When it comes to the topic of metabolism, it tends to grab attention—especially in a world obsessed with calories. As a nutrition coach or weight loss specialist, clients might often ask you about how to boost metabolism, as it's often linked to the idea of going from ‘fat-to-fit’ or ‘flab-to-fab’. This concept revolves around enhanced calorie burning, improved muscle mass, and greater fat utilization, ultimately leading to weight loss.
In this blog post, I'll teach you about your resting metabolic rat, how to calculate and improve yours.
One of the key players in this process is your resting metabolic rate (RMR). It plays a crucial role in determining how many calories your body burns at rest. Before diving deeper into RMR, let's first break down the elements of total daily energy expenditure (TDEE).
Total Daily Energy Expenditure (TDEE)
In scientific terms, metabolism refers to all the processes your body performs to sustain life. For most people, however, it boils down to total daily energy expenditure (TDEE) and how it influences weight gain or loss based on the balance of calories consumed versus calories burned.
Your TDEE consists of three main components:
Resting metabolic rate (RMR): The energy your body needs to function while at rest.
Thermic effect of food (TEF): The energy required for eating, digesting, and processing food.
Thermic effect of physical activity (TEPA): The calories burned through exercise and other forms of physical activity, including non-exercise activity thermogenesis (NEAT), which includes all movement that isn’t categorized as formal exercise, such as standing or fidgeting.
What is Resting Metabolic Rate (RMR)?
RMR refers to the number of calories your body uses while at complete rest. These calories support vital functions like breathing, blood circulation, organ function, and basic neurological tasks. Your RMR is closely tied to lean body mass and decreases slightly with each increase in body fat percentage.
How to Calculate RMR
There are various ways to calculate RMR, from precise methods like calorimetry to more accessible mathematical equations.
Calorimetry
Direct calorimetry measures the heat a person produces in a chamber to estimate energy expenditure.
Indirect calorimetry calculates energy expenditure by measuring oxygen consumption and carbon dioxide production.
While both methods are highly accurate, they are often expensive and difficult to access.
Mathematical Formulas
For a more practical approach, several mathematical formulas have been developed to estimate RMR. You can use calorie calculators online, fitness apps, or even wearable devices. Two of the most common methods are the Harris-Benedict and Mifflin-St Jeor equations.
Harris-Benedict Equation
The Harris-Benedict (H&B) equation, developed in 1918 and revised in 1984, remains a popular method for estimating basal metabolic rate (BMR), though BMR and RMR are often used interchangeably.
Men:
BMR=88.362+(13.397×weight in kg)+(4.799×height in cm)−(5.677×age in years)BMR = 88.362 + (13.397 \times \text{{weight in kg}}) + (4.799 \times \text{{height in cm}}) - (5.677 \times \text{{age in years}})BMR=88.362+(13.397×weight in kg)+(4.799×height in cm)−(5.677×age in years)
Women:
BMR=447.593+(9.247×weight in kg)+(3.098×height in cm)−(4.330×age in years)BMR = 447.593 + (9.247 \times \text{{weight in kg}}) + (3.098 \times \text{{height in cm}}) - (4.330 \times \text{{age in years}})BMR=447.593+(9.247×weight in kg)+(3.098×height in cm)−(4.330×age in years)
For example, a 38-year-old woman who is 5'6" (167.6 cm) tall and weighs 145 lbs (65.9 kg) would have an RMR of around 1,411 calories.
Mifflin-St Jeor Equation
The Mifflin-St Jeor equation, created in the 1990s, offers a more up-to-date estimate of RMR:
Men:
RMR=(10×weight in kg)+(6.25×height in cm)−(5×age in years)+5RMR = (10 \times \text{{weight in kg}}) + (6.25 \times \text{{height in cm}}) - (5 \times \text{{age in years}}) + 5RMR=(10×weight in kg)+(6.25×height in cm)−(5×age in years)+5
Women:
RMR=(10×weight in kg)+(6.25×height in cm)−(5×age in years)−161RMR = (10 \times \text{{weight in kg}}) + (6.25 \times \text{{height in cm}}) - (5 \times \text{{age in years}}) - 161RMR=(10×weight in kg)+(6.25×height in cm)−(5×age in years)−161
Using the same example, the same woman’s RMR using this formula would be around 1,356 calories—a 55-calorie difference from the Harris-Benedict calculation.
Potential Errors in Estimation
Though the difference between the two estimates may seem minor, even small miscalculations can lead to weight fluctuations over time. Additionally, these equations assume that individuals of the same gender, age, height, and weight have the same RMR, which isn’t always true. Factors like lean body mass significantly influence RMR.
Formulas like Katch-McArdle and Cunningham take lean body mass into account, but they require accurate measurements of body composition.
Studies show that while the Mifflin-St Jeor equation tends to be accurate within 10% of true RMR, errors with the Harris-Benedict equation can be as high as 36% in individuals with obesity.
Uncontrollable Factors Affecting RMR
While there are some things you can control, several factors that influence your RMR are out of your hands:
Age: RMR typically decreases by about 2% per decade after peak growth, which occurs in the late teens for females and early 20s for males.
Genetics: Your genes play a significant role in determining your metabolism. For instance, the FTO gene is linked to lower satiety and overeating, potentially altering RMR by up to 160 calories per day.
Epigenetics: Environmental factors and lifestyle choices can also affect how genes express themselves, potentially impacting your metabolic rate.
Controllable Factors to Boost RMR
While you can't control everything, there are ways to optimize your RMR through lifestyle choices:
Stimulants
Certain foods and ingredients, like caffeine and capsaicin, can give your metabolism a temporary boost of 4-5%, which equates to burning an extra 15-25 calories per day.
Lean Body Mass
Building muscle is one of the most effective ways to increase RMR. A gain of just 2-4 pounds of muscle can raise your metabolic rate by 7-8%, or an extra 90-110 calories per day.
Sleep and Calorie Intake
Sleep deprivation and extremely low-calorie diets can suppress your RMR. Chronically high cortisol levels (due to stress or very low-calorie diets) can interfere with thyroid hormone production, reducing your metabolic rate.
A 20% drop in RMR could mean burning 240-300 fewer calories per day, leading to weight gain over time.
Avoiding Starvation Mode
To avoid damaging your metabolism, you need to fuel your body properly. While formulas like Mifflin-St Jeor provide a solid guideline, keeping daily caloric intake above the recommended minimum (around 1,000-1,200 calories for women and 1,200-1,600 for men) is crucial.
If your calorie intake is too low, not only will your metabolism slow down, but you might lose muscle mass as well, which will further reduce your RMR.
The Hunger Scale
Another approach to maintaining an optimal calorie intake is to listen to your body's hunger cues. The hunger scale is a tool that can help you determine when it’s time to eat and when to stop.
Hunger Score | Description |
1 | Starving, weak, dizzy |
2 | Irritable, cranky, very hungry, low energy |
3 | Strong urge to eat, slight stomach growling |
4 | Slightly hungry, thinking about food |
5 | Feeling satisfied, neither hungry nor full |
6 | Pleasantly full, satisfied |
7 | Uncomfortably full, could still eat more |
8 | Stuffed, bloated, discomfort |
9 | Overstuffed, stomach pain |
10 | Sick from overeating |
Ideally, aim to stay between scores 4 and 6, eating when you're slightly hungry and stopping before you feel too full.
Hunger vs. Appetite
Lastly, it’s important to distinguish between hunger and appetite:
Hunger: A biological need for energy, usually indicated by physical symptoms like an empty stomach or low blood sugar.
Appetite: A desire to eat, often driven by emotions, thoughts, or environmental cues, regardless of actual hunger.
Conclusion
Resting metabolic rate is a vital component of total daily energy expenditure, but it’s not always easy to measure accurately. While formulas like Mifflin-St Jeor are useful, they come with limitations. By understanding the factors that influence your RMR and making adjustments where possible, you can make smarter choices to optimize your metabolism and support your fitness goals.
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