Estimating Daily Calorie Expenditure

Whether you’re trying to maintain your current bodyweight, gain muscle or lose fat, it is necessary to estimate daily calorie expenditure as a starting point for determining the calorie intake appropriate to your goals. The key words here are estimate and starting point. No formula or method of measurement is perfect. No matter how good something looks on paper, what ultimately matters is practical results. Whatever your initial estimate, you will need to keep records of calorie intake and goal-relevant measurements and adjust your intake accordingly.

Your body burns a number of calories every day just to sustain vital organ function – called basal metabolic rate (BMR) or basal energy expenditure (BEE).This can either be directly measured using indirect calorimetry or estimated using various formulas. Additional calories are burned during physical activity and during digestion, which is also referred to as the thermic effect of food (TEF). Total daily calorie expenditure can be estimated by multiplying BMR by an “activity factor”, which also takes TEF into account.

Estimating Basal Metabolic Rate

If measurement through indirect calorimetry is available to you, this is the best option. You may be able to find a local gym, training studio or university that provides this service. If not, there are several formulas that can be used to estimate BMR. I recommend using the Katch-McArdle Formula since it is based on lean mass rather than total body weight.

Katch-McArdle Formula:

• For men and women (metric): 370 + (21.6 x lean mass in kg)
• For men and women (standard): 370 + (9.82 x lean mass in lbs)

Many books and web sites recommend the more popular Harris-Benedict equation, however there are several problems with it, the biggest being a failure to account for body composition. There is a big difference between the basal metabolic rate of a 200 pound man with 10% bodyfat and 200 pound man with 25% bodyfat due to the difference in lean body mass. While the age factor may be intended to account for age-related decline in metabolic rate related to loss of muscle mass, this makes assumptions that are flat-out wrong when applied to people who regularly strength train and probably have better-than-average body composition for their age.

Also, the methods used when the Harris-Benedict equation was developed also failed to account for TEF, so it tends to overstate BMR slightly. Rather than BMR, the result is closer to resting metabolic rate (RMR)/resting energy expenditure (REE). In addition to calories burned sustaining vital organ functions, RMR also includes calories burned due to TEF, which can vary depending on the time between the last meal and testing as well as the macronutrient composition of the meal. If the subject does not fast for an adequate period of time before testing what is being measured is RMR and not BMR.

Harris-Benedict Equation:

• For men (metric): (13.75 x weight in kg) + (5 x height in cm) – (6.76 x age) + 66
• For men (standard): (6.25 x weight in lbs) + (12.7 x height in inches) – (6.76 x age) + 66
• For women (metric): (9.56 x weight in kg) + (1.85 x height in cm) – 4.68 x age) + 655
• For women (standard): (4.35 x weight in lbs) + (4.7 x height in inches) – 4.68 x age) + 655

The biggest downside of the Katch-McArdle formula is most methods of measuring body composition are off by at least a few percent, typically overstating the body fat percentage of very lean individuals and understating the body fat percentage of people with a high amount of body fat. However, the typical 3-4% error in body composition measurements when properly performed is lower than the potential error when using formulas based on total body weight as opposed to lean body mass.

For example, if we apply the Katch-McArdle formula to two 200 pound men – one with 10% body fat, one with 25% bodyfat – we get the following BMR estimates:

• 200 lbs at 10% body fat with 180 pounds lean mass: 2138
• 200 lbs at 25% body fat with 150 pounds of lean mass: 1843

Even if bodyfat percentage was off by 4 percent – high for the leaner man and low for the fatter man, the BMR estimates would be:

• 200 lbs at 14% body fat with 172 pounds lean mass: 2059 (79 lower)
• 200 lbs at 21% body fat with 158 pounds of lean mass: 1922 (79 higher)

Assuming both men are 30 years old and 5’10”, the Harris-Benedict equation would give each an estimated BMR of 2002, despite a significant difference in lean body mass. This results in an estimate that is off by an average of almost twice as much for both the leaner and fatter man (147.5 ± 11.5) than would result from a significant error in body composition measurement. The Harris-Benedict equation should only be used if you are unable to get your body composition tested.

Estimating Calories Burned Due to Activity and Thermal Effect of Food

After estimating your BMR you would need to determine the additional calories burned by activity and digestion. In Exercise Physiology: Energy, Nutrition and Human Performance, the authors provide several “activity factors” to multiply by your BMR to estimate your average daily calorie expenditure. These also account for TEF:

• 1.2 – Sedentary: Little or no physical activity.
• 1.375 – Lightly Active: Light exercise or activity 1-3 days per week.
• 1.55 – Moderately Active: Moderate exercise or activity 3-5 days per week.
• 1.725 – Very Active: Hard exercise or activity 6-7 days per week.
• 1.9 – Extremely Active: Hard daily exercise or activity and physical work

While these activity factors are pretty vague to say the least, keep in mind this is intended as a starting point, and that some adjustment is going to be required based on your results. If unsure of where you fit, it is better to err lower and gradually increase calories, especially if your goal is fat loss. Even if your goal is increased muscle mass, which requires a calorie surplus, it is better to err low at first and gradually adjust it upwards than to start high and find you’re gaining more body fat than muscle.

Making Adjustments

Although the above should provide a reasonably good estimate of your daily calorie expenditure, you will still need to track daily calorie intake and take skin fold or circumference measurements regularly and adjust accordingly. If the estimate is accurate and you are measuring and recording food intake accurately and consuming your estimated daily maintenance calorie level, your skin fold and circumference measurements should not change significantly.

If your goal is to lose fat and you have properly calculated your daily calorie deficit (the difference between your daily calorie expenditure and intake), your weekly fat loss should equal roughly your weekly calorie deficit (daily calorie deficit x 7) divided by 3,500 (the approximate number of calories stored in a pound of body fat). Keep in mind there are many factors such as muscle glycogen levels and hydration that can affect weight loss and gain, so don’t worry too much if you’re off a pound one way or the other as long as your measurements are consistently moving in the right direction.

If your goal is to gain muscle with minimal fat gain, I recommend first reducing your bodyfat to at least the low teens if it is not already there or lower. This will make it easier to determine whether weight being gained is coming from muscle or if you’re consuming too many calories and simply putting on fat. Start with maintenance calories and increase your daily calorie intake every week by 100 to 200 calories per day until your weight begins to increase. Take regular skin fold measurements at your fattest spot – usually the abdomen for men and the suprailiac or mid-thigh for women – and reduce your daily calorie intake to the previous level if the skinfolds go up over a few millimeters. Plan on gaining at least a little bit of fat while trying to increase muscle mass significantly, but do not allow yourself to become too fat.

References

Harris J, Benedict F. A biometric study of basal metabolism in man. Washington D.C. Carnegie Institute of Washington. 1919.

Katch, Frank, Katch, Victor, McArdle, William. Exercise Physiology: Energy, Nutrition, and Human Performance, 4th edition. Williams & Wilkins, 1996.

Alpert SS. A limit on the energy transfer rate from the human fat store in hypophagia. J Theor Biol. 2005 Mar 7;233(1):1-13.