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Metabolism

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What is Metabolism?

Metabolism is a reference to all the chemical reactions that occur within your body. More specifically, your metabolism is the sum of all energy transformations associated with each chemical reaction in your body. This is the reason why metabolism is commonly discussed from a fitness, exercise, and weight loss perspective, because it ultimately represents your body's regulation of energy or, in other words, calories. Keep reading to learn more about metabolism.

Important Concepts Related to Metabolism

To understand how metabolism works, it is first helpful to understand the simple but fundamental concepts listed below:

  1. Your body's energy source is the food you eat. When you eat food, you are consuming energy stored in a chemical form, the amount of which can be measured by the caloric content (read the Calories Explained article) of whatever you are eating.

  2. Energy can neither be created or destroyed, it can only change form. This is the first law of thermodynamics, and it governs your metabolism (along with everything else in the known universe).

  3. flames on a white background
  4. With respect to metabolism and the human body, when we say that calories (i.e. energy) are being "burned," it does not mean that the calories are disappearing. We know that calories can't disappear because calories are units of energy and so that would contravene the first law of thermodynamics (see #2 above). Essentially, calorie burn means that calorie energy is being transformed from the chemical form stored within your body to either a heat or mechanical form that is not stored within your body.

  5. All chemical reactions must either release energy or else absorb energy. Each chemical reaction within your body must be accompanied by a release of energy or an absorption of energy.

The Basic Physiology of Metabolism

Now that you understand the fundamental concepts that govern metabolism, we can review the specifics of metabolism. From item #4 on the list above, you know that there are two basic types of chemical reactions, ones that release energy and ones that absorb energy. When discussing metabolism, these types of reactions are classified as catabolic and anabolic reactions, respectively. They are described in more detail below:

Catabolism

Some chemical reactions in our bodies break down nutrient molecules (i.e. molecules from the food we eat) to release useable energy. This is called catabolism. The energy that is released during catabolic reactions is stored within adenosine triphosphate (ATP) molecules. You can learn more about ATP in the Exercise Energy Systems article, but for the purposes of this article you simply need to know that ATP is the energy molecule, generated through catabolic reactions from the food we eat, that is used to power all processes at the cellular level of our bodies.

Anabolism

Some chemical reactions in our bodies use the ATP energy made available through catabolism to construct the molecules that make up our bodies. This is called anabolism (also known as biosynthesis). The ATP energy absorbed in anabolic reactions is used to help build all the molecules in our bodies including, for example, our muscle, fat, and bone tissues.

Therefore, catabolism and anabolism are opposites of one another, and your overall metabolism is ultimately the sum total of all the anabolic and catabolic reactions that are happening within your body. Any chemical reaction that occurs within your body must be either catabolic or anabolic, there are no other alternatives. Essentially, you can think of catabolism as the body's release of energy in a form (ATP) that is optimized for use at our cellular level, whereas anabolism can be thought of as the utilization of that released ATP energy to build the molecules that are constructed within our bodies.

The ATP energy released through catabolism isn't always used for anabolism, however. After an ATP energy molecule has been generated through catabolic reactions it can be used in one of two ways:

  1. The ATP molecule can be used to power our energy absorbing anabolic reactions. As discussed, these are the reactions that construct all the molecules that make up our bodies. Therefore, the energy used in anabolism will remain "stored" inside our bodies within the chemical bonds of the molecules that have been synthesized, regardless of whether they are muscle, fat, bone, or any other type of molecules.

  2. OR

  3. The ATP molecule can be used to help power the movement of our muscles and support the non-anabolic aspects of our bodies' cellular level operation. This type of energy usage is what is meant by the term "burning calories." Energy used in this manner is converted to a form of energy, ultimately either heat energy or mechanical energy, that is not stored within our bodies and so it can be considered to have been "burned" off.

Metabolism as it Relates to Fitness, Weight Gain, and Weight Loss

Up to this point, this article has discussed the technically oriented scientific definition of metabolism and the related concepts of catabolism and anabolism. However, in the common usage context concerned with fitness, weight gain, and weight loss, it is the concept of basal metabolic rate, or BMR, rather than catabolism and anabolism, that is of greater importance in truly understanding your metabolism. Before discussing metabolism any further, you should read the definitions below to ensure a proper understanding of the discussion that follows:

Basal Metabolic Rate (BMR)

BMR represents the amount of energy, usually measured in calories, that your body must burn to keep itself alive while you are at rest and not digesting any food. The calories burned to meet your BMR requirement include only those used to keep your tissues alive and support the function of your vital organs. Any and all physical activity and digestion of food require additional calorie burn beyond your BMR requirements.

Daily Caloric Expenditure

Do not confuse BMR with total daily caloric expenditure. BMR represents only part (although usually the largest part) of the total number of calories that you burn in a given day. The other possible ways that your body can burn calories are through physical activity and digestion of food. Therefore, the total number of calories that you burn in a given day (i.e. your daily caloric expenditure) can be determined by adding your BMR, the calories that you have burned to support physical activity, and the calories that you have burned to support digestion of food (this is generally 10% or so of the total number of calories that you have consumed, read more about this in our Thermic Effect of Food article), as follows:

You can try our Daily Caloric Expenditure Calculator to estimate the total amount of calories that you burn in a given day.

What is a High Metabolism?

A person with what is commonly referred to as a "high metabolism" is someone that has a relatively high BMR. As such, a high metabolism means that, relative to a low metabolism, more calories must be burned to meet life sustaining energy requirements (i.e. to keep the tissues alive and support the function of vital organs). People with high metabolisms burn more calories while at rest than people with low metabolisms.

What is a Low Metabolism?

In contrast to a high metabolism, a person with what is commonly referred to as a "low metabolism" is someone with a relatively lower BMR. A low metabolism means that, relative to a high metabolism, fewer calories must be burned to meet life sustaining energy requirements (i.e. to keep the tissues alive and support the function of vital organs). People with low metabolisms burn fewer calories while at rest than people with high metabolisms.

High Metabolism versus Low Metabolism

Based on the definitions of "high metabolism" and "low metabolism" provided above we can illustrate the effects of a high metabolism versus a low metabolism. Consider the following scenario:

Let's compare two people that are identical in every possible way except for their metabolisms, or BMRs. One person has a higher metabolism and the other person has a lower metabolism. They both consume 2200 calories per day. The person with the higher metabolism has a BMR of 1700 calories per day. The person with the lower metabolism has a BMR of 1400 calories per day. Both people burn 200 calories per day due to the thermic effect of food during digestion. Both people burn 500 calories per day due to physical activity. Therefore, on a typical day the person with the high metabolism will burn 1700 + 200 + 500 = 2400 calories, whereas the person with the low metabolism will burn 1400 + 200 + 500 = 2100 calories.

Now, if we assume that both people wish to maintain their current weight, then it means that they will need to consume the same amount of calories that they burn or, in this example, 2400 calories for the person with the high metabolism and 2100 calories for the person with the low metabolism. If we instead assume that both people consume 2400 calories, it would mean that the person with the high metabolism will maintain a constant weight, whereas the person with the low metabolism will gain weight because their daily caloric consumption (i.e. 2400 calories) exceeds their caloric expenditure (i.e. 2100 calories).

How to Increase Your Metabolism

Based on the example provided above, one might be inclined to think that modification of their metabolism is the solution to their weight gain, weight loss, or weight maintenance efforts. Although it is certainly possible and often beneficial to increase your metabolism, it is important to understand that your metabolism does not dictate your weight. Rather, your metabolism simply determines the most substantial proportion of your daily caloric requirements. Instead of attempting a modification of your metabolism to achieve a desired weight gain or weight loss goal, the recommended and more effective approach is to adapt your caloric intake and/or your level of physical activity to attain the caloric balance that will allow you to achieve your goal (you can read more about this topic here: Slow metabolism - Is it to blame for weight gain?). You can also read the Calories Explained article to learn more about caloric balance and how it is the ultimate determinant of your weight.

a woman performing lat pulldown exercises

Okay, okay, so you still want to know how you can increase your metabolism? The answer to this question is quite simple... To increase your metabolism (i.e. BMR) you need to increase your muscle mass. Muscle burns calories while you are at rest and is therefore a contributing factor to your BMR. Although there is some confusion as to the exact amount of calories that muscle burns, the strongest evidence seems to indicate that each pound of muscle on your body adds roughly 5 calories per day to your BMR (you can read more about this topic here: The Myth about Muscle and Your Metabolic Rate). As you grow older your muscle mass gradually decreases and so your metabolism will decrease as well. This effect can be countered by adopting a resistance training program to maintain and/or increase your muscle mass. Not sure how to build muscle? Read our How To Build Muscle article to learn the basics.

Keep in mind, however, that increasing your metabolism through an increase in your muscle mass will not be the solution to your weight loss and/or weight maintenance goals. Resistance training to increase muscle mass is certainly very beneficial from an overall body health perspective, and as such it is a strongly recommended component of any fitness plan, but as far as your metabolism is concerned, consider the following:

Let's assume that you adopt a resistance training program and manage to increase your overall muscle mass by 5 pounds. This would take a significant amount of time and effort and, while it is certainly a recommended and worthy endeavour with respect to your overall health, from a purely metabolic perspective the end result would be that your BMR would increase by 5 pounds x 5 calories/pound•day = 25 calories per day. Since there are 3,500 calories in one pound of fat, this means that your increased metabolism would allow you to burn an extra 25 calories/day x 30 days/month x 1 pound of fat/3500 calories = .214 pounds of fat per month while you are at rest. So, to summarize, if you increase your muscle mass by 5 pounds it will increase your metabolism such that you are able to burn roughly an additional fifth of a pound of fat per month.

As you can see, from a metabolic perspective the return on your investment of time and energy into acquiring new muscle mass is less than overwhelming. In conclusion, therefore, you should now not only have a better understanding of metabolism and how it defines your energy requirements, but also of how its regulation is not the solution to your weight loss and/or weight maintenance efforts.

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