Calculating Metabolic Rate and Caloric Intake

Dear Lyle,

I am unclear on the determinants of metabolic rate and how to determine how many calories I should be taking in to either maintain my bodyweight, lose fat or gain muscle. What determines metabolic rate and how can I determine mine?

There are three major factors which determine how many calories we need on a daily basis.

1. Resting Metabolic Rate
2. Thermic Effect of Activity
3. Thermic effect of Food

Resting energy expenditure (REE).  REE represents the number of calories needed by the body to sustain itself at rest (I’ve joked that it should be called the couch energy expenditure since it’s how many calories you’d burn sitting on the couch all day). REE typically comprises approximately 60-75% of the total caloric expenditure per day. In general, REE is linked with total body mass as well as lean body mass. Due to difficulties in measuring lean body mass with 100% accuracy, I prefer to determine REE using total body mass. There are numerous equations to estimate REE. However the easiest method is to multiply total bodyweight in pounds by 10-11 calories per pound total body weight. Women should use the lower value, men the higher. Again, this number represents how many calories the body will burn assuming zero activity.

Thermic Effect of Activity (TEA).  TEA includes general moving around, shivering, AND exercise. Depending on the frequency, intensity and duration, exercise can increase total caloric expenditure by 15% (very sedentary) to 30% or more (very active) over baseline levels. Although it is possible to calculate the number of calories burned with varying types of exercise, it is generally sufficient to simply estimate the number of calories burned with activity. The level of activity in a day will determine the increase in caloric requirements over REE. Even someone who is totally sedentary will need to adjust REE upwards by at least 30%. See Fig 1 below for REE multipliers:

Fig 1: Multiplication modifiers for activity level Low activity: 1.3 Medium activity: 1.5 High activity: 1.7 Excessive activity: 2.0

The Thermic Effect of Feeding (TEF).  TEF represents the slight increase in metabolic rate which occurs when food is ingested. The term Specific Dynamic Action (SDA) of food is also used. The three macronutrients: carbohydrate, protein and fat have different SDA values. Protein has the highest SDA, burning off 25% of it’s total calories during digestion. That is, if 100 calories of protein is eaten, 25 calories will be burned during digestion. Carbohydrate is slightly less, having a SDA of 15-20%. Fat has the lowest SDA, approximately 3%. As a general rule, TEF will increase caloric requirements by roughly 10% per day.

So an example calculation for a male, with moderate activity (weight training 3 days per week) who weighs 170 lbs would be.

REE = 170 lbs * 11 cal/lb = 1870 cal/day TEA = 1870 * 1.5 = 2805 cal/day TEF = 2805 + 10% = 3080 cal/day

This is the number of calories needed to maintain body weight on daily basis.

Fat loss
Losing fat is a function of burning more calories than you consume. And to lose 1 lb of fat requires that you burn 3500 more calories than you consume. Thus the typical recommendations are to either decrease caloric intake by 500-1000 cal/day OR increase activity by the same token. Alternately some mix of caloric restriction AND increased activity is typically recommended. I generally find that dieters tend to decrease calories by too much AND increase activity (especially aerobics) far too much. And while this invariably works fine for fat loss initially, a plateau is quickly hit and fat loss slows. Frequently muscle is also lost, something which should be avoided. The absolute largest caloric deficit that should EVER be created is 1000 calories per day and this may even be too high for some. Please note that this is the TOTAL deficit from caloric restriction and increased activity. So you don’t get to cut calories by 1000 per day AND increase cardio at the same time. If you happen to be involved in an endurance sport and are burning 1500 cal/day with activity, this means that you would have to INCREASE caloric intake by 500 per day for optimal fat loss to occur. I realize that this is counter intuitive to everything dieters have been told but please trust me that it works. In practice, this works out to a low end caloric intake for optimal fat loss of around 11-12 calories per pound (women should use the lower number, men the higher). Going below that does not improve fat loss significantly but does increase metabolic slowdown and muscle loss.

For our same 170 lb person above, maximal fat loss would be achieved at 170 lb * 12 cal/lb = 2040 Which is right at a 1000 calories/day deficit.

Muscle gain
In the same way that fat loss requires a caloric deficit to occur, in all but a few cases (see next question), muscle gain requires a caloric excess of some sort. While it’s wonderful to hope that calories to support muscle growth will be pulled from fat stores, this tends not to happen except in beginners and those using repartitioning drugs like clenbuterol. Sadly there is no hard and fast rule for just how many calories an individual may need to support muscle growth. Someone with a very high metabolic rate may need in excess of 25 cal/lb. Someone with a lower metabolic rate may only need 20 cal/lb. I generally recommend 20% above maintenance as a good starting point which, in practice, works out to 18 cal/lb or so.

Adjusting caloric intake
Please note that all of these calculations are estimates only and there is a great degree of variability in metabolic rate, etc. I recommend that individuals use semi-frequent (every 2 weeks or so) body composition measures to adjust their caloric intakes. If you goal is fat loss, you want to find that ideal calorie level which maximizes fat loss while minimizing (or eliminating if possible) muscle loss. If muscle gain is you goal, there will be some optimal level where muscle gain is optimized by fat loss is minimized (keeping in mind that some fat must be gained as you are gaining muscle). If at all possible, try not to change too many variables at once when dialing in your caloric intake for either fat loss or muscle gain. Decreasing calories and adding a thermogenic like ephedrine will make it impossible to tell if the calorie change or the ephedrine caused the fat loss.rotein and fat so make the days ratios come out to Zone ratios

About the author

Lyle McDonald

Lyle McDonald+ is the author of the Ketogenic Diet as well as the Rapid Fat Loss Handbook and the Guide to Flexible Dieting. He has been interested in all aspects of human performance physiology since becoming involved in competitive sports as a teenager. Pursuing a degree in Physiological Sciences from UCLA, he has devoted nearly 20 years of his life to studying human physiology and the science, art and practice of human performance, muscle gain, fat loss and body recomposition.