WHY IS RESTING METABOLIC RATE IMPORTANT FOR FAT LOSS

In the last article I showed that protein increases your BMR by as much as 30%, 3 hours after a high protein meal. So let’s delve a bit deeper into BMR. What is BMR and why is it so important for fat loss? We hear all the time that as we age our BMR steadily declines and thats the main reason for age related weight gain. So what can be done to keep our BMR high?

 

Let’s first understand what the components of BMR are. Here is a definition from Wikipedia:

“Basal metabolic rate (BMR), and the closely related resting metabolic rate (RMR), is the amount of daily energy expended while at rest in a neutrally temperate environment, in the post-absorptive state (meaning that the digestive system is inactive, which requires about twelve hours of fasting in humans). The release of energy in this state is sufficient only for the functioning of the vital organs, the heart, lungs, nervous system, kidneys, liver, intestine, sex organs, muscles, and skin.”

Since BMR is such a restrictive definition a more useful definition known as the resting metabolic rate (RMR) is used. RMR, as the name suggests, is the calorie requirement for vital body functions while at rest. Both BMR and RMR require advanced calorimetric techniques to measure, so there have been many empirical equations that have been developed to estimate the RMR of individuals. The most popular one is the Harris-Benedict equation for normal healthy individuals:

• For men: (13.75 x w) + (5 x h) - (6.76 x a) + 66
• For women: (9.56 x w) + (1.85 x h) - (4.68 x a) + 655

And the Mufflin equation for RMR:

• For men: (10 x w) + (6.25 x h) - (5 x a) + 5
• For women: (10 x w) + (6.25 x h) - (5 x a) - 161

Where:

w = weight in kg

h = height in cm

a = age

The biggest drawback of these equations is they don’t take into consideration body composition. Studies have shown that there is as much as 80% variation in the RMR estimation of active individuals and athletes. This was taken into account by the Cunningham equation that took into consideration the fat free muscle mass present in the body. This is because lean muscle burns calories while fat doesn’t. So for active individuals a more accurate estimation is given by:

RMR = 500 +22*FFM (Kg)

Where:

            FFM = Fat Free Mass (lean muscle)

From this equation it should be pretty clear that the more FFM an individual has the higher his RMR is going to be.

It will be clear in a minute why all this math matters once we look closely at RMR. As I said, RMR is the calorie requirement of the body in order to maintain healthy vital functions. The body burns some calories daily for normal day-to-day activities. These maintenance calories are comprised of RMR, food thermogenesis and activity thermogenesis. The RMR is comprised of all the vital organ functional calorie requirements and the calorie requirement of skeletal muscles. In this sense the muscles behave as a vital organ that burn calories for their survival. Activity thermogenesis is comprised of exercise thermogenesis and non-exercise thermogenesis. This is illustrated in the figure below:

Non-exercise thermogenesis is the amount of calories burned during daily day to day activities like walking to the restroom, sitting down or getting up, fidgeting, brushing etc. It might seem insignificant but non-activity thermogenesis exceeds activity thermogenesis even in avid exercisers. So now two things should be clear from these pie charts, first, that RMR is a major constituent of our daily calorie requirement and the only knob that can be manipulated in order to boost RMR is increasing skeletal muscle mass. Second, that calories burned during exercise activity only form a small percentage (9%) of the total daily calorie requirement, even in active individuals. So relying on just exercise calories to burn fat would be very inefficient. Studies [1,2] have shown that the level of aerobic fitness doesn’t correlate with an increase in RMR where as anaerobic exercise such as weightlifting leads to a higher RMR via the addition of fat free muscle mass.

Did you know that in order to burn a pound of fat you would need to burn about 4100 calories which would require 7 hours of running at moderate pace? On the other hand if you had 5 extra lbs of muscle it would burn about 4000 calories in 20 days without lifting a finger (even in your sleep). Gaining 5 extra pounds is easy especially for beginners and will only take less than 3 months of weight training. And when I say weight training I don't mean working with big weights. It can be as simple as body weight exercises (lunges, squats, push-ups, pull-ups, dips, etc). Now while the calorie burn rate of muscle may not seem much, think of it as the race between the turtle and hare. The hare being the calories burned during aerobics and the turtle being the calories burned by muscles all day. The hare runs fast and burns a lot of calories while running but when it stops it burns no calories.  On the other hand the turtle runs slower and burns calories at a slower pace but it never stops running so the calories burn 24/7. So guess who’s going to win the race to burn calories? Slow and steady wins the race!

While aerobic exercise is good for cardiovascular fitness it does nothing to boost your RMR. Anaerobic exercise done in short interval bursts (such as in HIIT), on the other hand, will be good not only for cardiovascular fitness but also to boost your RMR which in the long run will burn fat more efficiently than aerobic exercise.  

REFERENCES:

1.       CE Broeder, KA Burrhus, LS Svanevik and JH Wilmore (1992). "The effects of aerobic fitness on resting metabolic rate". American Journal of Clinical Nutrition 55 (4): 795.

2.       D. A. Smith, J. Dollman, R. T. Withers, M. Brinkman, J. P. Keeves, and D. G. Clark(1997) (1997). "Relationship between maximum aerobic power and resting metabolic rate in young adult women". Journal of Applied Physiology 82 (1): 156.