3–3.1 g/kg of fat-free mass while cutting; ISSN position stand, Jäger et al., 2017; Morton et al., 2018).

protein (g/day) = c × fat-free mass (kg)

The coefficient *c* is 2.4 in fat loss mode and 2.0 in maintaining mode. The deficit is what raises the requirement, and only you know whether you are still in one, so the mode is yours to set (Goals → Mode). The app asks when your measured body fat has held at your goal for a week, meaning five of the last seven readings, and asks the other way when it has sat a full point or more above your goal. It is the same *c* for everyone: sex differences ride in the fat-free mass itself, not in a different multiplier.

Floors, stated: never below 60 g for women, 80 g for men (60 g when we don't know) — the bariatric-medicine absolute minimums for appetite-suppressed patients. The published number is rounded to the nearest 5 g; ties round up.

The number holds steady on purpose: we restate it only when the underlying target moves by at least 6 g, at most every 60 days — sooner only when a new DEXA scan moves the underlying numbers or you edit your goals. And if you hand-set a higher floor in Goals, the stated number never drops below it: the rule can raise your target, never undercut your own.

Before you adopt a goal, an interim target stands, computed from adjusted body weight — goal weight + 0.25 × (current weight − goal weight) — at 1.5 g/kg (1.8 g/kg if you're lifting), plus 0.1 g/kg from age 60, rounded to 5 g and held between the per-sex floor (80 g men, 60 g women, 60 g when sex is unknown — the same absolute floors as the adopted rule) and 240 g. Until you set a goal weight it equals your current weight, so set yours (Settings → Height & Goal Weight) and the interim target rebases immediately. Standard clinical "adjusted body weight" uses *ideal* weight; we use your *goal* weight because you chose it — that's our adaptation, not an established formula.

This is a nutrition target, not a prescription. If you have kidney disease or any condition that restricts protein, this number does not apply to you — ask your clinician for yours.

The same rule, with the method shown step by step: GLP-1 protein calculator.

Lifting target

Resistance training is the lever this app exists around: on a GLP-1 the weight comes off regardless, and lifting is what decides how much of it is fat rather than muscle. The floor is 2 sessions a week, which is where the major guidance lands for every adult — WHO's 2020 physical activity guidelines, the US Physical Activity Guidelines for Americans, and ACSM all recommend muscle-strengthening on at least 2 days a week, explicitly including adults 65 and over. Two is not a reduced target. It is the target.

Where you start depends on where you are. If you told us you're already lifting consistently, you start at 3. If you're not lifting yet or getting back into it, you start at 2 — and we offer the third session only after you've hit 2 in four of the last five weeks. That's roughly a month of established habit, which is deliberate: connective tissue adapts well behind muscle, and a caloric deficit with a suppressed appetite leaves less recovery capacity than the training literature's usual well-fed subjects.

We never raise it on a timer, and never without asking. A target that climbs on the calendar moves the bar away from the person already struggling to reach it. Yours changes when you accept the change, and you can set it anywhere from 2 to 7 yourself in Goals.

This is a general fitness target, not an exercise prescription. We don't know your joints, your heart, or your history. If you're new to resistance training, returning after a long gap, pregnant, or managing any cardiovascular or musculoskeletal condition, talk to your clinician about what's appropriate for you before you start.

Hydration target

Once you adopt a body-composition goal, your water target comes from your lean mass — the tissue that actually turns water over. The best available predictor of daily water turnover (Yamada et al., *Science*, 2022; ~5,600 people measured with doubly-labeled water) found that once fat-free mass is in the model, sex adds nothing — and that body *weight* over-prescribes for body fat, which barely uses water. So we don't scale by your weight, and we don't use the 30–35 mL/kg rules of thumb you'll see elsewhere.

fluids (mL) = 24.88 × fat-free mass (kg) + 1141

That line is our fit of the full turnover model at typical US climate and activity, in a caloric deficit: both halves of the line, slope and intercept, are struck after subtracting the food water you're no longer getting on a suppressed appetite, which is exactly why a GLP-1 raises the amount you need to *drink* even as total water need falls.

Then: never below 2.0 L (men) / 1.6 L (women) — clinical minimum drinks intakes (ESPEN) — never above 4 L, and rounded UP to the next 100 mL or fluid ounce, so the published number never under-states the target.

The number holds steady on purpose: we restate it only when the underlying target moves by at least 100 mL, at most every 60 days — sooner only when a new DEXA scan moves the underlying numbers or you edit your goals. Small day-to-day drift is noise, not a new prescription.

Before you adopt a goal, a simpler interim target stands: the NASEM/IOM reference intake (3.7 L men / 2.7 L women, treated as a beverage target given reduced food water) plus a small allowance for a high-protein intake, held between 2 and 5 litres.

Hydration gets its own protocol on a GLP-1 for a specific reason: appetite suppression cuts the food-water you'd normally get, GI side effects add losses, and these drugs blunt thirst — so the usual "drink when thirsty" signal is least reliable exactly when it matters most. This is general wellness information, not treatment for any condition.

The same rule, worked through: GLP-1 water calculator.

Goal weight

Earlier versions of Bild Health suggested a goal-weight range during onboarding. The current app does not: your goal weight is a number you set yourself, in Settings → Height & Goal Weight, and the app computes from it without second-guessing it. We removed the suggestion rather than ship one we weren't confident in; the age-adjusted BMI guardrails built for it are retained in our test suite and may return in a future version.

Because the app does not gate what you enter, it will accept a goal weight it would never recommend. A too-low goal weight quietly distorts your other targets: the interim protein target that stands before you adopt a goal is computed from it. Once you adopt a goal your protein comes from your fat-free mass and the mode you set, and your goal weight no longer steers it. Set it with your prescriber, not against them.

When you adopt a body-composition goal, the app also shows a projected weight — your target fat mass plus your measured lean mass and planned lean change. That is a projection for orientation, not a target; it can even rise while your body fat falls.

The right weight for you depends on things this app cannot see. Your prescriber sets your goals.

Body composition

Weight, body fat percentage, and lean mass are normally your measurements, not ours: values you entered, values read from Apple Health with your permission, or DEXA results you imported. We display them, chart them, and compute the changes between them. Their accuracy is the accuracy of whatever produced them — and the methods are not interchangeable. DEXA, bioimpedance scales, and Apple Health sources will disagree with each other, and comparing across them shows apparent changes that aren't real. Where we chart a trend, we're charting your measurements, not a model of you.

One consequence of not asking where a number came from: a body-fat percentage you type by hand is treated like a scale reading. If you have DEXA results, import them through DEXA Scans rather than typing the number, and the app will treat them with DEXA-grade confidence.

Which quantity each screen shows. The Results lean-mass chart plots fat-free mass: a body-composition scale's reading through Apple Health (labelled Lean Body Mass there) and, for each imported DEXA scan, the scan's lean soft tissue plus its bone mineral content, so that scan markers and scale readings count the same thing. The DEXA Scans list and the health report show each scan's lean soft tissue exactly as the report printed it. The lean-mass floor and ALMI use the scan's appendicular lean soft tissue, arms and legs, when the report carries regional data. All three appear under the words lean mass today.

The one thing we do estimate. If you have no body-fat reading, we estimate your lean mass from your height, weight, age and sex with a formula fit to 5,020 whole-body DEXA scans from the US national health survey (NHANES 2015–2018):

lean mass (kg) = 9270 × weight (kg) ÷ (A + B × BMI + C × age)

with the constants published in our methods paper. It is accurate to about ±5 points of body fat, which is too loose to show as a bare number, so the app never shows an estimated body-fat percentage; our web calculators print it only with that ±5 attached. The estimate does two jobs only: it sizes your protein and water targets, and at a BMI of 25 or above it tells you the direction the reference bands give (fat is the lever). Below 25 it asks for a measurement. The formula was fit on ages 20 to 59 and is held at that range's edge outside it. A smart scale or a DEXA scan unlocks the numbers. One caution when you compare: a clinic's DEXA report usually reads 3 to 4 points lower than the survey scans this estimate and our bands are built on, because clinic software skips a lean-mass correction the survey applies. That gap is method, not your body.

Body composition goals

If you adopt a body-composition goal, the target comes from a banded lookup table, not a formula fitted to you. For your sex, age band (under 40, 40–54, 55 and over), and objective (longevity or performance), we look up a healthy body-fat range drawn from population reference data — for example, a man of 45 with a longevity objective gets 15–20% — and set your target at the band's midpoint. The reference data are the age- and sex-specific healthy body-fat ranges of Gallagher and colleagues (2000) and the population cutoffs of Heo and colleagues (2012), both listed below; the bands are our own construction from them, and how the ranges become bands is shown on the body fat calculator. From age 50 we *recommend* the longevity bands — past 50, chasing performance-lean numbers costs muscle you should keep — but the objective you picked stands; nothing rewrites your choice.

target fat mass = lean mass × t / (1 − t)

where *t* is your target body-fat fraction and lean mass is your measured fat-free mass — not a projection. Fat to lose is your measured fat mass minus that target, never shown below zero. The planned lean change across a protocol is a flat +2 kg; we do not yet scale it by your training history.

A lean-mass floor guards the other side. Your floor is your current lean mass, screened against reference minimums — an appendicular lean-mass index of 7.0 kg/m² for men and 5.5 for women (the EWGSOP2 cutoffs, listed below), with a fat-free-mass-index screen at 18 and 15. Screening low raises a flag and a suggestion to get a DEXA scan. It is never a diagnosis.

The clamps in this engine, since we said every clamp is stated: body-fat targets will not go below 8% for men or 15% for women; the goal editor tops out at 35%; and projections carry a margin that widens with your measurement source — about ±0.5 kg from a DEXA, about ±3 kg from a body-fat scale. Without a measurement you get a direction, not numbers, as described above.

The protein number this engine produces is the fat-free-mass rule described under Protein target above — c × fat-free mass, c of 2.4 in fat loss mode and 2.0 in maintaining mode, floored per sex. Once you adopt a goal, that rule is the protein target you see, kept current by the same restatement policy.

Alongside lifting, the Today screen's cardio row defaults to 3 sessions a week, in line with general aerobic-activity guidance of 150 minutes of moderate activity spread across the week. It is a general fitness default, not an exercise prescription.

Sources

FDA labels (primary sources for half-life, Tmax, bioavailability, and dose ladders; the revision we read is named):

Peer-reviewed pharmacokinetics:

Body composition, protein, hydration, and goal weight:

Changes to this page

Model parameters are re-verified against current FDA labels when labels change and when we add a medication. If you believe a method or parameter is incorrect, tell us and show your source — we'd rather be corrected than incorrect: methodology@bildhealth.com.

September 6, 2026: the body-fat estimate paragraph notes that our web calculators print the estimate with its ±5 attached, and adds a caution that a clinic's DEXA report reads 3 to 4 points lower than the survey scans the estimate and the bands are built on.

September 5, 2026: the body-fat estimator changed. Where you have no body-fat reading, the app now estimates your lean mass with a formula fit to 5,020 whole-body DEXA scans from NHANES 2015–2018, in place of the Deurenberg equation, and it no longer shows an estimated body-fat percentage anywhere. Your protein and water targets are sized from the estimate; the direction comes from BMI; a measurement is what unlocks the numbers. Targets computed from the old estimate may have moved.

September 3, 2026: no formula or constant changed. Two things the app already did are now stated here: the interim protein rule adds 0.1 g/kg from age 60, and the investigational retatrutide profile is listed with its parameters. The label revisions under Sources were brought current, and the sources for the body-fat bands and the lean-mass floors were added.