What is a basal metabolic rate (BMR)?
Basal metabolic rate (BMR) is the amount of energy (calories) your body burns to maintain basic physiological functions at rest, such as breathing, blood circulation, cell production, and temperature regulation. It is the single largest component of your daily energy expenditure, accounting for 60% to 70% of the calories you burn every day. Your BMR is influenced by several factors, including age, gender, height, and body composition.
Natural and controllable factors that can reduce BMR
Yes, your BMR can be reduced, and it typically is in several scenarios, some of which are natural and others which are influenced by lifestyle. A significant decrease in BMR is not always a negative outcome, especially for those with conditions like hypermetabolism, but for most people, managing its reduction is crucial for effective weight management.
Aging and muscle mass
One of the most common causes of a reduced BMR is aging. As you get older, there is a natural decline in muscle mass, and since muscle is more metabolically active than fat, this loss leads to a lower BMR. A more sedentary lifestyle in later years often exacerbates this effect. For example, a study showed that BMR declines by 1-2% per decade after the age of 20.
Crash dieting and severe caloric restriction
When you drastically reduce your calorie intake, your body's survival instinct kicks in, slowing down your metabolism to conserve energy. This metabolic adaptation can cause a significant and long-lasting drop in BMR, making future weight loss more challenging and potentially leading to a weight loss plateau. A rapid weight loss plan results in a significantly greater reduction in BMR compared to a slow, gradual approach.
Sleep deprivation and stress
Poor quality or insufficient sleep can disrupt hormonal balance and negatively affect your metabolism. Lack of sleep is associated with lower leptin levels and higher cortisol levels, which can lead to increased appetite and a reduced metabolic rate. Similarly, chronic stress can cause the release of cortisol, which may slow down metabolism.
Changes in body composition
When you lose weight, especially quickly, you often lose both fat and metabolically active muscle tissue. This reduction in overall body mass, particularly lean mass, directly reduces your BMR because a smaller body requires less energy to function. However, losing fat mass has a much smaller effect on BMR than losing muscle mass, as fat tissue is less metabolically active.
Other factors that influence BMR
- Hormonal imbalances: Conditions like hypothyroidism, an underactive thyroid, can cause a slower metabolism because the thyroid gland does not release enough hormones to regulate energy expenditure.
- Medications: Certain drugs, while others can raise it, can affect BMR.
- Dietary deficiencies: Inadequate intake of certain nutrients, such as iodine, can impair thyroid function and lead to a slower metabolism.
How to mitigate BMR reduction
While some factors affecting BMR are outside of your control, you can take steps to manage or mitigate the decline, particularly during weight loss. The goal is to lose fat while preserving or even building muscle mass.
Incorporate resistance training
Strength training is one of the most effective ways to preserve and build lean muscle mass, which helps to counteract the natural decline in BMR that comes with age and weight loss. A higher muscle-to-fat ratio will help you burn more calories even at rest.
Consume enough protein
Eating adequate amounts of protein is vital, especially when in a calorie deficit. Protein has a higher thermic effect than fats or carbohydrates, meaning your body burns more calories to digest it. A high protein intake also helps preserve fat-free mass during weight loss.
Avoid crash dieting
Opt for a gradual, moderate calorie deficit instead of extreme restriction. This approach helps prevent the body from entering "starvation mode" and minimizes the loss of lean tissue, which in turn reduces the severity of the BMR drop.
Comparison of BMR-related strategies
| Strategy | Mechanism | Effect on BMR | Weight Management Impact |
|---|---|---|---|
| Crash Dieting | Severe caloric restriction triggers survival mode and muscle loss. | Significant and potentially long-term reduction. | Can lead to a plateau and difficulty maintaining weight loss. |
| Gradual Calorie Deficit | Moderate restriction minimizes lean mass loss and metabolic shock. | Smaller, more manageable reduction. | More sustainable weight loss and better long-term maintenance. |
| Resistance Training | Builds and preserves metabolically active muscle tissue. | Increases or maintains BMR despite weight loss. | Burns more calories at rest and supports fat loss. |
| High-Protein Diet | High thermic effect of food and preserves muscle mass. | Helps mitigate the decrease in BMR during dieting. | Aids fat loss and increases satiety. |
Conclusion
Yes, BMR can be reduced, primarily due to factors like aging, loss of muscle mass, and, most significantly, severe and rapid caloric restriction. While a decrease is a natural bodily response, it's not a fixed state. By prioritizing a balanced approach to weight management—including consistent exercise, especially strength training, a high-protein diet, and avoiding extreme dieting—you can effectively mitigate BMR reduction. Instead of fearing a slowing metabolism, focus on building sustainable, healthy habits that work with your body's physiology, ensuring a healthier and more balanced lifestyle over the long term.
