Understanding the Difference: RMR vs. REE
Before diving into the reasons for changes, it is important to clarify the terms used to describe calories burned at rest. While often used interchangeably, there is a slight distinction between Basal Metabolic Rate (BMR), Resting Metabolic Rate (RMR), and Resting Energy Expenditure (REE). BMR is a stricter measure, taken under very controlled conditions (e.g., fasting and lying down). RMR is a more common measure that accounts for minimal, daily movements and is about 10% higher than BMR. REE is functionally the same as RMR. For the purpose of this article, we will refer to them collectively as resting calories, since they all represent the energy your body needs just to exist.
Primary Reasons Why Resting Calories Change
Your body's energy needs at rest are not a fixed number. They are a dynamic value influenced by a combination of genetics, health status, and lifestyle choices. While some factors are out of your control, many others are things you can actively influence through your daily habits.
Age and the Gradual Slowdown
It's a common belief that metabolism drastically slows with age, and there is truth to this, though it is often less severe than perceived. The decline is primarily due to sarcopenia, the natural loss of muscle mass that occurs with aging, particularly after age 60. Since muscle is more metabolically active than fat, less muscle means fewer calories burned at rest. Hormonal shifts that occur with age, such as declining testosterone and estrogen, also play a role.
The Muscle Mass Factor
Muscle tissue burns more calories at rest than fat tissue, which is why building and maintaining muscle mass is a powerful way to influence your RMR. A person with more muscle mass will naturally have a higher RMR than someone of the same weight with a higher body fat percentage. This is because muscle is a more metabolically demanding tissue, requiring more energy to maintain.
Weight Fluctuations
Your body size and weight are significant determinants of your RMR. Larger bodies have more metabolizing tissue and, therefore, a higher resting energy demand. Conversely, as you lose weight, especially if it includes a loss of lean muscle tissue, your resting calorie needs will decrease. This can be one reason why weight loss plateaus; your smaller body requires less energy to function than it did before.
Hormonal and Thyroid Influences
Certain hormonal imbalances, especially those affecting the thyroid, can have a major impact on your metabolic rate. An underactive thyroid (hypothyroidism) can cause metabolism to slow, while an overactive thyroid (hyperthyroidism) can cause it to speed up. Additionally, other hormonal changes, such as those during pregnancy or menopause, significantly affect RMR.
Diet and Calorie Intake
Drastically restricting your calorie intake can cause your RMR to drop as your body enters a survival or "starvation" mode to conserve energy. This is why very low-calorie diets can backfire, making sustained weight loss difficult. The body learns to function on less energy, which can make subsequent weight gain easier once normal eating resumes. Regular, consistent eating patterns are more effective for maintaining a stable metabolism.
Environmental and Health Factors
Your body's RMR can also change due to factors like environmental temperature and illness. When you are in a very cold or hot environment, your body works harder to maintain its core temperature, temporarily increasing RMR. Similarly, fighting an infection or illness requires extra energy, causing a short-term rise in resting calorie burn. Stress can also play a role, as the body's hormonal response can influence metabolic function.
Comparison of Metabolic Influences
| Factor | How it Changes Resting Calories |
|---|---|
| Age | Decreases RMR, mostly due to loss of muscle mass and hormonal shifts. |
| Muscle Mass | Increases RMR; more muscle burns more calories at rest than fat. |
| Body Weight | Heavier individuals have higher RMR due to more metabolizing tissue. |
| Dietary Intake | Severe calorie restriction lowers RMR to conserve energy. |
| Illness/Infection | Increases RMR as the body expends more energy to fight infection. |
| Hormones | Thyroid and other hormonal imbalances can speed up or slow down RMR. |
| Genetics | Can predispose an individual to a naturally faster or slower metabolism. |
Managing Your Resting Calorie Burn
While you cannot change factors like genetics or age, you can significantly influence your RMR through lifestyle choices. Incorporating regular strength training into your routine helps build and preserve muscle mass, which in turn boosts your metabolism. Additionally, ensuring you get enough quality sleep and maintain a consistent, healthy calorie intake prevents your body from slowing its metabolism. The solution to a healthy metabolism lies in a balanced approach, not in extreme measures.
Conclusion
Your resting calories are not static and are influenced by a complex interplay of factors, from your age and body composition to your diet and overall health. Understanding why your resting calories change is the first step toward effective weight management and long-term health. By focusing on building muscle, maintaining a healthy body weight, and supporting your body with proper nutrition, you can have a positive influence on your metabolic rate, ensuring it works for you, not against you. For further reading, researchers have studied how age-related chronic diseases impact resting metabolic rate over time, providing deeper insight into the complexities of metabolism.
