DEXA and body composition for GLP-1 users: a complete guide

Weight loss on GLP-1 medications is usually rapid and large compared to lifestyle alone, and that is exactly why composition matters. Published trials and reviews of semaglutide and tirzepatide report that a meaningful share of weight lost can be lean mass, in some analyses up to roughly 40 percent without intervention. The percentage varies by age, training status, protein intake, and how aggressively the deficit runs, but the direction is clear: people who do nothing extra tend to lose meaningful muscle along with fat.

Bone is a second consideration. Rapid weight loss in any form, including bariatric surgery and dietary weight loss, is associated with bone density decline. The signal in GLP-1 cohorts is still being studied, but the same biology applies: less mechanical loading, lower calcium and vitamin D intake during low-appetite stretches, and changes in bone remodeling can shift T-scores over months. The USPSTF guidance on osteoporosis screening is a useful reference for who already qualifies for routine DEXA.

The upside is that both losses are largely addressable through habits. Adequate protein, resistance training two to four times a week, and a moderate rather than aggressive deficit help protect lean mass and bone. The only way to know whether those habits are working is to measure and compare over time. Talk to your doctor about whether scanning makes sense for you.

DEXA is not the only option, just one of the most accurate. The other two methods worth knowing are Bod Pod and bioimpedance.

Bod Pod uses air displacement plethysmography. You sit in a small egg-shaped chamber for a few minutes and the device measures how much air your body displaces, then calculates body fat from density. Accuracy is close to DEXA for body fat and lean mass, but Bod Pod gives you nothing on bone density and no regional breakdown. Cost is similar, around 50 to 100 dollars in the US, with more limited availability.

Bioelectrical impedance analysis (BIA) is what smart scales and devices like InBody and Withings use. A small current passes through your body and the device estimates fat and lean from how the current is conducted. BIA is fast, cheap, and great for trend lines, but the absolute numbers shift with hydration, recent meals, and time of day. For tracking change in your own data over weeks, BIA is fine. For comparing to a clinical reference value, it is less reliable.

There is also Phaze's own Body Composition feature (Beta), which estimates body fat percent and muscle mass from a progress photo with a confidence level and a margin of error. It is a quick, free way to watch a trend between more precise measurements, not a replacement for them. A practical combination on a GLP-1 is a DEXA scan a couple of times a year as the reference, with a quick photo estimate or smart scale for the weeks in between, all charted in one place.

There is no formal medical guideline for DEXA frequency in GLP-1 users, because GLP-1 weight loss is newer than the screening guidance. A practical cadence many people borrow from clinical practice and weight-loss research:

During active weight loss, every 3 to 6 months. The 3-month interval is what many bariatric surgery programs use, and it captures meaningful change in lean mass and bone while still being affordable. A 6-month interval is enough if you are losing slowly or if cost is a concern.

During maintenance, every 6 to 12 months. Once weight is stable, composition still drifts with training, age, and protein intake, but the changes are slower. An annual scan is a reasonable minimum for anyone keeping weight off long-term on a maintenance dose or after stopping.

A baseline scan before your first GLP-1 dose is the highest-leverage one you can get. Without it, you do not know whether the lean mass you have at month 6 is what you started with. If you are already a few months into the medication, a scan now is still useful as a starting line for the rest of your journey. Your doctor can help you decide what cadence fits your situation.

Your DEXA report has more numbers than you need. Three groups matter most.

Body fat percent and total fat mass tell you the obvious story: how much of your body is fat, in absolute pounds and as a share. Healthy ranges are roughly 10 to 20 percent for men and 18 to 28 percent for women, with athletic ranges lower. Watch the absolute fat-mass number, not just the percent, because losing fat while keeping lean mass changes the percent even when the lean number is steady.

Lean mass is the value to watch most carefully on a GLP-1. Total lean mass should hold or rise scan over scan if you are training and eating enough protein. A drop of more than a few pounds between scans is worth raising with your doctor, especially if it pairs with low protein intake or no resistance work.

Bone density is reported as a T-score (compared to a healthy young adult) and a Z-score (compared to your age group). T-scores between -1.0 and +1.0 are typically considered normal, -1.0 to -2.5 is osteopenia, and below -2.5 is osteoporosis. A drift over a year on a GLP-1 is a trend worth bringing to your doctor, especially if you are postmenopausal or have other risk factors.

What to bring to your visit: the scan itself, your weight trend, your protein intake, and your training log. The numbers are easier to interpret in context.

Body composition does not change because you scanned it. The work is everything you do between scans. Four habits carry most of the load.

Protein is the first. Research on muscle preservation during weight loss commonly points to 1.2 to 1.6 grams per kilogram of body weight per day, or roughly 0.55 to 0.75 grams per pound, spread across 3 to 4 meals so each meal lands at 25 to 40 grams. On a GLP-1 with low appetite, protein is the macro many people prioritize. Those who undershoot protein often show a lean-mass drop on the next scan.

Resistance training is the second. Two to four sessions a week, hitting major muscle groups, with progressive overload (a little more weight or a few more reps over time). Cardio is useful for cardiovascular health, but it does not build or protect muscle the way lifting does. The training does not have to be heroic, it has to be consistent.

A moderate deficit is the third. Aggressive deficits accelerate weight loss but can also accelerate lean-mass loss. On a GLP-1, where appetite is already suppressed, the deficit can run deep without you noticing.

Sleep, calcium, and vitamin D are the fourth, supporting bone remodeling and recovery. None of this is exotic, and all of it shows up in the next scan if you do it consistently. For more on the protein side, see the protein on GLP-1 guide. This is general information, not a personalized plan; check with your doctor or a dietitian.

Body Composition in Phaze is a Beta feature, part of the Labs & Scans area. You can log body fat percent, muscle mass, lean mass, and fat mass by hand at any cadence, and watch each value as a trend over time. Manual entry is part of the free core.

Phaze also offers an AI photo estimate: take a progress photo and Phaze estimates your body fat percent and muscle mass, with a confidence level (Low, Medium, or High) and a roughly 3 to 5 percent margin of error. It is a quick way to track a trend between more precise measurements, not a clinical reading. The photo is processed with your consent.

If you have a DEXA report, Phaze Pro can import it. Attach the PDF your scan provider emailed you, or take a photo of the printed report, and Phaze reads the values off the page so you can review and save them in seconds. Your numbers then sit on a trend chart with your weight overlaid, so a step down in body fat next to a stable lean-mass line tells you the loss is mostly fat. For a closer look, see the Body Composition page.

Body composition data is encrypted on your device with AES-256-GCM in the Apple Keychain and stays local-first; it is never sold, shared, or used to train AI. Phaze keeps body composition alongside Lab Tracker and Taper Coach, so scans, weight, labs, and doses live in one private app. Phaze is a wellness and habit-tracking app, not a medical device.