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  • Isaac J. Wedig MS, CSCS

Metabolism, Part One: What is metabolism?

Updated: Feb 20, 2018



Metabolism; it’s one of the most popular buzz words in the health and fitness industry.

If you’re overweight and seem to have a hard time losing body fat, you would typically be regarded as having a slow metabolism. Alternatively, we all know that one person who is able to eat whatever they want with reckless abandon and never gains an ounce of body weight. We tend to think of metabolism in terms how well we are able to get away with eating junk food. If you’ve got a slow metabolism, forget about pizza, ice cream, and potato chips, you better stick to your chicken breasts and vegetables.


We all have a general idea of what metabolism is and as a society we don’t have a problem in acknowledging its general importance when it comes to weight management, health, and fitness. However, after doing some polling and asking a few individuals how they would personally define metabolism, I’ve realized that our understanding is fairly vague, even in health and fitness professionals. Before researching the question, I’m not sure that I would have been able to provide a very specific and accurate definition either.


Understanding metabolism is key to understanding weight loss and weight gain. The metabolism is a very dynamic and complex system with many moving parts. However, it can be simplified and more easily understood by breaking it down into its component parts and then learning how each part is effected by genetics, the environment, the diet, and exercise.


Lets dive in! First off, lets get a general definition and explanation of what metabolism is.



What is metabolism?


The definition of metabolism that you’ll find in your biology textbook is;


“the sum of all chemical reactions that occur within a living organism”


Lets break that down and make some sense of it.


Growth, reproduction, maintenance of body structure, and the ability to respond to the environment are all fundamental components of sustaining life and each one of these vital processes is carried out through a series of complex chemical reactions that occur within the body. These chemical reactions require energy in order to occur. In other words, an organism requires energy to stay alive. If this comes as new information to you, you should probably stop reading now and reevaluate your life.


Metabolism, or metabolic rate, can be therefore be defined as the total amount of energy that is needed in order for these life-sustaining processes to take place. Another way to think of metabolic rate is in terms of energy expenditure. Commonly, when we think of energy expenditure, we think about the number of calories that we burn after running on the treadmill. Although exercise and physical activity is definitely a part of our daily energy expenditure, we often fail to realize that the largest portion of energy that we expend per day actually goes toward much more basic, yet necessary, bodily processes, such as breathing, circulating blood, repairing cellular damage, powering cellular growth, maintaining nervous function, hormonal production, and maintaining electrolyte balances inside and outside of cells.


These processes occur non-stop within our body, even when we are sleeping. The amount of energy that it takes to perform these basic life sustaining functions is termed your Basal Metabolic Rate or BMR and can be viewed more simply as the number of calories that you would expend if you were to sit motionless and relaxed in a chair all day long. In this state, your body expends the minimum amount of energy that is required to stay alive. This is the most fundamental and largest component of metabolism.


However, most of us don’t get the luxury of napping all day long. Therefore, BMR is not the only factor that contributes to our total daily energy expenditure or out metabolism. It takes energy to eat and digest food, go to work, walk to the car, play with your kids, exercise, and brush your teeth. All these tasks require energy above and beyond our BMR and thus result in an increased metabolic rate and total daily energy expenditure.


Now that you have a general idea of what metabolism is, lets look at each of its components.


Total Daily Energy Expenditure (TDEE)


Total Daily Energy Expenditure or TDEE, refers to the total amount of calories that an individual burns within a 24-hour period and is comprised of three different components:


1) Basal Metabolic Rate (BMR)


As already discussed, BMR refers to the number of calories that you expend at rest, or more precisely the number of calories required to undertake basic life-sustaining functions. BMR comprises the largest portion of your TDEE, about 60-75% of the calories that you burn per day, and varies from person-to-person depending on genetic factors, bodyweight, age, and sex.


BMR is most highly correlated to lean body mass, or the amount of bodyweight that is attributed to muscle, bone, connective tissue, and organ mass. In fact, about 80% of the variance observed in BMR between individuals is explained by lean body mass, leaving only 20% of this variance to be explained by alternative variables such as genetics and fat mass. Relative to lean body mass, BMR has little variance, varying a maximum by about 20% between two people with the same lean body mass. Intuitively this makes sense. An individual that has more metabolically active tissue, in other words, calorie consuming tissue, should have a higher energy demand in order to undertake all the processes that are needed to sustain it. After the age of 20, BMR typically reduces by about 1-2% per year and is due primarily to losses in lean body mass that occur with aging, particularly losses in skeletal muscle.


2) Thermic Effect of Food (TEF)


When we consume food, it takes energy to digest, absorb, process, and store it. The energy that is required for handling food is termed the Thermic effect of food (TEF). Each time we consume a meal, energy expenditure increases in proportion to the size of the meal. TEF accounts for a relatively small amount of your TDEE, only about 6-12%, and is directly proportional to how much overall food you consume per day. The higher your caloric intake, the higher your TEF. The macronutrient composition of the diet also effects TEF. The different macronutrients require variable amounts of energy to digest and process and therefore differing the proportions of the macronutrients within your diet can lead to different total daily thermic effects. Protein has the highest thermic effect, followed by carbs, and the lowest is fat. Dietary fiber has a high thermic effect as well, typically providing little to no calories itself, however, still requiring digestive efforts from the body.


3) Activity Thermogenesis (AT)


Activity Thermogenesis (AT) can be simply defined as the amount of energy that is needed to power all physical bodily movements each day. AT is the most variable part of TDEE, accounting for roughly anywhere between 10-40% of TDEE, depending on how active an individual is. AT is the most controllable and malleable aspect of TDEE and can be further broken down into two subcategories;


Exercise Activity Thermogenesis (EAT) which refers to formal physical activity that is undertaken for the purposes of health or sport, what most of us would consider “exercise”.


Non-Exercise Activity Thermogenesis (NEAT) is the energy expended during of all of the physical activities other than volitional sporting-like exercise, including walking to your car, using hand gestures as you speak, tapping your foot, playing an instrument, and using postural muscles to hold yourself upright.




Typically, we restrict formal exercise to a small window of time each day, usually 1-2 hours. NEAT, on the other hand, is comprised of all the physical activities that take place during the entire day. The average person usually spends about 18 hours awake each day, meaning that there are 18 hours for energy expenditure from NEAT to accumulate. NEAT takes up about 10x more time during day than formal exercise in the average person. Typically, NEAT is the result of unconscious physical activity and some individuals appear to display greater tendencies or genetic predispositions to higher levels of NEAT. However, these predispositions can be overridden by conscious interventions that attempt to increase physical activity levels throughout the day, perhaps through performing work at a standing desk, taking the stairs instead of elevator, or walking to work.


For the majority of “exercisers” who work out 3-4 times per week, exercise adds up to an average daily energy expenditure (average across 7-days) of about 100-400 calories per day depending on bodyweight, exercise intensity, activity type, and exercise duration. NEAT on the other hand can account for up to 2,500 calories of expenditure per day in a very active individual. That’s about 5x more calories per day. NEAT represents the most variable component of TDEE between individuals and can vary by as much as 2,000 calories per day depending on occupation, geographic location, and over lifestyle factors.



Variability in TDEE


According to the Dietary Guidelines of Americans, the average American male expends about 2,400 calories per day while the average female expends about 2,000. However, these TDEE averages can vary extensively, by as much as 2,000 calories, between two people of the same sex, weight, and body composition. That’s a lot of variability. But where does it come from? Why do some people have so called ‘fast metabolisms’ and other ‘slow metabolisms’? Knowing that BMR and TEF don’t vary significantly between two people of similar weight and lean body mass, we can see that most of the variation we observe in metabolic rate comes from physical activity levels, and most significantly, NEAT.


If we are familiar with the energy balance equation (‘energy in’ versus ‘energy out’) then we know that energy expenditure, is a pretty important part of weight management. In order for weight loss to occur, energy expenditure must exceed energy consumption. For weight maintenance, expenditure has to equal energy intake. Therefore, we can see the merit to having overall higher energy expenditures, or in other words, a faster metabolism. The question becomes, how do you increase your metabolism? And how much can you elevate it by?


In order to answer these questions, we have to firstly consider which factors of TDEE are even within our control. If we can’t control it and manipulate it, then it’s a lost cause. With genetics, for example, there isn’t much you can do to change them. While a lot of us like to blame a slow metabolism on genetics alone, it is only one piece of the puzzle. There are many other factors within your control that influence metabolism. You are not entirely doomed by genetics.



How can you increase your metabolism?


If we want to alter our metabolic rate, we have to first examine each of the components of TDEE, identify how we can alter each one, and by what extent.


Basal Metabolic Rate


We can’t change our sex, our age, or our genetics. However, we can change the amount of lean body mass that we possess. Since lean body mass is the highest correlated variable to BMR, then increasing it, primarily by means of increasing the amount of skeletal muscle mass, can improve our BMR. Studies have demonstrated that combing resistance training with an adequate intake of dietary protein can facilitate increases in muscle mass and raise BMR by up to 7-8%. This equates to an increased energy expenditure of approximately 100-200 calories per day for the average person. While building muscle does indeed increase metabolism, its effects on BMR are often a bit overstated. A common claim in the fitness world is that 1lbs of muscle burns an additional 50 calories per day, which simply isn’t true. This number is probably more like 7 calories per day. Although muscle mass has an effect on BMR, it is rather small. Possessing more muscle mass also has an effect on activity thermogenesis. Larger muscles have the potential to perform greater quantities of work and power during exercise and activity, thus potentiating greater overall TDEEs through enhanced formal exercise and NEAT.


One more factor that can effect BMR is environmental temperature. The body works to regulate a constant internal core temperature. When body temperature deviates from about 98.6 degree, the body has to work and expend energy to get back to its comfortable set-point. When we overheat, we sweat. When we get cold, we shiver. These compensatory adaptations are energy demanding processes that work to bring us back to our normal core temperature. The more time that you spend outside of a thermoneutral zone, and the more extreme the temperature shift is, the more energy that the body has to expend to regulate core temperature. Although this is a viable way to alter BMR, it isn’t a very practical tool for increasing metabolism, simply because its uncomfortable to be either extremely hot or extremely cold for a long enough period of time to cause a meaningful increase. The resulting BMR variations due to environmental temperature are usually small and often times are not under our control, depending on your job and geographic location in which you live. Also, in this day and age, with clothing, air conditioners, and heaters, its rare for humans be subject to extreme thermal neutral shifts for long periods of time.


Thermic Effect of Food


Quite simply, TEF can be increased by eating more food. However, eating more food isn’t the best thing to do if you are trying to lose weight. On the net balance, if we eat more food, we also take in more calories and we push the energy balance equation in the wrong direction. The increases in energy expenditure that we get from eating more do not outweigh the number of calories contained in the extra food. However, there are ways to increase TEF without manipulating calorie intake. This is by eating a greater proportion of our calories from protein and by also consuming higher amounts of dietary fiber.


Protein not only aids in muscle building, BMR improvement, and higher potential AT, but it also has the highest thermic effect out of the three macronutrients. Protein has a thermic effect of about 25-30%, whereas carbs and fats are 6-8% and 2-3%, respectfully. This means that if you consume 100 calories worth of protein, the actual net calorie yield in your body after its digestion and processing is only about 75 calories. Alternatively, 100 calories worth of carbs would yield a net 92 calories and fat would yield about 97 calories.


Often times fiber is poorly digested and absorbed within the body and usually provides little to no calorie yield. However, it still requires movement through the digestive track and an input of energy to handle, thus helping to increase energy expenditure without a proportional increase in calorie intake.


When it comes to metabolism, the concept of meal frequency is always at the forefront of most people’s minds. If you want to increase your metabolism its often suggested to eat smaller meals more frequently throughout the day. You have to keep stoking the fire! Right? This is not the case. Turns out your metabolism isn’t actually anything like a camp fire. When calorie, macronutrient, and fiber intake is equated across the day, meal frequency and meal number does not have a differential effect on TEF or total dialy energy expenditure. In other words, meal distribution does not significantly effect metabolism.


The effect of increasing your protein and fiber intake on metabolism is fairly minimal and most likely only represents an increase in total daily energy expenditure of maybe 100-200 calories per day, when total calorie intake is held constant. This represents a normal situation where optimal protein and fiber intake is achieved within the confines of your caloric intake. Like BMR, TEF is isn’t that variable within an individual if your controlling for calorie intake and bodyweight.


Activity Thermogenesis


Activity thermogenesis is the most variable aspect of TDEE and the one that you have the most control over. Therefore, AT is where you are best suited to direct your efforts. If you want to increase your TDEE, then increase your physical activity level! This seems pretty intuitive and straight forward, however many people seem to overlook its importance in increasing metabolism, especially when it comes to NEAT. While introducing formal exercise is a great idea, being a little more conscious of your daily NEAT levels is usually where you can have the greatest impact on your total daily energy expenditure. In the grand scheme, you can increase your energy expenditure by a far greater extent with NEAT than you can with formal exercise.


Walk or ride your bike to work, stand at your desk instead of sitting, go for a walk on your lunch break, use the stair instead of the elevator, set a daily step goal, focus on using good posture, park further away at the grocery store, play with your kids, and try to interject a little more physical activity into your daily routine wherever you can. If you can find a way to burn just 1 extra calorie per minute during the day, that equates to 60 extra calories per hour, and possibly 1,000 extra calories per day! This could represent a 30-50% increase in your TDEE if your current NEAT levels are relatively low. Now that’s a metabolism boost!


Next weeks blog will be a continuation of this article, discussing Metabolic Adaptation and how your metabolism changes in response to diet, exercise, and weight loss. Stay tuned!

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