The reason we need to eat is to supply our body with energy, which it needs to think, run, swim and much more. The obesity crisis proves there is a disparity between what we eat and what we need. There is always a focus on how much we eat and what we eat. But what about the way our body spends the energy? Actually understanding our bodies energy expenditure puts it all into place.
The energy we need each day is called the total daily energy expenditure (TDEE) and it is determined by three components. These are the basal metabolic rate, the energy cost of physical activity and diet-induced thermogenesis. To get a clear picture of how these relate we’ll follow the average man and woman from the USA. Let’s name them Joe and Anna. Joe is 175.7 cm tall, weighs 90.2 kg, and is 30 years old. Anna is 161.8 cm tall, weighs 78.4 kg, and is also 30 years old.
Basal Metabolic Rate
The basal metabolic rate (BMR) is the energy expended by an organism when it is at complete rest. Complete rest is when there is no physical activity or digestion going on. BMR can be determined by measuring the heat production of an individual when he or she is at complete muscular repose, and when enough time has passed after the last meal. The meal must be digested and fully absorbed, which is after a time of 12-14 hours. This state is the so-called ‘post-absorptive state’.
The BMR is the largest fraction the daily amount of energy expenditure. There are multiple equations to approximate its value. The latest Harris-Benedict equation is one of them:
- Men: BMR = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) + 5
- Women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) – 161
Now we can determine how high the BMR is for Joe and Anna. Filling in the formula gives 1855 kcal/day for Joe and 1484 kcal/day for Anna.
Energy cost of physical activity
Muscular activity belongs in the second category, but how does physical activity compare to BMR? Any physical activity, whether you are walking to the supermarket or doing a marathon, uses energy. Muscles burn fuel to keep the body in motion. The amount of energy that the muscles require is highly dependent on the type and length of an activity. Sprinters need vastly more energy per minute than marathon runners. Yet marathon runners run for hours, while the record for a 100-meter dash is 9.58 seconds.
Besides the activity itself the state of the body also contributes. The heavier you are the more energy something usually costs. A rock climber of 80 kg will have to use more energy than a climber of 50 kg to climb the same rock. This is why, for an obese person, simply walking a couple kilometers consumes a lot of energy; all the extra mass that has to be transported makes a stroll through the park an intense exercise.
To get an idea of how much the energy cost of physical activity contributes, let’s assume both Joe and Anna run for 30 minutes three times a week. Joe will burn approximately 451 kcal/run and Anna 392 kcal/run. Multiply each by 3 for the number of runs in a week and divide by 7 for days in a week and the daily energy expenditure from physical activity is 193 kcal for Joe and 168 kcal for Anna.
Diet-induced thermogenesis (DIT) is the smallest component of daily energy expenditure. It has the following definition:
Diet induced thermogenesis (DIT) can be defined as the increase in energy expenditure above basal fasting level divided by the energy content of the food ingested and is commonly expressed as a percentage.
– Westerterp 2004
The amount of DIT after a meal depends on the macronutrient composition of that meal. Carbohydrates and fats don’t affect the DIT that much, but protein content is positively correlated with the amount of DIT. A regression analysis of 19 studies showed that an increase of the protein fraction of 1%, results in an average increase of 0.22% DIT (in percentage of the meal intake). Most studies find values of 5-9% of intake for meals of common macronutrient composition.
To find out how much Joe and Anna burn due to DIT, let’s assume they burn 7% of their meal intake as DIT. To calculate this we take the sum of basal metabolic rate and energy cost of physical activity and multiply by 1.07:
TDEE = 1.07 x ([Energy from BMR] + [Energy from physical activity])
For Joe, we had 1855 from BMR and 193 from physical activity. Together with DIT, this becomes a total of 2191 kcal/day.
For Anna, we had 1484 from BMR and 168 from physical activity. Together with DIT, this becomes a total of 1767 kcal/day.
Now that Joe and Anna know roughly the amount of energy they use every day, they also know how much they can eat every day to maintain their weight.
Your dietary requirement
The amount of energy we need is different for everybody, depending on sex, height, weight, age, level of physical activity, and a variety of other things. The values in this post for the average man and woman in the USA are examples. I cannot claim they will work for you! Just as an example: Hafþór Júlíus "Thor" Björnsson, Icelandic Strongman and Gregor ‘The Mountain” Clegane in the show Game of Thrones, needs a 10,000 kcal/day diet to maintain his physique!
This post in movie form:
J. Arthur Harris – A biometric study of human metabolism
Latest Harris-Benedict equation
Height and weight statistics in the USA
Diet induced thermogenesis
Hafþór Júlíus "Thor" Björnsson