We often hear about the word, “metabolism,” but do we know exactly what this word means? People usually use the word in phrases such as, “I want to lose weight, but my metabolism is so slow,” or, “my friend has a fast metabolism and can eat as much food as he wants and not gain any weight,” I am sure you have all heard these phrases, or variations of them. These phrases all refer to our metabolism, and they are all correct in what people are trying to portray. But, what is the actual definition of metabolism?
DEFINITION
Metabolism is defined as, “All the physical and chemical processes in the body that convert or use energy from food.” It involves converting food into fuel to perform all the functions that keep us alive.
In order for us to live, we need to eat. We cannot survive very long without a regular supply of nutrients. Every part of the body needs nutrients to perform its job. Our body converts the food we eat into energy. This energy is measured in Calories. A Calorie is actually a measurement of heat. A special device called a Calorimeter is used to determine how much heat energy is contained in food. When a sample of food is burned in a Calorimeter, the amount of heat it creates determines its caloric content. One Calorie is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius. The amount of calories in our food determines how much energy it provides to our body, to allow it to perform all the necessary functions to stay alive. The higher the amount of calories in a specific food, the more energy is available for our body’s needs. We can easily calculate the amount of calories in our food, either from the nutritional label on the food product or referring to nutritional charts.
Therefore, food provides us with the essential nutrients that are critical for a healthy metabolism. Some of these nutrients include: minerals, protein, essential fatty acids, probiotics, enzymes, vitamins, phytonutrients, and fiber. Not only are the calories from food necessary, but we need to ensure that our diet contains all the vital nutrients we need. We can consume food containing a large amount of calories and yet still be nutritionally deficient if we don’t eat a diet that supports our metabolism.
Some of the metabolic processes that our body performs include: breathing, blood circulation, control of body temperature, muscle contraction, digestion of food and nutrients, elimination of waste through urine and feces, and functioning of the brain and nerves. In fact, all the functions of our body that keep us alive are part of the metabolic process.
HYPOTHALAMUS
A small area in our brain known as the hypothalamus is the organ most responsible for regulating our metabolism. This organ controls heart rate and body temperature, the movement of food in our body, bladder control, and the function of our thyroid gland. It controls the sensation of hunger and thirst. If our body doesn’t let us know when it needs more nutrients, we can easily die from lack of nutrients, known as malnutrition. These functions are considered “survival mechanisms” to keep us alive. They hypothalamus is key for our survival.
METABOLIC RATE
Definition: This is the speed or rate at which our body uses the energy from our food. The speed of metabolism determines how much food we require. The faster our metabolism, the quicker we use (burn) the calories from our food, and the more food we require. Slower metabolic rates require less food energy.
Do we all have the same metabolic rate? No. Some people naturally have a faster or slower metabolism compared to others. We can also have fluctuations in our metabolic rate depending on many factors.
THREE COMPONENTS OF METABOLIC RATE
- RESTING METABOLISM
The term, “resting metabolism,” refers to the amount of energy our body needs for basic functioning. It is the minimum amount of energy needed just to keep us alive. Think of it as the amount of food needed for us to just lie in bed without moving. Our resting metabolism uses up about 70% of our energy that we get from food. Therefore, the majority of the food we consume is used by our body to maintain life. This is the largest component that influences our metabolic rate, but the one we have the least control over.
There are many factors that can affect our resting metabolic rate. Let us learn about these in this next section:
Genetics: Some people are just born with a faster metabolism than others. Those are the people that seem to be able to eat anything, and are yet very slim. Then there are those people who just “look at food” and gain weight. They were born with a much slower resting metabolic rate. Unfortunately, we have no control over our genes.
Body Composition: People with a large body mass from excess weight, usually from excess fat, have higher metabolisms because the body needs to work harder just to sustain itself, even at rest. However, because overweight people are also sedentary, calorie expenditure is still low over all. Metabolism may be up to 25% higher in obese persons. Weight loss is easier at the beginning of a diet because your metabolism is already running higher with the higher total body mass. Even small reductions in food will result in weight loss. As one loses weight and body mass decreases, metabolism also slows down and many people reach a “plateau” that is difficult to get past, unless they reduce their calories even more.
Metabolic Syndrome: Excess body weight, especially around our abdomen creates a group of health problems that include elevated blood pressure, high triglycerides, low LDL levels (good cholesterol) and high HDL levels (bad cholesterol). A person becomes insulin resistant. The body is not able to respond normally to insulin, causing a rise in blood sugar. The changes in one’s body, caused by this excess weight is referred to as “metabolic syndrome.” This syndrome is typically caused by a sedentary lifestyle, lack of exercise, obesity (Body Mass Index greater than 30), poor nutrition, smoking, and a diet that is high in fatty foods. These people have a higher risk of diabetes, heart attacks and strokes. Some races are at higher risk than others. They especially need to be careful about excess weight. These include Africans, Hispanics, native groups, and Asians. Decreases in weight will reduce all the other risk factors for metabolic syndrome, which includes decreasing the amount of food as well as eating healthier foods and increasing exercise—an overall healthier lifestyle.
Muscle vs. Fat: Muscle burns more calories than fat, even at rest. Every pound of muscle in our bodies burns 35 calories a day, whereas every pound of fat in our bodies only burns 2 calories a day. Therefore, the more muscle mass you have, the higher your metabolism. With extra muscle, you can eat more without gaining weight. Even people who are not overweight but have little muscle mass have slower metabolic rates than those who have larger muscles.
Gender: Who has the faster metabolism, men or women? In general, men have a faster metabolism than women.
In general, women have a higher percentage of fat and less lean muscle mass, whereas men have a higher percentage of lean muscle mass and less body fat. Women’s bodies were designed to hold onto fat to bear children.
The thyroid gland is also significant in regulating metabolism. Thyroid hormones influence energy levels, weight management, body fat percentage, insulin production, body temperature, muscle growth, and emotional health. Low thyroid levels will slow the metabolism down. Women typically have more problems with low thyroid hormone levels than men do, and these problems increase with age.
Also, menopause causes a decrease in estrogen which also is associated with weight gain and muscle loss. Women who keep physically active can counteract the effects of the decrease in estrogen during menopause.
Age: We have the highest metabolic rate as we are growing. Babies and children have the highest metabolic rates overall. Once we stop growing, our metabolic rate slows. And, as we age, our metabolism slows down even more. We experience a decrease in our metabolism by 5% each decade after the age of 40. Many people complain that they gain weight after the age of 40, because they do not reduce their calories to compensate for the slower metabolic rate. At the age of 40, you need to reduce your daily food intake by 100 cal in order not to gain weight; maintaining the same amount of calories equates to a 10 pound weight gain per year.
Another factor that is related to aging is that after the age of 40, if we do not maintain a regimen of physical exercise, we lose 10% of our muscle mass per decade. And as we know, lower muscle mass equates to a slower metabolism. From age 30 to 50, men can lose 5 to 10 pounds of muscle, and women will lose twice that amount if nothing is done to maintain muscle mass.
A slower metabolism does not need to be an inevitable consequence of aging; if you keep physically active and have a proper diet, you can maintain your metabolic rate. Physical exercise needs to be a daily commitment, even as we get older.
Climate: Those in very cold or very hot climates have faster metabolic rates, either to cool the body down or to increase the body’s temperature to the optimum level for all body functions to perform at their peak.
Exercise: The amount of exercise we do can increase our resting metabolic rate. We will learn more about this in the next section.
Stress: When we are dealing with an acute stressful situation such as writing an exam, or jumping out of the way of a moving car, our metabolism increases greatly for us to respond to this stressful situation. We need to use much more energy to think, to move, to react. However, chronic stress has the opposite effect on our body and metabolism. Chronic stress releases high levels of cortisol from our adrenal glands. Chronically elevated cortisol levels disrupt our metabolism, decreases our thyroid function, causes weight gain, poor sleep, decreased muscle mass, increases our blood sugar, poor memory, depression and increase our craving for sugar.
Sleep: During sleep our body uses 10% less calories than when we are awake. However, people who are chronically sleep deprived tend to have slower metabolisms because of the elevated levels of cortisol. Sleep deprivation is considered stressful to our body.
- THERMIC (HEATING) EFFECT OF ACTIVITY
Since we all don’t spend our days lying in bed, there is more to metabolism than just our resting metabolism. Therefore, the amount of energy we need is equivalent to the energy we need for basic functions, plus the amount of energy needed to perform all the activities that we engage in throughout the day.
The thermic effect of activity is defined as the rate at which we burn calories with normal movements and with exercise. This accounts for about 30% of our body’s calorie needs.
However, the more we increase our level of activity, the more energy our body needs in the form of food. Someone who is mostly sedentary requires about 30% more food than their resting metabolic rate to supply the body’s need for energy. Someone who does light activity requires about 50% more calories above the resting metabolic rate. With moderate activity, we are looking at 75% more calories. With vigorous activity, it doubles the amount of energy requirement, needing 100% more calories than our resting metabolic rate. In terms of food consumption, the vigorously active person will need to double the amount of calories above the resting metabolic intake in order to supply the body with enough calories.
Exercise: There is one sure-fire method for increasing our metabolic rate, and that is through exercise. Exercise is the most effective method for instantly boosting metabolism. One author quoted, “essentially, we know of no way to burn more calories or up our metabolism than to move more.” The increase in our metabolism with exercise can last many hours after we stop exercising, anywhere from 6 to 36 hours. And the longer that we exercise the longer period of time our metabolic rate is increased. However, we must include a proper diet with our exercise in order to gain the maximum metabolic effect. Without proper nutrition, you won’t be able to access proper energy stores, and you won’t be able to replace nutrients lost during exercise. This will lead to a slowing down of the metabolic rate.
There is also a different effect whether you engage in aerobic (cardio—involving increasing our heart rate) exercise, or anaerobic (strength) exercise. 30 minutes of cardio training burns more calories than 30 minutes of weight lifting. However, in the hours after stopping exercise, weight training has a longer lasting effect on boosting metabolism. Why is this? Because weight training increases muscle mass, which overall increases our metabolic rate.
- THERMIC (HEATING) EFFECT OF DIGESTION
It takes energy to digest our food. A certain amount of the calories that we eat is necessary to actually digest the food that we eat. The thermic effect of food is the energy required for digestion, absorption, and disposal of ingested nutrients. This accounts for approximately 10% of our calorie use. Water is also essential for digestion of food. If we are even mildly dehydrated, our metabolism will slow down. Another 10% of the energy in the food we consume is actually needed to digest our food.
EATING FOR A HEALTHY METABOLISM
In order to sustain a healthy metabolism we need to eat regular meals throughout the day. When we put too many hours in between our meals our metabolism slows down, and our body goes into “starvation mode.” People who try to skip meals in order to lose weight are creating the opposite effect. Their metabolism will slow down because they are not getting the nutrients needed to maintain their usual metabolic rate. The body wants to hang on to every calorie of energy possible because it is not being fed regularly. So metabolism slows to conserve calories.
Breakfast is the most important meal of the day. It jump-starts our metabolism for the day. Skipping breakfast is connected to weight gain as we then tend to eat more calories throughout the rest of the day, as well as our metabolism has slowed down from lack of food.
ENERGY IN FOOD
As we know, every food contains differing amounts of calories. If we look at the three main nutrients that our body takes in, we can compare the amount of energy that each provides for us. 1 gram of protein and carbohydrates, each, provide the body with 4 calories of energy. Fats are more calorie dense, therefore 1 g of fat provides the body with 9 calories of energy.
Each of the three main nutrients also need differing amounts of energy for digestion. Proteins are the highest in that 20 to 35% of the calories in the protein that we eat is actually needed to digest the protein. Carbohydrates require 10 to 15% of its calories to be digested. And fats only require 5%. The nutrient that provides us with the most calories (fats), is also the easiest for our body to digest in that it requires the least amount of energy. This is why a diet that is high in fats can also contribute to increased weight—easy to digest and high in calories. A recent study shows that processed foods use less calories to digest than whole foods by up to 50% less. This is because much of the food is already broken down and processed into its smaller components and doesn’t need as much energy to break down in our body. Studies indicate this could be contributing to overall increases in weight of the population in developed countries.
OUR AND HEALTHY WEIGHT LOSS METABOLISM
The basic weight control formula goes like this: Eat more calories than your body needs, you gain weight. Eat less calories than your body needs, you lose weight. Excess calories are stored by the body in the form of fat. For healthy weight loss, you should not lose more than 1 to 2 pounds per week. Any more than that and we are breaking down proteins in our body to use for energy. As muscle is made up mainly of protein, you lose muscle mass, which will then slow down your metabolism.
Do not eat less than 1200 calories per day, as then you are putting yourself into “starvation mode” and metabolism will slow down dramatically. To lose one pound per week, you must consume 500 less calories a day than you need to maintain your current weight. To lose one pound of body weight means eating 3500 calories less than we use. Fad diets tend to be unsuccessful in keeping weight off once it is lost. We return back to our normal eating patterns and put the weight back on. Also fad diets tend to be deficient in a number of nutrients.
A decrease in food consumption automatically slows down metabolism as your body tries to conserve energy. Therefore, just decreasing the amount of food you eat will make it difficult for you to lose weight. So how does one lose weight successfully? Exercise is critical to weight loss, as exercise boosts metabolism and burns calories.
The most efficient way to lose weight is a combination of diet and exercise. Exercise should be done on a daily basis, if possible. Do cardiovascular exercise at least five days per week. A brisk 30 minute walk every day has been shown to be the most beneficial exercise for most people to lose weight, but it must be at a pace that increases your heart rate in order for it to be of any benefit. Strength training should be done at least two times a week to build muscle.
If you don’t maintain your exercise program, even after you are at your target weight, most people will gain the weight back and even more. People who get slim and stay slim have changed their eating habits and their attitude to food, as well as have stayed committed to an exercise program.
For successful weight loss, the program of eating and exercise must also be compatible with your lifestyle, otherwise you will not be able to maintain it. If you decide to exercise by running, and you hate running, you will not do it for very long. You need to find a form of exercise that you enjoy.
Because every person is different in how their metabolism works—how much energy is burned during rest, exercise, and eating—it is difficult to calculate the dietary needs of a specific person with any accuracy. A Scottish study showed that the resting metabolic rate can vary anywhere from 1000 to 2500 calories a day. Even in individuals who are the same size, with the same amount of muscle mass, the metabolic rate can vary by up to 30%.
If we adhere to a diet of proper nutrition, maintain adequate levels of both exercise and rest, stay at a healthy weight and avoid stress, we will have optimal metabolic health.