Fusing Body Composition and Activity into Biological Age

1. What is biological age?

Chronological age is the number of birthdays you have had. Biological age is an estimate of how old your body looks on the inside relative to the average person of the same chronological age. Two people who turn 47 on the same day can have very different biological ages: one may have the body of someone in their early forties, the other may already show changes typical of someone in their fifties.

Biological age is not a single, universally agreed measurement. It is an estimate produced by a model, and different models look at different signals. Petal focuses on signals you can measure at home, without needing a clinic visit or a blood test.

2. Signals used by Petal

Petal measures three signals unobtrusively and quietly in the background: your body composition, your resting heart rate (RHR) and your movement.

Body composition. Petal’s safe medical-grade sensors and low-power electronics analyze bioelectrical impedance (BIA) - resistance that tissue offers to small, micro-ampere currents - with personalized and custom-developed models to estimate trunk fat or midsection fat that is stored around the abdomen and chest. Midsection fat tends to accumulate with age, particularly after the perimenopausal transition, and is more closely linked to metabolic health than total body weight or the commonly used body mass index (BMI). 

Resting heart rate. Petal uses the same BIA sensors to measure miniscule electrical signals that are generated during heart beats and estimates your resting heart rate, a simple marker of cardiac fitness. A younger, well-conditioned heart tends to show a lower resting rate for its age.

Daily activity. The number of steps you take on a typical day is a simple, well-studied marker of long-term health. It is compared against how much people of your age usually walk. Petal’s 3-axis accelerometer of inertial motion unit (IMU) allows step counting.

For all of these, Petal compares your values to age-matched averages for women drawn from the US National Health and Nutrition Examination Survey (NHANES). Lower-than-average trunk fat, a healthier resting heart rate, or more daily steps than your peers all point to a younger-looking result; the opposite points the other way.

3. Petal estimates your Biological Age (BioAge)

Petal turns these signals into three related numbers.

Body-age — from your body composition (BIA scan) and resting heart rate (ECG). It reflects how your body is made up and how your heart is doing, compared with people your age. This is representative of gradual changes to your body and wellness due to lifestyle and diet improvements

Step-age — from your daily activity. Consistently walking more than the typical person your age reads younger; walking less reads older. This is representative of short-term improvements that you can achieve by being mindful of your sedentary lifestyle and being more active.

BioAge — a single combined number that brings your body age and your step age together into one overall age, capturing both what your body is made of and how active you are. This is the headline number Petal shows.

4. How the calculation works

Each Petal age compares your signals against age-matched norms and expresses the result as an age. For body age, your body composition and resting heart rate are compared with what is typical for women of different ages. For step age, your daily steps are compared with the typical walker of your age group, using an established relationship between activity and long-term health (Paluch et al., 2022). The fused age combines the two. Each result is expressed as your chronological age plus or minus a bounded adjustment (max. 7 years) so that  your BioAge can track your wellbeing.

5. How each input affects the result

The chart below shows how the estimate changes when one input is varied while the others are held at a representative reference point (a woman of age 47). Where a line rises above the dashed reference, the estimate reads older; where it falls below, it reads younger.

Figure 1. How the two main signals shift the estimated age. Higher trunk fat reads older; more daily steps read younger. The dashed line marks the reference age.

A few practical observations emerge. Body composition is the strongest driver of body age — higher trunk fat pulls the estimate upward. Walking more than your age group pulls step age downward. A lower resting heart rate nudges it down more subtly. These directions match what is known about healthy aging.

6. How we know it is trustworthy

A biological-age number is only useful if it tracks something real. We checked BioAge against the heart age from QRISK-lifetime — the cardiovascular model behind the UK NHS "Calculate your heart age" tool, developed by the Joint British Societies (JBS3). It expresses cardiovascular risk as a heart age: a heart age above your real age means your risk resembles that of an older person. QRISK-lifetime is an independent, clinically established model, so close agreement with it is meaningful evidence that Petal is measuring genuine, health-relevant aging rather than an arbitrary score.

Across our tested users, Petal body age and the QRISK heart age agree very closely — a correlation of 0.97, with a typical difference of about 1.6 years (n = 52 women aged 30 and over). Two independently built models arrive at essentially the same number.

Figure 2. Petal body age plotted against the clinical QRISK heart age for tested users. Points cluster along the diagonal (equal-value) line — correlation 0.97.

7. References

  1. Paluch AE, Bajpai S, Bassett DR, et al. Daily steps and all-cause and cardiovascular mortality: a meta-analysis of 15 international cohorts. The Lancet Public Health 2022; 7(3):e219-e228.
  2. QRISK-lifetime cardiovascular model, © ClinRisk Ltd (GPLv3); heart age as implemented in the JBS3 calculator (Joint British Societies 3 / British Cardiovascular Society). Underlying QRISK methodology: Hippisley-Cox J et al.

  3. National Center for Health Statistics. National Health and Nutrition Examination Survey (NHANES), cycles 2005-2006 and 2011-2018. Centers for Disease Control and Prevention.  https://wwwn.cdc.gov/nchs/nhanes/