So you say you want to lose weight? One would think that lowering your caloric intake would help do the trick. Yes, this is true, but if you lower it too much you do more harm than good. How low can you go? This number is best determined by figuring out your Metabolic Rate.
In order to figure out your metabolic rate you must know your Resting Metabolic Rate. Resting Metabolic Rate can be defined as the energy required staying alive with absolutely no activity. RMR is sometimes confused with Basal Metabolic Rate. Basal Metabolic Rate can be defined as “the minimal caloric requirement needed to sustain life in a resting individual. This includes the body functions such as circulation, breathing, generating body heat, transmitting messages to the brain, cellular metabolism, and the production of body chemicals.”(Robert Adams, CPT)
An easy way to figure out your RMR is by multiplying your weight by 10. For example someone weighing 110 pounds would have a RMR of 1000.Your RMR can more accurately be determined by using the Harris-Benedict Equation.
| Men: | RMR = 66 + (13.7 x Weight) + (5 x Height) – (6.8 x Age) | |
| Women: | RMR = 655 + (9.6 x W) + (1.7 x H) – (4.7 x A) | |
| Where:W = weight in kg-(weight in lb/2.2/b/kg) H= height in cm-(height in inches x 2.54 cm/in) A = age in years |
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My Example:
RMR=655 + (9.6) x (50.5kg) + (1.7 x 160.2cm) – (4.7 x 26years) ->
=655 + (484.8) + (272.34) – (122.2) = 1289.94 kcal
Now that you know your RMR you can multiply that by your daily activity level and that will give you your metabolic rate. Ex: 1289.4 x .35 =451.5 this makes my BMR 1740.9. This number does not include exercise. Usually this number is too low for someone with a lot of muscle and too high for someone who is fairly over-weight.
| Activity Level | Men | Women |
| Sedentary (Inactive) | 15% | 15% |
| Lightly Active (most professionals, office workers, shop workers, teachers, homemakers) |
40% | 35% |
| Moderately active (workers in light industry, most farm workers, active students, department store workers, solders not in active service, commercial fishing workers) |
50% | 45% |
| Very active (full-time athletes and dancers, unskilled laborers, forestry workers, military recruits in training, soldiers in active service, mine workers, steel workers) |
85% | 70% |
| Exceptionally active (lumberjacks, blacksmiths, female construction workers) |
110% | 100% |
As early as 1991, there was evidence to show that highly restrictive diets were not as effective for weight loss. Frost, et al. (1991) showed that highly restrictive diets tend not to be followed as closely, that people tend to underreport what they are eating (“a few Twinkies don’t really count”), and that people whose diets were based on BMR rather than diet history lost more weight.
Somehow, though, the myth still persists that the less you eat, the more you will lose.
If you eat less than your (BMR) you sacrifice all of your hard earned muscle. The more muscle you have the higher your metabolism will be. When you don’t eat enough your body goes into starvation mode and will use your precious muscle as fuel. Your metabolism will sink faster than the Titanic when you don’t eat the proper amount of food. Every time you eat you increase your metabolism due to the thermic effect of food. When you eat often you greatly increase the number of calories you burn. That sounds great doesn’t it? The days of eating like Paris Hilton are over!
Before you go too crazy remember that there is a catch. You cannot eat too much or you will put your metabolism to sleep. You want to eat until you are no longer hungry, not necessarily until you are full and definitely not stuffed! Your metabolism is like a fire. When you don’t eat enough your metabolism slows down because of a lack of fuel. If you eat too much you put out the fire. Try to find a happy medium and keep that metabolism revved.
Putting together information from several sources (Robert Adams, CPT; ShapeFit.com, and Johnstone et al.), we can determine a fairly exhaustive list of the factors that affect BMR including both normal day-to-day factors and external or event-based factors, sometimes called short-term factors:
| Genetics: | Some people are born with faster metabolisms |
| Age: | As you age your metabolism slows down. It reduces at a rate of about 2-3% per decade. This is due to the loss of lean muscle mass as you age. |
| Weight: | Higher weight = higher BMR and thus higher calorie requirements. |
| Gender: | Men are higher than women overall but not on a muscle-for-muscle basis. Gender is a factor because men are usually larger and have a greater proportion of lean muscle mass. |
| Malnutrition: | Reduces size, may lower BMR, seems to predispose towards gaining fat easier when fed more. |
| Body Composition: | The more lean tissue you have the greater the BMR. This is probably the single-most important factor, since most others relate back to this in some way. |
| Exercise: | Weight lifting can lead to a constant increase in BMR due to the rise in muscle mass. Aerobics do not have the same effect. Cardio does boost total energy expenditure, but does not raise BMR. |
| Hormones: | Raises BMR, particularly the thyroid hormone thyroxin. |
| Sickness (Fever): | Raises BMR because there is an increase in the body’s temperature. The temperature increases in the body because it is trying to battle an illness, which causes a person’s metabolism to increase. |
| Environmental: | Temperature: Heat and cold both raise BMR. |
| Fasting or starvation: | Lowers BMR because your body will try to conserve more energy since it is not receiving the proper amount of fuel. |
| Supplements | Some supplements can raise metabolism. Ephedrine can boost metabolism 3-16% while 5mg to 100mg per kg of body weight of caffeine per day speeds metabolism up to 7%. |
| Stress: | Stress boosts BMR because of an increase in the use of stress hormones. The release of these hormones is due to an increase in heart and respiration rate. |
| Pregnancy | Raises BMR because your body has to work harder to take care of two people. |
Final Thoughts
Understanding how your metabolism works is imperative to achieving your fitness goals. By computing your caloric requirements you can salvage your hard earned muscle by making certain that you eat enough. You will also save yourself from wasting time with uneducated guesses. Some people have the tendency to overestimate their needs while others severely underestimate their needs. The formulas above will be a valuable guide for you as you persist on your fitness journey. Now the only thing left to do is go to the gym and put in the hard work!
References:
Adams, R. (n.d.). What is Basal Metabolic Rate?. Retrieved April 24, 2009 from
http://www.body-perfect-fitness.com/Free-Fitness-Articles.html
Mardiastuty, T. (n.d.). Resting Metabolic Rate Calculator. Retrieved May 1, 2009 fromhttp://www.bodybuilding.com/fun/calrmr.htm
“Factors That Affect Your Metabolic Rate” http://completefitnessconcepts.com/BodyGem/index_files/MetAffects.htm
Johnstone, A., Murison, S., Duncan, J., Rance, K., & Speakman, J. (2005). Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine1–3. The American Journal of Clinical Nutrition. 82, (5). Retrieved April 30 from http://www.ajcn.org/cgi/reprint/82/5/941
Frost, G., Masters, K., King, C., Kelly, M., Hasan, U., Heavens, P., White, R., & Stanford, J. (1991). A new method of energy prescription to improve weight loss. Journal of Human Nutrition and Dietetics. Vol. 4
Also see:
Frost, G., (1997). Commentary on Frost, G., Masters, K., King, C., Kelly, M., Hasan, U., Heavens, P., White, R. & Stanford, J. (1991) A new method of energy prescription to improve weight loss. Journal of Human Nutrition and Dietetics; 4, 369–373. Journal of Human Nutrition & Dietetics, Vol. 20 Issue 3. Retrieved May 1, 2009 from Health Source: Nursing/Academic Edition database
http://www.shapefit.com/basal-metabolic-rate.html
