What Are Basal & Resting Metabolic Rates (RMR)?
These two terms are used interchangeably, although they are not technically the same. Resting metabolic rate is really what most lay people mean when they say basal metabolic rate, and I talk here only about resting metabolic rate (RMR). Basal metabolic rate is a precise calculation with a precise definition; RMR is close enough for practical purposes.
Resting metabolic rate is the energy required by an animal to stay alive with no activity. Therefore, your real metabolic rate is always significantly higher than your RMR. Calculating RMR is a very useful first step in calculating your real metabolic rate.
Your Resting Metabolic Rate (RMR) is one of the main contributing components of energy expenditure (around 70%).
What Determines Resting Metabolic Rate?
A very small number of people have physical conditions that give them strange resting metabolic rates.
However, for the vast majority of people, resting metabolic rate can be calculated knowing a few key variables. They are age, sex, weight, height, and fat-free body mass. Fat-free mass is a very important variable. Weight and height are used in one formula to determine body surface area.
Things That Affect Your Metabolic Rate
MUSCLE: More muscle increases your RMR.
AGE: Your RMR decreases with age.
GENETICS: A decrease in your RMR can be due to genetics.
THE WEATHER: Living in a cold enviroment can increase your RMR. I know this sounds strange, but you expend more energy while moving around in cold weather. It's a lot easier to move around in summer but more of an effort to "get going" in winter.
MEALS: Small regular meals will increase your RMR.
PREGNANCY: It can increase your RMR.
CRASH-DIETING: It will decrease your RMR.
SUPPLEMENTS: Some supplements can raise your RMR.
When Does The Body Change Its Resting Metabolic Rate? Does Cutting Your Food Intake Reduce Resting Metabolic Rate?
The body cannot change its resting metabolic rate per unit of fat-free body mass. Studies have shown this.
Your resting metabolic rate will decrease as you lose muscle. Losing fat alone will not lower your RMR (and note that you will need to follow a very sensible program to lose fat without losing muscle). You have probably heard that people who go on crash diets end up lowering their metabolic rate, which means when they go off the diet, they put on fat more easily than before they started. This is because they have lost muscle, so they have lowered their metabolic rate.
However, the amount of energy burnt per unit of fat-free weight does not change; poor dieters end up with fewer units of fat-free weight, and that's where their vicious cycle comes from.
Are Some People's Digestive Systems More Efficient Than Others?
No. And your system does not become more or less efficient in response to changing food intake. Even obese people rarely have more efficient bodies. Researchers inspect the energy value of feces to determine this.
Given The Same Values For The Variables, How Much Does Resting Metabolic Rate Vary Between People?
In other words, what is the error in the formulas used to calculate RMR?
The latest research indicates there is a low variance in resting metabolic rate (RMR) between individuals who have the same values for the key variables. That is, given someone's age, their fat-free mass, their height, and their sex, the formulas are accurate.
"Recent evidence thus supports the conclusion that within-subject variations in BMR [more or less the same as RMR] are small and insignificant, even when energy intake and physical activity are uncontrolled (Shetty & Soares, 1988). This effectively refutes the Sukhatme-Margen hypothesis."