Biological Age

What biological condition is your body in, relative to your calendar age?

How old are you? Most people will answer this question with their date of birth. Modern medicine, however, poses another question — what biological condition is the body in relative to its calendar age? At first glance these two questions seem similar, but in reality they describe different dimensions of aging.

Two people who are both 60 years old can differ considerably in metabolic health, physical capacity, muscle strength, cognitive function, burden of chronic disease, and ability to recover. One may be active, strong, and without significant limitations. The other may have multiple chronic diseases, reduced fitness, loss of muscle mass, and a markedly smaller physiological reserve. Differences like these lie at the heart of the concept of biological age.

What Is Chronological Age?

Chronological age is the time that has elapsed since birth. It is precise, simple to determine, and independent of the measurement method. On its own, however, it cannot fully describe current health status, functional capacity, physiological reserve, the presence of chronic disease, recovery speed, or an individual's risk of future health problems.

Chronological age is indeed a significant risk factor for many diseases, but it does not explain why people of the same age can have very different health statuses.

What Is Biological Age?

Biological age is an estimate of the extent of the changes that have developed over a lifetime in cells, tissues, and organs. It attempts to answer the question of whether the body functions better, similarly to, or worse than would be expected on average for a given chronological age.

Biological age is not a single, directly measurable parameter. It is a model derived by combining different types of data — clinical biomarkers, metabolic and cardiovascular health, physical capacity, muscle mass and strength, cognitive function, inflammatory markers, epigenetic and other molecular changes, the presence of disease, and functional limitations. For this reason, a test result stating that someone is "biologically 52 years old" should not be interpreted as an absolute fact — it is an estimate based on a specific algorithm, a chosen set of biomarkers, and a reference population.

A 60-year-old may have blood pressure, body composition, fitness, muscle strength, and a metabolic profile that are more favorable than the average for their age group. Another person of the same chronological age may have characteristics associated with a higher risk of disease and functional decline. This does not mean that all of the first person's organs correspond to those of an average 50-year-old — biological age is a statistical and biological estimate, not a new date of birth.

Why Do People Age at Different Rates?

The rate and pattern of aging result from a complex interaction of genetics, lifestyle, environment, disease, treatment, and random biological events.

Health during aging is also influenced by smoking, excessive alcohol use, air pollution, occupational exposures, UV radiation, and socioeconomic conditions. Healthy aging is therefore not solely a matter of an individual's biology — it also depends on the environment in which a person lives.

How Is Biological Age Assessed?

Science does not yet have a single, universal, and perfect test for biological age. Available methods measure different aspects of aging, which is why it is more useful to speak of a profile of biological and functional aging than of one final number.

1. Clinical and Phenotypic Clocks

These combine multiple routine parameters — blood pressure, glucose and HbA1c, lipid status, kidney and liver function, blood count, inflammatory markers, body composition, and the presence of chronic disease — to assess how far a person's profile deviates from the values expected for their chronological age. The advantage of this approach is practicality; its limitation is that the result depends on the population on which the algorithm was developed.

2. Epigenetic Clocks

These analyze DNA methylation patterns at selected sites in the genome. Newer models attempt to estimate outcomes related to health and disease risk, rather than chronological age alone. They represent an important research tool, but are not yet a universal standard for routine clinical decision-making — a change in the result does not automatically mean that a person has extended their life expectancy or reduced their risk of a specific disease.

3. Proteomic, Metabolomic, and Glycan Clocks

These analyze patterns of proteins, metabolites, lipids, and sugar structures attached to proteins. They can provide important information about changes in metabolism, inflammation, and organ function, but most of these tests are still primarily intended for research.

4. Functional Age

For the patient, what often matters most is not the age shown by a molecular clock, but what they can do in everyday life. Functional assessment can include grip strength, gait speed, the ability to rise from a chair, balance, aerobic capacity, mobility, cognitive function, and the ability to perform activities independently.

Which Indicators Have the Greatest Practical Value?

Grip strength is measured with a dynamometer — it is simple, quick, and associated with overall muscle strength and functional status, although the value depends on sex, body build, and measurement technique.

Gait speed reflects the combined functioning of muscles, joints, the nervous system, balance, vision, and the cardiovascular system. Tracking changes over time in the same person has particular value.

Cardiorespiratory fitness and VO₂max represent one of the strongest indicators of long-term cardiovascular and overall health risk — the American Heart Association has proposed that it be regarded as an important clinical vital sign.

Muscle mass and muscle quality — mass alone is not sufficient, since a person may have relatively preserved mass but reduced strength. It is best to combine an assessment of body composition with strength testing.

Body composition and visceral fat — body mass index does not distinguish between muscle and fat tissue; assessing waist circumference and visceral fat provides a better picture of metabolic risk.

Can Accelerated Aging Be Recognized by Symptoms?

Chronic fatigue, declining fitness, loss of muscle mass, poor sleep, slower recovery, and reduced concentration can point to impaired health or a diminished physiological reserve. None of these symptoms, however, is specific to accelerated biological aging — similar complaints can be associated with anemia, thyroid dysfunction, depression or anxiety, sleep apnea, cardiovascular or pulmonary disease, infection, medication side effects, nutrient deficiency, or hormonal changes.

Symptoms should therefore not be attributed to aging before an appropriate medical evaluation has been carried out. The first goal is not to calculate biological age, but to rule out a disease that requires diagnosis and treatment.

Where Do Genetic Tests Fit In?

A genetic test cannot reliably predict how long someone will live or how fast they will age. It can point to certain inherited variants associated with lipid and glucose metabolism, the inflammatory response, oxidative stress, drug metabolism, the risk of specific diseases, and the response to certain dietary components.

Its clinical value depends on the strength of the evidence for the specific variant and on whether the result changes a decision. Analyzing individual polymorphisms in genes such as MTHFR, COMT, CRP, or SOD2 cannot, on its own, determine biological age or justify a complex treatment protocol. Genetic results should be interpreted in the context of family history, clinical status, standard biomarkers, and lifestyle factors.

Functional Tests and Biological Age

Specialized functional tests can help answer certain clinical questions, but they do not represent a direct measurement of biological age. The DUTCH test can provide additional insight into the excretion and metabolism of steroid hormones, and DUTCH Plus into the daily rhythm of free cortisol as well — useful in carefully selected situations where the result can change a decision.

Microbiome analysis changes with age, diet, medications, and health status. Certain patterns are associated with health and frailty syndrome, but there is no single, universally "young" or "healthy" microbiome, so the result should be interpreted together with symptoms and a standard gastroenterological evaluation.

A specialized test has the greatest value when there is a clearly defined clinical question, the method has adequate analytical validity, the result is interpreted within a broader medical context, and the finding can change a concrete decision.

Can We Reduce Our Biological Age?

It depends on what we mean by biological age. Changes in body weight, blood glucose, blood pressure, fitness, or inflammatory markers can change the result of some biological clocks, but this does not automatically prove that the overall process of aging has been reversed. It is far more important to ask concrete questions: Has cardiovascular risk been reduced? Is blood pressure better controlled? Has muscle strength increased? Does the person move more easily? Has fitness improved? Is sleep better quality? Has independence been preserved? And has the risk of falls and disability been reduced?

The strongest evidence currently exists for interventions that are already available: regular physical activity (150–300 minutes of moderate aerobic activity per week plus strength training at least twice a week), preserving muscle mass, a high-quality diet modeled on the Mediterranean pattern, quality sleep, controlling known risk factors, and social connection.

What Is the Medicine of the Future Trying to Achieve?

Current research is not only trying to find yet another way to treat each individual disease — it is trying to understand the shared processes that precede multimorbidity, frailty syndrome, and the loss of independence. In the future, the assessment of biological aging will likely combine clinical data, molecular biomarkers, body composition, physical capacity, digital indicators, cognitive function, and changes over time.

The greatest value will not belong to the test that produces the most attractive number, but to the model that reliably predicts a clinically important outcome and helps select a safe and effective intervention. Until then, biological clocks should be regarded as promising supplementary tools, not as a final diagnosis.

Conclusion

We may not be able to change our date of birth. But we can influence a great many factors that determine how we will live, function, and age.

Biological age is one of the most important, yet also one of the most frequently misunderstood, concepts in longevity medicine. It does not denote the literal number of years of our body — it represents an attempt to estimate changes related to health, function, physiological reserve, and disease risk. There is no single test that can fully describe the way we age.

The most realistic picture is obtained by combining the medical history and clinical examination, standard laboratory parameters, metabolic and cardiovascular risk, body composition, muscle strength, gait speed, cardiorespiratory fitness, cognitive and everyday functioning, and carefully selected molecular tests. The goal is not simply a lower estimated biological age — the goal is a longer life with preserved health, function, and independence.

AFLM — From Data to a Responsible Clinical Decision

The Academy for Functional and Longevity Medicine develops training programs that help healthcare professionals critically evaluate and responsibly apply new biomarkers, genetic tests, and functional tests. The goal is not to collect as much data as possible, but to identify the information that can improve prevention, risk assessment, and individual clinical decisions.

Knowledge before technology.
The clinical question before the test.
The patient before the result.

Sources

• Moqri M, Herzog C, Poganik JR, et al. Biomarkers of Aging for the Identification and Evaluation of Longevity Interventions. Cell. 2023.
• López-Otín C, Blasco MA, Partridge L, Serrano M, Kroemer G. Hallmarks of Aging: An Expanding Universe. Cell. 2023.
• Warner B, Ratcliff J, Stovold E, et al. A Systematic Review of Phenotypic and Epigenetic Clocks Used to Measure Ageing. 2024.
• Ross R, Blair SN, Arena R, et al. Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign. Circulation. 2016.
• World Health Organization. Guidelines on Physical Activity and Sedentary Behaviour. Geneva: WHO; 2020.

Note: This content is educational in nature and does not constitute individual medical advice. Symptom assessment, laboratory and genetic testing, and the introduction of therapy or supplements require individual medical evaluation.