Metabolism refers to biochemical processes that occur within any living organism - including humans - to maintain life. These biochemical processes allow us to grow, reproduce, repair damage, and respond to our environment.
It is a common belief that slim people have a higher metabolism and overweight people have a slower metabolism. This is very rarely the case.
In this article, we will discuss the facts behind metabolism, what it is, what it does, and how it is influenced.
Contents of this article:
Fast facts on metabolism
Here are some key points about metabolism. More detail and supporting information is in the main article.
- Catabolism is the breaking down of compounds to release energy
- Our body weight is a result of catabolism minus anabolism
- One gram of fat produces 9 calories, 1 gram of protein or carbohydrate produces 4 calories
- As we age, we need fewer calories
- A healthy sleep pattern can help maintain the correct weight
Anabolism and catabolism
Most people use the term "metabolism" incorrectly for either anabolism or catabolism:
Anabolism is the building up of things - a succession of chemical reactions that builds molecules from smaller components; anabolic processes usually require energy.
Catabolism is the breaking down of things - a series of chemical reactions that break down complex molecules into smaller units; catabolic processes usually release energy.
During anabolism, simple molecules such as amino acids are built up into larger and more complex molecules.
Anabolism, or biosynthesis, allows the body to grow new cells and maintain all the tissues.
Anabolic reactions in our bodies use a few simple chemicals and molecules to manufacture (synthesize) a vast array of finished products. Examples include the growth and mineralization of bone and increases in muscle mass.
Classic anabolic hormones include:
- Growth hormone - a hormone made by the pituitary gland that stimulates growth.
- Insulin - a hormone made by the beta cells in the pancreas. It regulates the level of sugar glucose in the blood. Cells cannot utilize glucose without insulin.
- Testosterone - causes the development of male sex characteristics, such as a deep voice and facial hair. It also strengthens muscles and bone.
- Estrogen - involved in strengthening bone mass, as well as developing female characteristics such as breasts.
Adrenaline, a hormone produced by the adrenal gland, plays a part in the process of catabolism.
Catabolism breaks things down and releases energy; it uses larger compounds to create smaller compounds, releasing energy in the process.
Catabolism provides the energy our bodies need for physical activity, from the cellular level to whole body movements.
Catabolic chemical reactions in the living cell break down polymers (long chains of molecules) into their monomers (single units). For example:
- Polysaccharides are broken down into monosaccharides - for instance, starch is broken down into glucose
- Nucleic acids are broken down into nucleotides - nucleic acids, such as those that make up DNA, are broken down to purines, pyrimidines, and pentose sugars, which are involved in our body's energy supply
- Proteins are broken down into amino acids - sometimes, protein is broken down into amino acids to make glucose, which appears in the blood
When we eat, our body breaks down the organic nutrients - this breaking down process releases energy, which is stored in molecules of adenosine triphosphate (ATP) in the body. ATP is considered to be "the energy currency of life."
The energy stored in ATP is the fuel for anabolic reactions. Catabolism creates the energy that anabolism consumes for synthesizing hormones, enzymes, sugars, and other substances for cell growth, reproduction, and tissue repair.
Metabolism and body weight
Despite the common belief that metabolism determines whether someone is fat or thin, weight gain is primarily due to energy imbalance.
In simple terms, our body weight is a result of catabolism minus anabolism - the amount of energy we release into our bodies (catabolism) minus the amount of energy our bodies use up (anabolism).
The excess energy is stored either as fat or glycogen in the muscles and liver. Glycogen is the main storage form of glucose in the body.
One gram of fat produces 9 calories, compared with 4 calories from a gram of protein or carbohydrate.
Although becoming overweight is generally a result of the body storing excess energy as fat, sometimes hormonal problems or an underlying medical condition may affect metabolism.
If you are very overweight or obese, it may be advisable to have a medical evaluation to determine whether a medical condition is affecting your weight.
Despite what promoters of certain brands of "health" foods say, there is little we can do to significantly change our resting metabolic rate (basal metabolism).
Long-term strategies, such as increasing muscle mass, may eventually have an effect. However, determining a body's energy needs, then adapting lifestyle accordingly, will have a quicker effect on altering body weight.
Body size and composition
A larger body mass requires more calories than a smaller body mass. People with more muscle in relation to fat will require more calories than individuals who weigh the same but have less muscle in relation to fat.
Therefore, people with a higher muscle-fat ratio have a higher basal metabolic rate than people with a lower muscle-fat ratio, if their weight is the same.
As we get older, several factors emerge which result in a lower calorie requirement. Our muscle mass drops, resulting in a higher fat-muscle ratio. Our metabolism alters slightly, resulting in a lower calorie requirement.
The following age-related factors reduce our calorie requirement:
- Hormones - men produce less testosterone and women produce less estrogen with age - both hormones are involved in anabolic processes that consume energy. Human growth hormone also decreases as we age.
- Menopause - as women approach menopause, there is a drop in hormones that normally promote energy use. Many women find it harder to lose weight during this time.
- Physical activity - older adults tend to be less physically active than when they were younger.
- Gender - men have a higher basal metabolic rate than women because their muscle-to-fat ratio is generally higher than women's. This means that an average man will burn more calories than an average woman of the same age and weight.
How can I lose weight?
When you have determined your daily calorie requirements and have confirmed that you do not have an underlying condition contributing to your weight gain, you should focus on three crucial factors associated with weight loss and maintenance of your ideal body weight - exercise, dietary intake (nutrition), and sleep.
The impact of sleep
Lack of sleep can contribute to a disturbance in neuroendocrine control of appetite. This may lead to overeating, altered insulin resistance, and an increased risk of developing type 2 diabetes - all of which can lead to weight gain.
Getting enough exercise
A 6-month trial carried out by researchers from Duke University Medical Center studied the impact of exercise on 53 participants who had led a sedentary lifestyle.
The trial measured four levels of physical activity: the equivalents of 12 miles of walking per week, 12 miles of jogging per week, 20 miles of jogging per week, and inactivity. Participants worked out on treadmills, elliptical trainers, or cycle ergometers in a supervised setting.
Significant benefits were noted in the exercise groups. Only a moderate amount of exercise was needed to show benefit.
Diet and nutrition
Calorie restriction: Managing the number of calories consumed each day is an important factor in weight control, especially if attempting to lose weight.
Severe calorie restriction: This has been shown to be ineffective for long-term success. A severe drop in calories may trigger the body to alter its metabolism so that much less energy is burned, while at the same time storing any energy it can find.
Very low-calorie diets commonly undermine motivation, often resulting in overeating when the diet is abandoned. Unless the very low-calorie diet is being supervised by a well-qualified dietitian, nutritionist, or doctor, there is a significant risk of malnutrition, which is not only detrimental to health but may also affect metabolism in a way that makes it more difficult to achieve weight loss.