They say age is just a number. But what if your body is actually older or younger than the amount of candles on your birthday cake? That's the idea behind metabolic age, which compares your body's metabolism to that of other people who have made the same number of trips around the sun.
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Or at least that's the promise. At this time, there's not much research around the concept of metabolic age. So while a younger metabolic age seems to indicate a lower risk of health problems, it's best to take all of this with a grain of salt.
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Here's what we do know: The things that appear to lower your metabolic age are healthy for you in general, so there's no reason not to do them.
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Although metabolic age can be measured in a few ways, it most commonly refers to how your basal metabolic rate (BMR) compares to the average BMR for people of your chronological age (how long you've been living), explains Catherine J. Andersen, PhD, RDN, associate professor in the Department of Nutritional Sciences at the University of Connecticut.
So what's basal metabolic rate? That's how many calories you need to sustain your basic vital functions while at rest without any external influences. For example, it doesn't take into account any extra energy you need to move or do physical activity, Andersen says.
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This differs slightly from resting metabolic rate (RMR), which is the number of calories you burn outside of physical activity, according to the American Council on Exercise (ACE).
One way to distinguish BMR vs. RMR? While they're similar, BMR tends to be measured in more controlled settings, whereas RMR may be more representative of your body at rest in everyday life, according to ACE.
Knowing what your metabolic age is can be an indicator of physical health. Having a metabolic age lower than your chronological age indicates good health, while a higher one suggests you may have some health problems, according to a May 2017 study in Transplantation.
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"A lower metabolic age is typically found in those with more lean muscle tissue, better diet choices and a more active lifestyle, all of which typically indicate a healthier lifestyle," dietitian Lauren Minchen, MPH, RDN, tells LIVESTRONG.com.
While much more research is needed, one small study of 19 people in Current Developments in Nutrition in June 2019 found a possible health perk: having a metabolic age lower than your actual age was associated with lower blood pressure.
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And in Andersen's lab, "we see an increased metabolic age associated with a higher BMI, waist circumference, body fat mass and blood pressure," she says. "All of these are risk factors for cardiovascular disease, metabolic disease and diabetes."
First of all, BMR is based on your sex, weight, height and age. So carrying some extra pounds for your height may lead to a higher metabolic age.
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More advanced and accurate ways of measuring BMR also take into account your body composition. "The biggest thing that results in a higher metabolic age would most likely be lower lean body mass relative to overall body weight," Andersen says.
That's because having a higher ratio of lean body mass to fat mass often means you burn more calories at rest than someone of the same weight who has more fat, per the Mayo Clinic.
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And of course your diet and exercise habits influence your weight and body composition. So, "your metabolic age may be higher than your actual age if you don't participate in regular exercise, make poor diet choices habitually and don't manage stress well," Minchen says. "These can lead to a loss of lean body tissue, fat gain and a general decline in overall health."
Healthy lifestyle factors can help lower your metabolic age.
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Lean mass is a big factor in metabolic age. To build muscle and gain lean body mass, hit the weights for some strength training.
Staying active in general can also help keep your weight in check and bring down your metabolic age.
Not sure where to start? Here's everything beginners need to know about building muscle.
Protein supports muscle growth. Eating a high-protein diet helps you maintain and build muscle, especially if you're also trying to lose weight, according to a December 2019 analysis in Advances in Nutrition, which suggests aiming for about 1.3 grams per kilogram of your body weight daily.
Keep in mind that 1 pound equals about 0.45 kilograms. So if you weigh 200 pounds, that's about 90.7 kilograms. Multiply that by 1.3, and voila: You should aim to eat about 118 grams of protein each day.
Foods high in protein include:
Eating too much protein can be harmful if you have certain medical conditions, such as kidney disease. If you're unsure, talk to your doctor or a registered dietitian about what amount of protein is right for you.
"Sleeping at least seven hours per night is essential for supporting a healthy BMR, recovering from exercise, digesting food well and becoming stronger," Minchen says.
"We see the greatest amount of muscle building during sleep," Andersen adds. "So if your goal is to increase lean body mass, adequate sleep will better support that."
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Struggle with zzzs? Learn how to fix the 10 most common sleeping mistakes.
"If we don't find ways to properly manage stress, we sleep poorly, skip workouts, make quick and convenient food choices that often make us feel worse later and run our bodies into the ground," Minchen says.
All of that can increase your metabolic age. Plus, too much stress can put your body into a catabolic state where it breaks down nutrient stores, which can affect skeletal muscle, Andersen adds.
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Eating a plant-based diet is associated with a lower metabolic age compared to chronological age, according to the Current Developments in Nutrition study.
Ready to change your eating habits? Get started with a seven-day plant-based meal plan.
Wondering how to find out your metabolic age? Well, there's currently no standardized way to determine it, Andersen says. Instead, various calculations and types of proprietary software are used. "I haven't found any that have been truly validated by research," she says.
That said, if you want to calculate your metabolic age, you need access to data of the metabolic age of other people who have the same chronological age as you, because you want to know how you stack up compared to them.
Only some personal trainers, registered dietitians and other experts at medical or fitness centers have the technology to determine your metabolic age. If you're curious to know yours, search online for providers in your area or call around.
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These experts may calculate your metabolic age using bioelectrical impedance scales, Andersen says. You stand or hold onto sensors that send a low-level electrical current through your body, which measures your body fat and lean body mass to compute metabolic age.
You can also find bioelectrical impedance scales online to buy for your home, but Andersen says these vary in accuracy.
With that said, if you have access to a metabolic age chart that shows you the BMR or RMR of other people in your chronological age group, you can calculate your own metabolic rate for comparison. You can do that using several different equations:
The Mifflin-St. Jeor equation to calculate BMR is derived from the Harris-Benedict formula, per a February 1990 announcement in The American Journal of Clinical Nutrition.
Let's say you're AFAB and 30 years old, 150 pounds (68 kilograms) and 5 feet tall (152.4 centimeters). Your BMR would be 1,321.5. As a reminder, that's an estimate of the calories your body burns while carrying out basic functions like breathing and circulating blood, not including moving or any physical activity.
Manual calculations are not 100 percent accurate. Getting a completely accurate BMR calculation requires sophisticated equipment used in a highly controlled testing environment.
This equation harnesses lean body weight to calculate BMR, according to ACE. You must know your body fat percentage to determine your BMR using this formula, so if that information is unavailable to you, stick with the Mifflin-St. Jeor equation.
For example, a 185-pound person with 10 percent body fat weighs about 84 kilograms, 75.5 kilograms of which is lean tissue. Plug in those numbers, and you'll find that this person has a BMR of about 2,000.8 calories per day.
If you're wondering how to calculate RMR specifically, the Cunningham equation can help. It uses lean body mass to provide a higher gauge of how many calories you burn per day than the Katch-McArdle equation, which is why it's used as an RMR formula.
However, it may overestimate your actual RMR, according to September 2013 research in Topics in Clinical Nutrition.
For the 185-pound person mentioned above, this equation puts RMR at 2,161 calories per day.
Your metabolic age tells you how your BMR compares to others of your same age. Having a younger metabolic rate seems to indicate a healthier lifestyle and may mean you're at lower risk for certain health conditions. However, we need more research to determine the best way to calculate metabolic age and what exactly it means.
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