The Unchangeable Factors Influencing BMR
Some of the biggest determinants of your basal metabolic rate are elements that are entirely out of your control. Understanding these foundational factors is the first step toward realistically managing your health and weight. Genetics, for example, play a significant role in your metabolic efficiency. Some individuals are simply born with a naturally faster or slower metabolism. Research on twins has shown that a significant portion of BMR variation is heritable, highlighting the genetic blueprint that influences how your body uses and stores energy.
Age
One of the most well-known factors affecting BMR is age. Your metabolic rate tends to decrease as you get older, and this is largely due to a natural loss of muscle mass. As early as your 30s, adults can begin to lose muscle mass, which is more metabolically active than fat tissue. Additionally, hormonal and neurological changes that occur with aging contribute to a slower BMR. This is one of the main reasons why maintaining weight can become more challenging with age, even if diet and activity levels remain consistent.
Gender
On average, men tend to have a higher BMR than women. This is primarily because men typically have a greater proportion of lean muscle mass and a lower percentage of body fat compared to women of a similar age and weight. Lean muscle tissue burns more calories at rest than fat tissue, leading to a higher baseline energy expenditure. However, once adjusted for differences in body composition, the BMR between men and women with similar fat-free mass is not significantly different.
Controllable Factors for Your BMR
While genetics, age, and gender provide a baseline, several controllable factors can influence your BMR. Focusing on these areas can give you a degree of control over your metabolism.
Body Composition
This is arguably the most significant modifiable factor for BMR. The ratio of lean muscle mass to body fat directly impacts your metabolic rate. Muscle tissue is far more metabolically active, burning more calories at rest than adipose (fat) tissue. Therefore, increasing muscle mass through resistance or strength training is one of the most effective ways to increase your BMR. A 2018 study, for instance, suggested that resistance training effectively boosts BMR levels in sedentary women.
Diet and Nutrition
What and how you eat has a large influence on your metabolic rate. Eating too few calories, also known as crash dieting or starvation, can significantly lower your BMR. Your body perceives this as a threat and slows your metabolism to conserve energy. The thermic effect of food (TEF), or the energy required to digest and metabolize food, is also influenced by diet. Proteins, for example, require more energy to digest than fats or carbohydrates, meaning a higher protein diet can slightly boost your metabolism.
Exercise and Activity Levels
Regular physical activity, especially strength training, increases your BMR indirectly by building muscle mass. More intense workouts, like high-intensity interval training (HIIT), can also cause a temporary increase in your metabolic rate long after the workout is over, a phenomenon known as Excess Post-exercise Oxygen Consumption (EPOC). A consistently active lifestyle keeps your metabolic engine running more efficiently.
Hormonal and Environmental Influences
Thyroid Hormones
The thyroid gland secretes hormones, such as thyroxine, that are crucial for regulating metabolic processes. An overactive thyroid (hyperthyroidism) can cause an unnaturally high BMR, leading to weight loss and anxiety. Conversely, an underactive thyroid (hypothyroidism) can cause a slow metabolism, leading to weight gain and lethargy.
Environmental Temperature
Your body works to maintain a stable internal temperature. If you are in a very hot or cold environment, your body has to expend extra energy to either cool down or warm up, which in turn increases your BMR. For example, shivering in the cold is a metabolic process to generate heat. A thermoneutral environment, typically used during BMR measurements, is one where the body does not have to expend this extra energy.
A Comparison of BMR-related Metabolic Concepts
| Feature | BMR (Basal Metabolic Rate) | RMR (Resting Metabolic Rate) | TDEE (Total Daily Energy Expenditure) |
|---|---|---|---|
| Measurement Condition | Strict, thermoneutral lab setting after 12+ hours of fasting and sleep. | Less strict conditions; measured at rest but not necessarily under thermoneutrality. | All-inclusive daily energy use, including BMR, TEF, and activity. |
| Energy Use | Minimum calories needed for basic life functions at complete rest. | Energy used at rest, but includes some low-effort activities. | Total calories burned from all metabolic processes and physical activity. |
| Calculation Method | Indirect calorimetry in a lab, or estimated via formulas like Harris-Benedict or Mifflin-St Jeor. | Often estimated with the same formulas as BMR, or measured by calorimetry. | BMR or RMR multiplied by an activity factor. |
Conclusion
Your BMR is a complex metric determined by a multifaceted combination of factors. While you can’t change your age, gender, or genetic predisposition, you can influence your BMR by focusing on modifiable factors. By increasing your muscle mass through strength training, eating a balanced diet, and maintaining a consistently active lifestyle, you can effectively manage your metabolic rate. Recognizing the role of all these elements, from genetics to diet, allows for a more comprehensive and realistic approach to health and weight management. For more on metabolism, you can consult reliable sources like the Mayo Clinic for further reading.
