Diet & Weight Loss – Metabolism, Muscle, Exercise & Aging

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How does age impact Resting Metabolic Rate?

Resting Metabolic Rate (RMR) is highest during infancy when considered as calories per lb (or kg) of body weight. At this stage resting metabolism not only reflects normal life-sustaining operations of the infant but also must power the building of new tissue. The same can be said for growth spurts in children and teens. Conversely, as we age, our basal metabolism seems to slow down. Some researchers have estimated the slow down to be on the order of 2-3% in each decade. This downward progression of RMR in later life can be largely attributed to the loss of fat-free mass due to physical inactivity.

While researchers agree that some of this is related to changing hormone levels, much of it is related to changing body composition. As we age we become less active and thus lose muscle mass and gain fat mass. In fact, when older individuals are placed on an exercise program that includes resistance exercise for muscle development they tend to gain muscle and increase their RMR.

Can we determine Resting Metabolic Rate based on muscle mass?

The equation above is appropriate for inactive adults. However, for leaner, muscle muscular people such as athletes and fitness enthusiasts estimating RMR based on body composition is more appropriate. The equation below is the Cunningham equation and uses fat free mass (FFM) to estimate RMR.  

Estimating FFM is simple once %BF has been determined (below). Begin by calculating Fat Mass (FM) which is body weight time %BF. Then subtract FM from body weight to determine FFM. Assuming our example man (82 kg, 180 cm) from above is also an athlete with 15% BF lets use the Cunningham equation to estimate his RMR.

   Step 1.   Determine %FFM:   100% – 15% = 85% FFM

   Step 2.   Determine FFM:       82kg  x .85 = 70 kg FFM

   Step 3.   Determine RMR      500 + 22 (70) = 2040 calories

You see that the estimate of RMR for our example athletic man, using the Cunningham equation, is much higher than using the Mifflin-St. Jeor equation, which doesn’t factor in body composition.

What is the physical activity factor and how do we estimate it?

The physical activity factor is the energy used by skeletal muscle activity. Simply stated, the more we contract our skeletal muscle the more calories will be used to power this activity. Physical activity includes everything from every day basic tasks such as showering, loading the dishwasher and driving to work to exercise such as running, swimming and dancing.

You can use the physical activity factors (PAF) presented in the PAF Table below to estimate of total calories burned daily. Let’s apply these factors to estimate total calories burned daily for our example man as either an inactive person and as an athlete training most days of the week. 

                        Click for PDF of Physical Activity Factors

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