Lean body mass is everything in your body that isn't fat: muscle, bone, organs and water (Holmes and Racette, 2021). It matters because muscle and strength fall with age, and strength is closely tied to health. Home scales only estimate it. In one study, three smart scales misjudged muscle by a median of 4.0 to 6.6 kg (about 9 to 15 lb) (Frija-Masson et al., 2021).
Key takeaways
Lean mass includes muscle, bone, organs and water, and lean tissue is about 73% water (Holmes and Racette, 2021).
In adults aged 70 to 79, leg lean mass fell about 1% a year, and strength fell faster (Goodpaster et al., 2006; Delmonico et al., 2009).
Strength training two to three times a week for about 20 weeks added about 1.1 kg (2.4 lb) of lean mass in adults aged 50 to 83 (Peterson et al., 2011).
Smart scales are best for trends measured the same way each time, not single readings (Siedler et al., 2023).
What's lean body mass?
It's everything in you that isn't fat. Researchers often call it fat-free mass: muscles, bones, organs, ligaments, tendons and water. Lean tissue is about 73% water, which is why your reading can move with what you drink and eat (Holmes and Racette, 2021).
So lean mass isn't the same as muscle. A drop on a scale can be water, not muscle loss.
How is lean mass measured?
With a scan in a clinic, or an estimate at home. A DXA scan, a low-dose X-ray, measures bone, fat and the lean tissue that isn't bone. Bioimpedance, used in some home scales, estimates lean mass from how a small current passes through your body water. That makes it very sensitive to hydration (Holmes and Racette, 2021).
Apple's list of what iPhone and Apple Watch record doesn't include lean mass. Apple Health stores it when a connected app or device, like a scale, adds it, or when you enter it by hand (Apple).
How accurate are smart scales?
Not very, for a single reading:
Study | What it tested | What it found |
|---|---|---|
Three smart scales against DXA scans in hospital patients | Weight close; muscle off by a median of 4.0 to 6.6 kg (about 9 to 15 lb) | |
15 bioimpedance devices against a more detailed method, in healthy adults aged 19 to 50 | Body fat from 3.5 points too low to 11.7 too high; each device consistent with itself | |
Readings before and after a day's eating and drinking | Fat-free mass up about 2% by the afternoon, up to 4.5% in some people |
In the second study, 9 of the 15 devices tracked average changes over 12 to 16 weeks within about 1 point (Siedler et al., 2023). The researchers suggest measuring in the morning, after an overnight fast and using the bathroom. Wear minimal clothing, use the same device, and average several days.
Like your weight, a lean mass reading swings from day to day (reading weight trends without the noise).
Why does lean mass matter?
More muscle and more strength are both linked with a lower risk of dying early as people age. One study followed older adults. Those with the most muscle had about a 20% lower risk of dying during the study than those with the least (Srikanthan and Karlamangla, 2014).
Strength may matter even more: losing strength is a more consistent warning sign for disability and death than losing muscle (Mitchell et al., 2012). Across nearly 140,000 adults aged 35 to 70 in 17 countries, each 5 kg (11 lb) lower grip strength was linked to a 16% higher risk of dying (Leong et al., 2015). These studies show links, not proof.
European experts now define age-related muscle loss first by low strength, then confirm it with low muscle amount or quality. Several of the authors report fees from nutrition and drug companies (Cruz-Jentoft et al., 2019).
How fast does lean mass drop with age?
Slowly at first. Studies comparing younger and older adults put average muscle loss at under half a percent a year. By about age 75, studies that followed people over time found muscle shrinking about 0.6% to 1% a year, while strength fell 2.5% to 4% a year (Mitchell et al., 2012).
In adults aged 70 to 79, leg lean mass fell about 1% a year and leg strength 2.6% to 4.1% a year (Goodpaster et al., 2006; Delmonico et al., 2009). People whose weight held steady still lost thigh muscle and gained fat inside their muscles. Gaining weight didn't stop strength loss (Delmonico et al., 2009).
How do you build or keep lean mass?
Regular strength training, with enough protein, is how the studies below added lean mass.
Strength training. A review of 49 studies looked at adults aged 50 to 83. Resistance training two to three times a week for about 20 weeks added 1.1 kg (2.4 lb) of lean mass on average (Peterson et al., 2011). The World Health Organization suggests muscle-strengthening activity on two or more days a week. For adults 65 and older, it adds balance and strength activities on three or more days (WHO, 2020). That advice includes people living with chronic conditions or disabilities, so the right exercises are the ones that fit your body. There's more in cardio vs strength.
Protein. In trials of healthy adults doing strength training, protein supplements added about 0.3 kg (0.7 lb) of fat-free mass on average. Gains leveled off around 1.6 grams per kilogram a day, and the benefit shrank with age. Two authors reported industry ties, one to a dairy council and one to a supplement maker (Morton et al., 2018). For healthy older adults, experts suggest at least 1.0 to 1.2 grams per kilogram a day (Deutz et al., 2014). See how much protein you need.
How do lean mass, BMI and body fat fit together?
BMI uses only your height and weight, so it can't tell muscle from fat (Holmes and Racette, 2021; BMI's limits). Body fat percentage and lean mass split your weight into fat and everything else, though a home scale only estimates both (body fat percentage). Water moves all of them from day to day (muscle, water and fat).
Who should be careful?
Check with your care team first if you have an implanted electronic device, or have kidney disease and are changing your protein.
Implanted devices: bioimpedance is considered unsafe with implanted electronic devices (Holmes and Racette, 2021). In one small study, the body-composition features of a smartwatch and a smart scale interfered with 5 of 27 people's pacemakers or defibrillators, with the implants set to maximum sensitivity (Hansen et al., 2025).
Kidney disease: ask your doctor or dietitian how much protein is right for you. Some people with kidney disease need a moderate amount, while people on dialysis are encouraged to get enough (NIDDK; NIDDK).
Frequently asked questions
Is lean mass the same as muscle? No. It also includes bone, organs and water (Holmes and Racette, 2021).
Why did my lean mass change overnight? Usually water or food, not muscle. In one trial, a day's eating and drinking raised bioimpedance fat-free mass readings about 2% by the afternoon (Tinsley et al., 2017).
Can a scale tell me if I have muscle loss? No. Doctors test strength first, then confirm with measurements (Cruz-Jentoft et al., 2019).
How Knit shows your lean mass
When a connected scale or app records lean body mass in Apple Health, Knit shows it in the Record tab, under Body & metabolic. Watching the trend over weeks, measured the same way, tells you more than any single day (why Knit compares you to you).
