What Is Metabolic Age?
Metabolic age is a way of comparing how efficiently your body burns energy at rest — your Basal Metabolic Rate (BMR) — against the average BMR for people of different chronological ages. If your BMR is as high as the average for someone 10 years younger than you, your metabolic age is said to be 10 years younger than your actual age. Conversely, if your BMR is lower than average for your age, your metabolic age may be older.
The concept is used as a fitness and health indicator. A younger metabolic age typically correlates with higher muscle mass, lower body fat percentage, better cardiovascular health, and greater overall energy expenditure. It is not a medically certified diagnostic tool, but it provides a useful perspective on metabolic health beyond simple weight or BMI measurements.
The Mifflin-St Jeor Equation
Our calculator uses the Mifflin-St Jeor equation, currently considered the most accurate standard BMR formula for the general population. It was developed in 1990 by M. D. Mifflin and S. T. St Jeor and published in the American Journal of Clinical Nutrition. The formulas are:
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
This equation predicts the number of calories your body needs to maintain basic physiological functions at complete rest — breathing, circulating blood, maintaining body temperature, and repairing cells. It does not include the calories needed for any physical activity.
How We Calculate Metabolic Age
Once your personal BMR is calculated, we compare it to the average BMR for your gender across different age groups. We find the age at which the average person of your gender would have the same BMR as you. That age is your metabolic age.
For the comparison, we use reference BMR values derived from UK population averages for height and weight, applying the Mifflin-St Jeor formula. The average UK adult male is 175.3 cm tall; the average UK adult female is 161.6 cm tall, according to the ONS Health Survey for England.
Why BMR Declines With Age
BMR naturally declines as we age for several interconnected reasons. The primary factor is sarcopenia — the age-related loss of skeletal muscle mass. From around age 30, adults typically lose 3–8% of muscle mass per decade, with the rate accelerating after 60. Since muscle tissue is metabolically active (it burns calories even at rest), losing muscle directly reduces BMR.
Hormonal changes also play a significant role. Testosterone levels in men decline gradually from about age 30, reducing the stimulus for muscle protein synthesis. In women, the menopause transition causes a drop in oestrogen that is associated with increased fat accumulation, particularly around the abdomen, and a reduction in lean mass. Growth hormone secretion also declines with age, further reducing the body's capacity to build and maintain muscle.
Mitochondrial efficiency — the ability of cells to generate energy — also declines with age. Mitochondria, often called the "powerhouses of the cell," become less numerous and less efficient as we age. This reduces the overall rate of cellular metabolism, contributing to a lower BMR.
Metabolic Age vs Chronological Age vs Biological Age
Chronological age is simply how many years you have been alive. It is fixed and unchangeable.
Metabolic age is a specific measure of your BMR relative to age-group averages. It reflects your body composition (muscle-to-fat ratio), hormonal status, and cellular energy efficiency.
Biological age is a broader concept that encompasses many health markers beyond metabolism — including telomere length, inflammatory markers, organ function, cognitive function, and more. Some researchers argue that biological age, rather than chronological age, is the best predictor of longevity and health outcomes. Metabolic age is one component of biological age.
It's entirely possible to have a chronological age of 50 but a metabolic age of 35 if you have maintained high muscle mass, low body fat, and excellent cardiovascular health. Conversely, a sedentary lifestyle, high body fat, and low muscle mass can result in a metabolic age significantly older than your chronological age, even in younger adults.
Factors That Affect Your Metabolic Age
Muscle mass: The most significant factor. Each kilogram of muscle burns approximately 13 kcal per day at rest, compared to about 4.5 kcal per kilogram of fat. Building and maintaining muscle through resistance training is the most effective way to keep BMR high and metabolic age low.
Body fat percentage: Higher body fat relative to muscle mass lowers BMR. Reducing excess body fat — particularly visceral (abdominal) fat — while preserving or building muscle improves metabolic age.
Physical activity level: Regular exercise, especially strength training, preserves and builds muscle. Cardiovascular exercise improves mitochondrial density and cardiovascular efficiency, both of which support a higher BMR.
Diet quality: Adequate protein intake (1.6–2.2g per kg of body weight for active individuals) is essential for muscle protein synthesis. Severe caloric restriction can reduce BMR — this is why crash diets are counterproductive for long-term metabolic health.
Sleep: Poor sleep disrupts hormones that regulate metabolism, including cortisol, growth hormone, insulin, and leptin. Chronic sleep deprivation is associated with reduced muscle mass and increased fat storage, both of which worsen metabolic age.
Thyroid function: The thyroid gland regulates metabolic rate. Hypothyroidism (underactive thyroid) significantly reduces BMR, while hyperthyroidism elevates it. If your metabolic age seems significantly older than your chronological age despite a healthy lifestyle, it may be worth discussing thyroid function with your GP.
How to Improve Your Metabolic Age
The good news is that metabolic age is not fixed — it can be improved at any age with the right interventions:
- Strength training: Resistance exercise 2–4 times per week stimulates muscle protein synthesis and prevents sarcopenia. Even modest strength gains significantly increase BMR. NHS guidelines recommend muscle-strengthening activities on 2 or more days per week.
- High-protein diet: Consume 1.2–2g of protein per kg of body weight daily. Protein has a high thermic effect (it costs more energy to digest than carbs or fat) and supports muscle maintenance and growth.
- HIIT (High-Intensity Interval Training): Short bursts of intense exercise followed by rest periods have been shown to boost metabolism for hours after the workout ends — a phenomenon called excess post-exercise oxygen consumption (EPOC).
- Avoid prolonged sitting: Stand up and move regularly throughout the day. Non-Exercise Activity Thermogenesis (NEAT) — the calories burned through incidental movement — can account for 15–50% of total daily energy expenditure.
- Prioritise sleep: Aim for 7–9 hours of quality sleep per night. Consistent sleep supports growth hormone secretion, which peaks during deep sleep stages.
- Stay hydrated: Even mild dehydration can temporarily reduce metabolic rate. Cold water may slightly boost BMR as the body works to warm it to body temperature.
- Avoid severe caloric restriction: Eating too little triggers adaptive thermogenesis — the body reduces BMR to conserve energy. Instead, aim for a moderate caloric deficit of 300–500 kcal/day when losing weight.
- Consider your gut health: Emerging research links gut microbiome diversity to metabolic rate. A diet rich in fibre, fermented foods, and diverse plant foods supports a healthier gut microbiome.
Metabolic Age and the NHS
While the NHS does not use "metabolic age" as a clinical metric, it does focus extensively on BMR-adjacent concepts in health assessments. The NHS Health Check (available to adults aged 40–74) assesses several metabolic health indicators including blood pressure, cholesterol, blood glucose, and BMI. These metrics together paint a picture of metabolic health.
The NHS Active 10 programme and NHS Better Health app promote the physical activity that directly improves metabolic health. For those concerned about their metabolic health, speaking to a GP can lead to referrals for body composition analysis, nutritional advice, and exercise programmes on prescription.
Frequently Asked Questions
What is metabolic age?
Metabolic age is a comparison of your Basal Metabolic Rate (BMR) to the average BMR for people of different chronological ages. If your BMR matches the average of a 30-year-old but you are 40, your metabolic age is 30 — 10 years younger than your actual age.
How is metabolic age calculated?
Your BMR is first calculated using the Mifflin-St Jeor equation (based on weight, height, age, and gender). This is then compared against population-average BMR values by age group to find the age at which the average person would have the same BMR as you. That matching age is your metabolic age.
What is a good metabolic age?
A metabolic age equal to or lower than your actual age is considered good. The lower your metabolic age relative to your chronological age, the more efficiently your body is burning energy at rest — typically indicating higher muscle mass and healthy body composition.
What is the Mifflin-St Jeor equation?
For men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5. For women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161. It is the most widely recommended BMR formula for the general adult population.
How can I improve my metabolic age?
Key strategies include: strength training to build muscle (the biggest driver of BMR), eating adequate protein, performing HIIT workouts, getting 7–9 hours of sleep, staying hydrated, avoiding prolonged sitting, and not crash dieting.
Why does metabolic age increase as we get older?
Primarily because of sarcopenia (age-related muscle loss), hormonal changes (declining testosterone and growth hormone), and reducing mitochondrial efficiency. These factors collectively lower the body's resting energy expenditure over time.
Is metabolic age the same as biological age?
No. Metabolic age specifically measures your BMR relative to age-group averages. Biological age is broader, incorporating telomere length, organ function, inflammation, cognitive function, and more. Metabolic age is one component of biological age but not equivalent to it.