Metabolism and Daily Caloric Requirement


 by Max Wettstein, Copyright 2012

“I have the metabolism of a hibernating bear.”  This is how one of my fellow pilots describes himself, as he psyches up for another round of cardio on the hotel elliptical-trainer.  Does this sound like a fitting description for you?  Perhaps you’ve been genetically blessed with a fast metabolism, more like that of a hummingbird, and your only issue with calories is consuming enough of them.  Our metabolism is the single biggest factor in determining our daily caloric requirement.  Our caloric requirement does vary day to day depending on our activity level, but our ‘resting’ caloric requirement can only be changed by altering our metabolism.  Genetics play a huge role in determining metabolism, but there are many other factors that affect metabolism that you can take advantage of.

    Metabolism is defined as the sum of all physical and chemical changes involving energy and material transformations that occur within all our living cells.  Each cell has its own energy production center known as the mitochondria, and it is this collective energy production of all cell mitochondria that determines metabolic rate.  Along with energy production, two fundamental processes of metabolism are anabolism and catabolism: building up tissue and breaking it down, respectively.

    When discussing metabolism, it makes sense to begin with our resting metabolism for simplification.  Our metabolism while at complete rest is known as our Basal metabolic rate, or BMR.  Our BMR is affected by many factors, many of which we can influence.  These factors in no particular order include but are not limited to: genetics, gender, age, body-type, diet, body temperature, environment, hormones, and body-fat percentage/lean-body mass.  More discussion on this later, but you’ll see why your lean-body mass has the greatest effect of all, aside from genetics, in determining your BMR.  Muscle burns calories at rest, while fat does not.

    For simplification, we’re focusing on ways to increase BMR, or resting metabolic rate, so we can burn more calories while sitting around playing Solitaire.  But let’s take a moment to discuss exercise as a factor in increasing BMR.  Although we’re clearly not at rest during exercise, a long-lasting effect on our BMR occurs after we finish our session.  Exercise provides a 3-fold, lasting benefit to boost metabolism.  First you burn calories during the exercise session itself.  Secondly, your metabolism stays slightly elevated for several hours after your session depending on the intensity and duration.  Thirdly, after a nutritious recovery meal and a good night’s sleep, you build muscle tissue while your body repairs itself, further increasing your lean-body mass.  Once again we see the fundamental role of exercise in our overall health and fitness.

                Factors affecting BMR:


  1. Genetics. Genetics have the greatest role in determining our BMR, however, we can not change our genetics, so nothing to discuss there.
  2. Gender.  Because men generally have a less body fat and a greater lean-body mass, men generally have a higher BMR than women.
  3. Age.  As we age our BMR tends to decrease about 2% per decade after age 20.  This slowing primarily occurs due to loss of muscle mass and lower hormone levels.  Unfortunately, if we change nothing as we age, we get fat.  What a deal, huh?!
  4. Body Type.  This refers to the Ecto-, Meso-, and Endomorph classification.  For simplification, it is theorized that taller, thinner body-types have a higher BMR than shorter, heavier body-types.   
  5. Body Temp.  For every increase of 0.5 degree C in internal temp. BMR increases by about 7%.  The chemical reactions in your body actually occur more quickly at higher temps.  When you have a fever, your caloric requirements go up considerably, but nobody wants the flu!
  6. Environment.  Exposure to cold temperatures causes an increase in BMR as the body creates extra heat to maintain its core temp.  Prolonged exposure to heat can also raise BMR.
  7. Hormones.  Many of the body’s hormones influence BMR, but by far the Thyroid gland plays the biggest role in regulating metabolism.  Thyroxin secreted by the thyroid, can alter BMR by 50%, faster or slower.  If you experience rapid weight loss or gain without a concurrent change diet or exercise, you should have your thyroid levels checked.  Likewise, if your temperature seems to chronically run low or high and you are not ill, it could be a sign of improper thyroid regulation.  The adrenal glands which secrete epinephrine, (adrenalin), among other hormones, can significantly alter BMR as well.
  8. Exercise.  See earlier discussion.
  9. Diet.  Eating frequent, smaller meals is an excellent way to boost your metabolism.  Careful though, because this does not mean eat additional calories.  Divide up your daily caloric requirement into 5 to 6 smaller meals/snacks each day to keep your furnace stoked.  This really works.  None of your meals should be large enough to make you feel ‘stuffed’, yet you should never have to feel hungry either.  Furthermore, fasting or serious abrupt calorie-reduction will dramatically slow your BMR by up to 30%, as your body prepares for potential starvation.  Thanks to our ancient hunter-gatherer ancestors, our bodies are evolved to efficiently store surplus calories as fat and slow the metabolism whenever starvation is perceived.
  10. Lean-body Mass.  This is essentially the combined mass of all the tissue in your body minus the fat tissue.  It is determined by obtaining your percentage of body fat through hydrostatic weight measurement or through special measurements with calipers by a Personal Trainer.  The greater your lean-body mass, (muscle-to-fat ratio), the higher your BMR will be.  Again, it is the muscle cells that have the mitochondria energy production centers that continuously burn calories, 24-7, not your fat cells.  This is good news since you can easily use this to your advantage, simply by adding strength/resistance exercise to your lifestyle.  This is especially important as we age to offset the natural loss of muscle tissue that occurs, not to mention strength training increases bone density.  As a general reference, men should strive for 15% body-fat or less, while estrogen-challenged women should strive for 25% body-fat or less.  (No offense girls.  Estrogen promotes fat storage.)

 Determining your BMR and total daily caloric requirement:

    1. ‘Harris Benedict’ Formula.  Calculates BMR, (daily resting caloric requirement), using age, gender, height and weight:


Women: BMR = 655 + (9.6 X weight in kg) + (1.8 X height in cm) – (4.7 X age in years).


Men: BMR = 66 + (13.7 X weight in kg) + (5 X height in cm) – (6.8 X age in years)


*1 inch = 2.54 cm   *1 kilogram = 2.2 pounds

    1. ‘Katch-McArdie’ Formula.  Calculates BMR using lean-body mass, (obtained through body composition analysis using hydrostatic weighing or calipers).  This formula is more accurate:

Men and Women: BMR = 370 + (21.6 X lean mass in kg)


    1. Total Daily Caloric Requirement.  The above two formulas give you your resting caloric requirement (BMR), factoring in NO activity.  To calculate total daily caloric requirement, using BMR from above:

For both men and women, Total Daily Caloric Requirement = BMR multiplied by …

1.2 if you’re sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, and 1.9 if you’re extra active.

*Sedentary = little or no exercise.  Lightly active = light exercise/sports 1-3 days per week.  Moderately active = moderate exercise/sports 3-5 days per week.  Very active = hard exercise/sports 6-7 days per week.  Extra active = very hard daily exercise/sports & physical job or 2X day training.

So you may have been born an ‘Endomorph/Neanderthal-type’ with the metabolism of a hibernating bear, but there is still hope for you even if you can’t change your genetics.  The moral of the story is, exercise to maintain your muscle mass so you will burn more calories at rest, and divide your total daily caloric intake into smaller more frequent meals.  Careful that you’re not just eating more food in the process though!  Keep the furnace stoked but not stuffed, with quality food choices.  As we age, keeping our body-fat down gets more and more challenging, but it really is a matter of maintaining your lean-body mass that ultimately determines your metabolic rate. 


Sources:; ‘FitnessWave’ Mobile Hydrostatic Body Fat Testing; ‘Encyclopedia of Modern Body Building’ by Arnold Schwarzenegger; ‘The Complete Nutrition Guide’ by the MT Sinai School of Medicine.

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