Metabolic age is a reflection of physical health in the form of a calculation based on the base metabolic rate (BMR). If someone's metabolic age is lower than his or her actual age, it suggests that the body is in good health, while a metabolic age higher than the actual age indicates that someone may be experiencing health problems. This measurement is one among many which can be used to assess health, and people should avoid putting too much weight on it; a more complete assessment of physical health can be obtained from a doctor, who may also have suggestions to improve health.

A number of different calculations can be used to determine metabolic age. They all rely on some combination of the subject's height, actual age, and weight. However, metabolic age is a bit more complicated than this. It is impacted by activity level, genetics, and a wide variety of other factors. Estimates of metabolic age can vary depending on which formula is used, reflecting the fact that it is a bit difficult to pin down a single number in discussions of metabolic age.

As people age, their BMR changes. Older individuals tend to have lower caloric requirements, for a variety of reasons, ranging from a reduction in physical activity which reduces the demand on calories for energy to a reduction of muscle which also reduces the body's need for energy. The BMR plays an important role in metabolic age, making it important to periodically recalculate to see if circumstances have changed with age.

Some fitness centers perform this calculation for their clients as a courtesy, and there are also numerous calculators available on the Internet which allow people to plug in values and obtain their metabolic age. People should be aware that health is a complicated issue, and the human body is quite diverse, which means that a calculation may not always be accurate. For example, a young athlete may appear to have a skewed metabolic age and actually be in excellent health. A full physical exam is a much better way to learn about general health.

The BMR can also be used to help people calculate appropriate caloric intake. People who are interested in calorie restriction as a facet of a weight loss program can calculate their BMR and determine how many calories below that number they need to eat to lose weight. Likewise, people who want to bulk up can eat over their BMR. When the desired weight is reached, the subject can return to a maintenance level, eating their BMR in calories every day.