WHAT IS METABOLISM? - by Wayne Ho, M.D.
Every time you swallow a bite of sandwich or slurp a smoothie, your body works hard to process the nutrients you've eaten. Long after the dishes are cleared and the food is digested, the nutrients you've taken in become the building blocks and fuel needed by your body. Your body gets the energy it needs from food through a process called metabolism.
What Is Metabolism and What Does It Do?
Metabolism is a collection of chemical reactions that takes place in the body's cells. Metabolism converts the fuel in the food we eat into the energy needed to power everything we do, from moving to thinking to growing. Specific proteins in the body control the chemical reactions of metabolism, and each chemical reaction is coordinated with other body functions. In fact, thousands of metabolic reactions happen at the same time - all regulated by the body - to keep our cells healthy and working.
Metabolism is a constant process that begins when we're conceived and ends when we die. It is a vital process for all life forms - not just humans. If metabolism stops, living things die.
Here's an example of how the process of metabolism works in humans - and it begins with plants. First, a green plant takes in energy from sunlight. The plant uses this energy and a molecule called cholorophyll (which gives plants their green color) to build sugars from water and carbon dioxide. This process is called photosynthesis, and you probably learned about it in biology class.
When people and animals eat the plants (or, if they're carnivores, they eat animals that have eaten the plants), they take in this energy (in the form of sugar), along with other vital cell-building chemicals. The body's next step is to break the sugar down so that the energy released can be distributed to, and used as fuel by, the body's cells.
After food is eaten, molecules in the digestive system called enzymes break proteins down into amino acids, fats into fatty acids, and carbohydrates into simple sugars (e.g., glucose). In addition to sugar, both amino acids and fatty acids can be used as energy sources by the body when needed. These compounds are absorbed into the blood, which transports them to the cells. After they enter the cells, other enzymes act to speed up or regulate the chemical reactions involved with "metabolizing" these compounds. During these processes, the energy from these compounds can be released for use by the body or stored in body tissues, especially the liver, muscles, and body fat.
In this way, the process of metabolism is really a balancing act involving two kinds of activities that go on at the same time - the building up of body tissues and energy stores and the breaking down of body tissues and energy stores to generate more fuel for body functions:
* Anabolism (pronounced: uh-nah-buh-lih-zum), or constructive metabolism, is all about building and storing: It supports the growth of new cells, the maintenance of body tissues, and the storage of energy for use in the future. During anabolism, small molecules are changed into larger, more complex molecules of carbohydrate, protein, and fat.
* Catabolism (pronounced: kuh-tah-buh-lih-zum), or destructive metabolism, is the process that produces the energy required for all activity in the cells. In this process, cells break down large molecules (mostly carbohydrates and fats) to release energy. This energy release provides fuel for anabolism, heats the body, and enables the muscles to contract and the body to move. As complex chemical units are broken down into more simple substances, the waste products released in the process of catabolism are removed from the body through the skin, kidneys, lungs, and intestines.
Several of the hormones of the endocrine system are involved in controlling the rate and direction of metabolism. Thyroxine (pronounced: thigh-rahk-sun), a hormone produced and released by the thyroid (pronounced: thigh-royd) gland, plays a key role in determining how fast or slow the chemical reactions of metabolism proceed in a person's body.
Another gland, the pancreas secretes (gives off) hormones that help determine whether the body's main metabolic activity at a particular time will be anabolic or catabolic. For example, after eating a meal, usually more anabolic activity occurs because eating increases the level of glucose - the body's most important fuel - in the blood. The pancreas senses this increased level of glucose and releases the hormone insulin, which signals cells to increase their anabolic activities.
Metabolism is a complicated chemical process, so it's not surprising that many people think of it in its simplest sense: as something that influences how easily our bodies gain or lose weight. That's where calories come in. A calorie is a unit that measures how much energy a particular food provides to the body. A chocolate bar has more calories than an apple, so it provides the body with more energy - and sometimes that can be too much of a good thing. Just as a car stores gas in the gas tank until it is needed to fuel the engine, the body stores calories - primarily as fat. If you overfill a car's gas tank, it spills over onto the pavement. Likewise, if a person eats too many calories, they "spill over" in the form of excess fat on the body.
The number of calories a person burns in a day is affected by how much that person exercises, the amount of fat and muscle in his or her body, and the person's basal metabolic rate. The basal metabolic rate, or BMR, is a measure of the rate at which a person's body "burns" energy, in the form of calories, while at rest. The BMR can play a role in a person's tendency to gain weight. For example, a person with a low BMR (who therefore burns fewer calories while at rest or sleeping) will tend to gain more pounds of body fat over time, compared with a similar-sized person with an average BMR who eats the same amount of food and gets the same amount of exercise.
What factors influence a person's BMR? To a certain extent, a person's basal metabolic rate is inherited - passed on through the genes the person gets from his or her parents. Sometimes health problems can affect a person's BMR (see below). But people can actually change their BMR in certain ways. For example, exercising more will not only cause a person to burn more calories directly from the extra activity itself, but becoming more physically fit will increase BMR as well. BMR is also influenced by body composition - people with more muscle and less fat generally have higher BMRs.
Things That Can Go Wrong With Metabolism
Most of the time your metabolism works effectively without you giving any thought to it. But sometimes a person's metabolism can cause major mayhem in the form of a metabolic disorder. In a broad sense, a metabolic disorder is any disease that is caused by an abnormal chemical reaction in the body's cells. Most disorders of metabolism involve either abnormal levels of enzymes of hormones or problems with the functioning of those enzymes or hormones. When the metabolism of body chemicals is blocked or defective, it can cause a buildup of toxic substances in the body or a deficiency of substances needed for normal body function, either of which can lead to serious symptoms.
Some metabolic diseases and conditions include:
Hyperthyroidism is caused by an overactive thyroid gland. The thyroid releases too much of the hormone thyroxine, which increases the person's basal metabolic rate (BMR). It causes symptoms such as weight loss, increased heart rate and blood pressure, protruding eyes, and a swelling in the neck from an enlarged thyroid (goiter). The disease may be controlled with medications or through surgery or radiation treatments.
Hypothyroidism is caused by a nonexistent or underactive thyroid gland, and it results from a developmental problem or a destructive disease of the thyroid. The thyroid releases too little of the hormone thyroxine, so a person's basal metabolic rate (BMR) is low. Not getting treatment for hypothyroidism can lead to brain and growth problems. Hypothyroidism slows body processes and causes fatigue, slow heart rate, excessive weight gain, and constipation. Teens with this condition can be treated with oral thyroid hormone to achieve normal levels in the body.
Inborn errors of metabolism.
Some metabolic diseases are inherited. These conditions are called inborn errors of metabolism. When babies are born, they're tested for many of these metabolic diseases. Inborn errors of metabolism can sometimes lead to serious problems if they're not controlled with diet or medication from an early age. Examples of inborn errors of metabolism include galactosemia (babies born with this inborn error of metabolism do not have enough of the enzyme that breaks down the sugar in milk called galactose) and phenylketonuria (this problem is due to a defect in the enzyme that breaks down the amino acid phenylalanine, which is needed for normal growth and protein production). Teens may need to follow a certain diet or take medications to control metabolic problems they've had since birth.
Type 1 diabetes mellitus.
Type 1 diabetes occurs when the pancreas doesn't produce and secrete enough insulin. Symptoms of this disease include excessive thirst and urination, hunger, and weight loss. Over the long term, the disease can cause kidney problems, pain due to nerve damage, blindness, and heart and blood vessel disease. Teens with type 1 diabetes need to receive regular injections of insulin and control blood sugar levels to reduce the risk of developing problems from diabetes.
Type 2 diabetes.
Type 2 diabetes happens when the body can't respond normally to insulin. The symptoms of this disorder are similar to those of type 1 diabetes. Many children and teens who develop type 2 diabetes are overweight, and this is thought to play a role in their decreased responsiveness to insulin. Some teens can be treated successfully with dietary changes, exercise, and oral medication, but insulin injections are necessary in other cases. Controlling blood sugar levels reduces the risk of developing the same kinds of long-term health problems that occur with type 1 diabetes.
Human metabolism is basically made up of three parts:
I. Physical activity: 20-25%
II. Thermic effect of food: 10%
III. Resting Metabolism: 70%
1. Resting Metabolic Rate (RMR)
This is the amount of calories needed to run all essential functions and chemical reactions while in a rested and quiet state. RMR is the largest part of total metabolism and accounts for 65 - 75% of calories burned in a day.
If lean weight is lost from the body through increased protein metabolism the RMR decreases. This often happens when people go on a strict diet, the body is forced into what's known as a "negative nitrogen balance" which means more protein is lost than what is replaced due to less protein/energy intake. This imbalance causes a gradual loss in lean weight thus lowering the RMR.
Many dieters limit the amount of lean weight loss with some type of intense exercise in order for muscles to develop a need to hold onto more protein forcing the body to take more energy from fat stores.
2. Thermic Effect of Food (TEF
The body uses energy to digest and absorb the nutrients present in the food we eat. The rate of energy used for the TEF is about 10%, it can be increased depending on the composition of each meal.
The TEF causes much confusion when dieters calculate calories in and out. For example;
If we overeat the TEF actually increases due to more food to digest, the stomach and intestines have to work harder and longer. It means if we ate an extra 3500 calories ( number of calories per pound of fat ) we wouldn't actually gain 1 pound of body fat because the TEF has to be accounted for, we would gain less.
The opposite also happens if we cut 3500 calories to lose 1 pound. The TEF decreases because there would be less food/nutrients to process so energy expenditure would reduce thus we would lose less than a pound in weight.
Calories do count but our body has sophisticated mechanisms to balance energy within the body to enable us hold onto as much energy as possible for a time when starvation may occur!
3. Physical activity
The amount of energy the body burns during daily activities such as exercise, recreation, work, housework, etc. Daily physical activities account for 20 - 40% of calories burned each day. This part will vary depending on the individual and how active they are each day. A sedentary person will require less calories to maintain weight than a busy worker in a construction site!
It is here where we can have the greatest effect on metabolism. The intensity, frequency and duration of any activity all have an effect on metabolism.
METABOLISM EXPLAINED by Jerry Kennard, PhD.
What is metabolism? Although there are many scientific ways for me explain it, and I could make it seem really confusing like most of the so-called experts do, but I won't. I'm going to give you my extremely simple and easy to understand definition... metabolism is the rate at which your body burns calories to sustain life. I should also note that your body, yes yours, burns calories 24 hours a day, everyday - regardless of whether or not you workout. Remember that your body needs energy all the time, even while you're asleep and that is why skipping meals is the absolute worst thing you can do if your goal is to lose weight (body fat). Before we go any further let's talk about what affects metabolism...
What affects metabolism? What do you think has the biggest impact on your metabolism? Activity levels? Your Thyroid? Age? WRONG! WRONG! and WRONG! Activity levels, Thyroid function, and age do affect metabolism but not nearly as much as...any idea? It's muscle tissue! The more muscle you have the more calories you burn regardless of how active you are, how old you are, etc.
It's live tissue and it's there working for you and burning calories 24 hours a day - each and every day! Here's a list of some of the factors affecting metabolism in order of greatest impact to least:
1. Muscle tissue (you already know why this is on the top of the list)
2. Meal frequency (the longer you go between meals the more your metabolism slows down to conserve energy)
3. Activity level (important but doesn't make any difference if you don't match your eating to your expenditure)
4. Food choices (ex. low-fat diets tend to result in poor hormone production which leads to a slower metabolism)
5. Hydration (over 70% of bodily functions take place in water - not enough water causes all your systems to slow down and unnecessary stress)
6. Genetics (some people are born with higher metabolisms than others)
7. Hormone production and function (think you have a slow thyroid? it's not likely - before you go blame it on the thyroid first stabilize your blood sugar and throw in some progressive exercise 2-3 times each week)
8. Stress (stress also can slow metabolism by placing extra stress and strain on numerous systems.)
THE BASICS OF ABSORPTION AND METABOLISM OF A MEAL by James Loeser
Americans typically eat 2-3 meals per day. The food we take in during each meal is digested by the body, by a process called metabolism, a term most people are familiar with; it can refer the to overall functioning of the body or it can refer to specific bodily function, such as food metabolism. Food taken in during a typical meal in absorbed by the body in approximately 2-4 hours, which is referred to as the absorptive state. During this time there are transient changes in the levels of glucose (a form of sugar that serves as the body’s most abundant energy source), amino acids (which are the building blocks of proteins), and triacylglycerols (the body’s storage form of fat). Also during this time, insulin (an anabolic hormone secreted by the pancreas) is released into the blood stream in response to eating. Insulin promotes the synthesis of triacylglycerols (fat stores), glycogen (sugar stores), and proteins (structural and functional molecules of the body). During the absorptive period virtually all tissues primarily use glucose as a fuel. Pretty dry stuff so far, but, it is my goal to introduce some commonly used dietary terms and to lay down a basic foundation of food metabolism so that the reader may be able to make more informed decisions about nutrition.
The first and most important organ in food metabolism is the liver. One of the largest organs in the body, it is located just below the diaphragm, on the upper right side of the abdomen. Food ingested during a typical meal is chewed up in the mouth, further broken down in the stomach, and delivered to the small intestine where most of the nutrient absorption takes place. Absorbed nutrients are sent directly to the liver via a special vein, called the hepatic portal vein.
Because nutrients go directly to the liver via the hepatic portal vein and not into the general blood circulation, the liver gets first crack at processing absorbed nutrients. The liver takes up carbohydrates, fats, and amino acids. These nutrients are then metabolized or stored by the liver, or they are put into the general blood circulation for use by other tissues of the body.
The liver serves a vital function by smoothing out a rapid influx of nutrients during the absorptive state, that is, 2-4 hours after a meal. Because your body is working hard to absorb nutrients from a meal, blood is diverted to the digestive tract and away from other organs, such as muscles and the brain, which is why you may feel sleepy after a big meal.
The liver is normally a glucose producing organ. The liver stores glucose (sugar) in long, branched chemical chains, called glycogen. When the body needs energy and is in a state of starvation, the liver breaks down glycogen and delivers glucose to the tissues of the body via the blood stream, thus defining its glucose producing function. However, during the absorptive state, the liver does the reverse: it builds up and stores glycogen from sugar taken in during a meal for use in a fasting state.
The liver also builds fat molecules (triacylglycerols) from absorbed fat and delivers these molecules to other tissues of the body, in particular, to fat cells and muscle cells where they are stored and metabolized, respectively. Although not a major source of body fat, the liver builds fat from the building blocks of carbohydrates and proteins (simple sugar and amino acids) absorbed from a meal. The liver builds fat for a simple reason: dietary intake exceeds the body’s energy expenditure. In other words, the body takes in more fuel than it can use, so the body stores the energy as fat. What does this mean? If you eat meals containing large amounts of carbohydrates, then some of this fuel will be converted into fat. Many consumers do not know that the liver does this. However, many dieters who have tried one of the fad diets that promote very high carbohydrate, very low protein, and virtually no fat intake have discovered--among other things like reduced energy levels--weight gain, especially around the area where body fat tends to accumulate: on the hips and legs of women and around the waists of men.
Also important in food metabolism is amino acid metabolism. During the absorptive period, more amino acids are delivered to the liver by the hepatic portal vein than the liver can use. The liver uses as many amino acids as it is capable of to build proteins for the body. The body does not store protein, per se, but merely replaces those proteins lost during normal body function. So the proteins delivered to the liver in excess of its capacity are either delivered in turn to the general blood circulation where they are picked up and used by other tissues of the body, or they are degraded by the liver into urea and excreted in the urine. Therefore, a high protein diet will show up as a high urea concentration in the urine. Overall, the liver is the main conductor and processor of nutrients absorbed after a meal, but also important in food metabolism is adipose (fat) tissue, skeletal muscle, and the brain.
Adipose tissue is next in line after the liver in its ability to distribute fuel molecules, namely fat. Adipose tissue is made up of adipose cells, in which triacylglycerol (fat) droplets fill nearly its entire volume. In a 70 kg (155 lbs) man, adipose tissue weighs approximately 14 kg (31 lbs) or about half as much as the total muscle mass. In morbidly obese individuals adipose tissue can constitute up to 70% of body weight and around 50% of body weight for severely obese individuals. Therefore, in a severely obese man weighing 113 kg (250 lbs.), adipose tissue constitutes approximately 57 kg (125 lbs.), that’s about half, of his total body weight.
After consumption of a fat containing meal, triacylglycerols not processed in the liver are sent into the general circulation to be utilized by other tissues of the body, namely, fat tissue. In the absorptive state, elevated levels of glucose and insulin promote storage of triacylglycerols into fat tissue for use in a state of starvation. Therefore, if the absorptive state is frequently maintained and if total energy consumption is more than energy expenditure, then elevated levels of glucose and insulin would continually favor storage of triacylglycerol, resulting in a increase of adipose tissue and an overall gain in weight. While diet determines energy consumption, skeletal muscle determines the greatest portion of energy expenditure.
Skeletal muscle during a resting state accounts for approximately 30% of the total oxygen consumption of the body; however, during vigorous exercise skeletal muscle consumes up to 90% of the total oxygen consumption. Since the energy requirements vary so greatly between different levels of activity, muscle is unique in its ability to respond to substantial changes in energy requirements. Oxygen consumption of a specific tissue, such as muscle, is directly related to its energy requirement, and ultimately related to the number of calories it burns.
In order to meet increased energy demands, muscle tissue maintains its own surplus of glycogen, similar to liver glycogen but exclusive for use in the muscle. After a meal, carbohydrates passed on by the liver are readily taken up by muscle tissue and subsequently stored in the form of muscle glycogen. In the well-fed state, glucose broken down from muscle glycogen is primarily used to fuel muscle tissue during exercise. Fat is secondary to glycogen usage for increased energy requirements in muscle in the well-fed state. After a meal, depleted muscle glycogen stores are readily replenished, and as long as glucose is available to the muscle it will use it and not fat. Thus, in order for muscle tissue to burn fat as an energy source, glucose circulating in the blood and muscle glycogen stores must first be exhausted. Also important after a protein containing meal is the muscle’s uptake of amino acids from the general circulation that are used to replace degraded or broken down muscle protein –the structural and functional molecules of muscle. While muscle tissue constitutes the majority of oxygen consumption and thus energy requirements, the brain is the single most important consumer of energy.
Although the brain contributes only 2% of the total body weight of an adult, it consumes 20% of the total oxygen consumption of the body in a resting state. Because the brain never rests, it uses energy at a constant rate. In fact, brain cells will begin to die after just a few minutes of oxygen or energy deprivation. Therefore, special metabolic pathways exist to provide the brain with a steady source of oxygen and energy to ensure its continual function. The brain is restricted to glucose as its principal energy source; it cannot use fat or amino acids as an energy source. In addition, no appreciable stores of glycogen or fat are found in the brain. Therefore, the brain must have a constant supply of glucose from the general circulation, and the body will break down stores of energy in other tissues, such as liver glycogen stores and muscle protein, in order to supply the brain with energy. The basics of energy metabolism during a state of starvation will be discussed in an upcoming article.