Metabolic Rate and the Calorie Conundrum

Your calorie intake is a very important part of achieving and maintaining your body weight goals. Knowing a target calorie level is helpful especially when your body composition goals are specific. Unfortunately, the educated guesses that many individuals rely on are typically way off. Some end up taking in far too few calories, while others are well above their actual needs. The end result is frustration and a lack of goal attainment.

It should be clear that figuring out your estimated daily calorie requirement is pretty important. To that end, let’s briefly review the difference between basal and resting metabolic rate and look at the other factors that affect your caloric needs. Hopefully you can then apply this information to your specific goals.

Basal metabolic rate (BMR) is the lowest rate of energy expenditure aside from sleeping. It is generally measured after a 12-hour fast, in a reclining position and in a climate-controlled setting. It is essentially the amount of calories the body needs to function physiologically on a daily basis. Resting metabolic rate (RMR) is slightly higher than BMR and includes some calories used for both eating and previous muscular activities. The two values are thought to differ by less than 10%, though they are often used interchangeably.

There are numerous ways to figure out an estimate of your RMR. One involves simply adding a zero to your weight. This very simplistic method can be relatively accurate for small and lean individuals, but becomes very inaccurate with larger folks. There are also several equations one can use, which use height, weight and age as the primary variables. These include the World Health Organization, Harris-Benedict, Owen and Mifflin equations, with Mifflin winning out as the most accurate across a range of different body types and sizes. The Mifflin equation appears below:

Men: (10 x wt) + (6.25 x ht) – (5 x a) + 5 =

Women: (10 x wt) + (6.25 x ht) – (5 x a) - 161 =

wt = weight in kg ht = height in cm a = age in years

There are several other equations that were developed for more active individuals, and they use lean body mass as the primary determinant for calorie needs. Remember, organ tissue and muscle mass are highly metabolic tissues and contribute significantly to daily RMR, so excluding body fat from the calculation for athletic populations may be reasonable since it is less metabolically active. The two most popular are the Cunningham equation and the Katch equation. The Katch equation appears below. Notice that the same equation is used for both men and women.

(LBM x 21.6) + 370 =

LBM = lean body mass in kg

The best method for determining your RMR is to get it checked by a qualified professional using an indirect calorimetry device. This generally involves a basic breath test to measure oxygen and carbon dioxide use by the body. There are plenty of options for this including Medgem, Korr, New Leaf and other more traditional metabolic cart systems that you might find in hospitals or university settings.

Once an estimate of RMR has been established, you have to factor in several other variables to figure out your total daily calorie needs. Activities of daily living, the thermic effect of exercise and the thermic effect of food (digestion and assimilation) account for the remainder of your calorie goal. The thermic effect of food often accounts for about 5-10% of total daily energy expenditure, though it is slightly higher for those that consume a high-protein diet. The calories needed for exercise and daily activities can be difficult to estimate, but there are several charts and websites that can be used. You can enter your weight and the duration of an activity to get an estimate of calories expended.

If you’re looking to gain or lose weight, you obviously need to adjust your calorie intake in one direction or the other. Five hundred calories up or down is a good start, since it takes a 3500 calorie deficit to lose one pound of fat per week and between a 2500 and 3500 calorie surplus to gain about a pound of lean weight per week. As always, the best way to determine the appropriateness of your estimation is to test it out for a week or two, monitoring your calorie intake and overall weight balance. Then, make any necessary adjustments on an as needed basis. If you need help figuring out where should be in relation to your health and fitness goals, feel free to email me for guidance.