By Elena Voropay

Trying to determine your caloric need for the day? Daily caloric need represents the number of calories a person needs to sustain current body weight and activity levels, and consists of the resting or basal metabolic rate (BMR), the thermic effect of food, and the calories used in daily activities. Remember that resting metabolic rate is the largest contributor to total energy expenditure, accounting for approximately 60% to 75% of daily energy expenditure. It is a measure of the calories required for maintaining normal body functions, such as heart beat, breathing, thinking, and even regulating body temperature.

Age

The younger you are, the more energy you expand at rest. The reason being is the decline in lean body mass with age which slows the resting heart rate. Remember that muscle mass is your metabolically active tissue that requires oxygen.

Body Composition

As previously mentioned, the amount of lean tissue is one of the major contributors in calorie burning mechanism of the body. To speed up your metabolism, try to add or at least maintain your muscle mass regardless of age or other factors. (BMR is generally reported in kilocalories per kilogram of fat-free mass per minute).

Height/Body Surface

Tall people have a larger surface area, so more heat loss occurs across the skin. Because more energy is needed to maintain their body temperature, tall people have higher BMRs. (For this reason, the resting metabolic rate is often reported in kilocalories per square meter of body surface area per hour).

Growth

Children and pregnant women burn more calories at rest due to energy required for development of new tissues

Body Temperature

The higher the temperature of your body - the greater is your BMR. Exercise may increase the temperature by up to 7 degrees Celcius, thus speeding up metabolism. As a result of sickness or other factors, fever raises resting metabolic rate.

Stress

Stress, both mental/emotional and physical, increases activity of the sympathetic nervous system which increases the BMR. Several stress hormones, such as cortisol, also play a role in body's metabolism.

Thyroxine

This hormone produced by the thyroid gland is a key BMR regulator. There is a direct correlation between higher thyroxine levels and greater basal metabolic rate.

Fasting/Starvation/Malnutrition

Lowering your caloric intake brings about slower metabolism by the mechanism of fasting/starvation hormones.

Environmental Factors

Adjusting to either heat or cold raises the BMR.

The BMR may vary between 1,200 and 2,400 kcal/day. You can increase your resting metabolic rate by adding lean tissue (or building muscle) and by engaging in physical activities. If Individual engaged in normal daily activity ranges from 1,800 to 3,000 kcal. The energy expenditure for very large athlete who trains intensely can exceed 10,000 kcal/day!

There are several formulas used to assess the BMR.

1. One of the simplest formulas, but it does not take into consideration such factors as lean mass, sex, age, height.

BMR = your bodyweight (in pounds) x 12

Example:

For a 200-pound male BMR = 200 x 12 = 2,400 kcal/day

2. BMR = gender factor (1.0 kcal/kg/hr for men; 0.9 kcal/kg/hr for women) x weight in kilograms (1kg = 2.2 lb) x 24 hours

Example:

a) Convert bodyweight in pounds to kilograms by dividing by 2.2: 200 pounds / 2.2 pounds per kilogram = 90.9 kg
b) Multiply weight in kilograms by the BMR factor: 90.9 kg x 1 kcal/kg/hr = 90.9 kcalories per hour
c) Multiply the calories used in one hour by the hours in a day: BMR = 90.9 kcal x 24 hrs = 2,182 kcal/day

3. Harris-Benedict equation provides two formulas for different genders: Men BMR = 66 + 13.8(weight in kg) + 5 (height in cm) - 6.8 (age)

Example:

200-pound (90.9 kg) male who is 30 years old and 6 foot tall:

a) Calculate height in foot to inches: 6 x 12 = 72 inches
b) Convert inches to centimeters (one inch = 2.5 cm) : 72" x 2.5 cm = 180 cm
c) BMR = 66 + 13.8 x 90.9kg + 5 x 180cm - 6.8 x 30yrs = 66 + 1254 + 900 - 204 = 2016 kcal/day

Women BMR = 655 + 9.6 (weight in kg) + 1.8 (height in cm) - 4.7 (age)

Example:

120-pound female who is 30 years old and 5 foot 5 inches tall:

a) Convert bodyweight in pounds to kilograms by dividing by 2.2: 120 lb/ 2.2 = 54.5 kg
b) Calculate height in inches: 5 foot x 12 + 5 = 65 inches
c) Convert height in inches to centimeters: 65" x 2.5 cm = 162.5 cm
d) BMR = 655 + 9.6 x 54.5kg + 1.8 x 162.5cm - 4.7 x 30yrs = 655 + 523.2 + 292.5 - 141 = 1,330 kcal/day

4. If the information about fat-free mass (muscle/lean tissue) is available, the following equation can be used to predict BMR.

BMR = 370 + 21.6 x fat-free mass in kg

Example:

A 200-pound male who has 10% bodyfat

a) Calculate total fat in pounds: 200lb x 10% = 20 lb of fat
b) Assess the amount of lean tissue: 200lb - 20 = 180 lb
c) Convert muscle mass in pounds to kilograms: 180lb / 2.2 = 81.8 kg
d) BMR = 370 + 21.6 x 81.8kg = 2,137 kcal

These formulas will help you determine how much energy you need to maintain the functionality of your body when at complete rest. In other words, BMR in calories shows the amount of food you need to eat in 24-hour period just to keep yourself alive. Note, that 1kcal = 1000 cal, but for ease of use contemporary literature utilizes a simplified version of measure in calories, even though it is inaccurate and should be stressed that the actual numbers are kilocalories (or the above estimated BMR would be 2,137,000 cal/day).

References:

ACSM's Resource Manual for Guidelines for Exercise Testing and Prescription, 3d ed., (Baltimore: Williams and Wilkins, 1998), pp. 390-391.

Cunningham, J.J. (1991). Body composition as a determinant of energy expenditure: A syntheric review and a proposed general prediction equation. American Journal of Clinical Nutrition, 54, 963-969.

Schutz, Y., Jequier, E. Energy needs: Assessment and requirements, in M.E.Shils, J.A.Olson, and M.Shike, eds., Modern Nutrition in Health and Disease (Philadelphia: Lea & Febiger, 1994), pp. 101-111.

Sizer, F., Whitney, E., Nutrition: Concepts and Controversies, 7th ed., (International Thomson Publishing Componay, 1997), pp.338-340.

Wilmore, J., Costill, D., Physiology of Sport and Exercise, 2nd ed., (Human Kinetics, 1999), pp.138-139.