Metabolism
Many people have probably heard this line at least once in their lives: “I just have a really fast metabolism, so I don’t tend to gain weight!” This line typically stems from the public’s general understanding of metabolism, that it is the process by which food is broken down by the body for energy and that, due to varying metabolic rates, some people get hungrier faster than others. While there is truth to this sentiment, a few key components should be addressed.
Metabolism:
Metabolism refers to all the chemical processes that occur throughout the body, starting from the breakdown of food into usable energy and nutrients, to the growth and repair of the body. Energy converted from food fuels all the subsequent chemical processes. This division is made more clear through the two types of metabolism: catabolism and anabolism. This metabolism cycle is always at work, even when the body is at rest.
The basic body functions it fuels include:
Breathing
Circulating blood
Digesting food
Growing and repairing cells
Managing hormone levels
Regulating body temperature
Diagram demonstrating the basics of metabolism
Catabolism
Catabolism refers to the breakdown of food components, namely proteins, lipids (fats), and polysaccharides (carbs), into smaller, energy-rich compounds, such as ATP. The energy is then transported and utilized in anabolism.
Anabolism
Anabolism refers to the part of metabolism in which energy from catabolism is used to synthesize more complex molecules that act as the building blocks of growth and repair throughout the body. Excess nutrients left unused by the body are typically converted and stored as fat, the most energy-efficient form of energy. If the body has a nutrient deficit, it will take energy from the stored fat to fuel anabolic processes.
Metabolic Rate:
The metabolic rate refers to the body's total energy expenditure, and it falls into three main components: the basal metabolic rate, thermogenesis, and expenditure from physical activity.
Basal Metabolic Rate
The basal metabolic rate (BMR) refers to the minimum amount of energy needed by the body to function or maintain homeostasis through the systems listed above. BMR makes up around 60 to 70% of the body’s total energy expenditure, and the average man has a BMR of 7,100 kJ per day and for women, 5,900 kJ per day. However, the rate at which it occurs differs based on time of day, mass and composition, dieting, growth, etc.
Specific factors include:
Body Mass and Composition: Larger bodies naturally require more energy because they have more metabolizing tissues. Bodies with more muscle will burn through more energy rapidly, increasing BMR, whereas bodies with more fat will burn far fewer kJs of energy, decreasing BMR.
Time of Day: The rate of energy expenditure varies throughout the day. Typically, it is lowest in the morning and highest in the afternoon and evening.
Dieting: Diets, such as fasting, that lead to calorie and, therefore, energy deficiencies encourage the body to slow BMR to conserve energy. Statistics have shown that BMR can drop 15%, and even more so if lean muscle tissues are lost.
Age: Decreases in muscle tissue, as well as hormonal and neurological changes, lead to a decreased BMR with age.
Growth: When the body is still in development, more energy is needed to supplement growth. Because the body is also under-developed, there is a higher energy demand to maintain homeostasis.
Environmental Temperature: Higher or lower environmental temperatures push the body to work harder to maintain an internal temperature of 98.6 degrees. This increases BMR.
Genetics: BMR may be higher or lower based on genetics.
Infection or Illness: BMR increases due to increased energy expenditure in activating and maintaining an immune response and repairing and growing new tissues.
Degrees of Fitness and Activity: Increased levels of physical activity typically means that there is an increase in muscle mass, which again, means that there is a greater energy demand.
Diagram of Basal Metabolic Rate
Thermic Effect of Food
Also known as diet-induced thermogenesis, this component of energy expenditure refers to the normal byproduct of metabolic processes that occurs in all warm-blooded animals. Body heat is generated as food is ingested, digested, and metabolized. It takes around 10% of food to process into energy, but it may vary slightly based on the levels of fats, carbohydrates, and proteins consumed.
Physical Activity
Energy used for physical activity of any degree accounts for the remaining energy. Engaging in weight lifting or strenuous activities will expedite energy resources to sustain the level of vigor and to grow and repair muscles (think soreness). Exercise is the only source of energy expenditure that humans have significant control over.
Conclusion:
In reality, weight gain or loss isn’t truly linked to the basal metabolic rate, which is what most people link metabolism to. It simply follows the fundamental idea that for weight loss, a calorie deficit is needed; for weight gain, a calorie surplus is needed. Increasing levels of physical activity will result in higher energy expenditure in that category, and while increased muscle mass will, therefore, lead to an increased BMR, on the whole, BMR itself does not share a direct relation with weight. In fact, those with obesity often have fast metabolisms.
Sources:
Better Health Channel. “Metabolism.” Accessed August 17, 2024. https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/metabolism.
professional, Cleveland Clinic medical. “Metabolism.” Cleveland Clinic, September 6, 2023. https://my.clevelandclinic.org/health/body/21893-metabolism.
Mayo Clinic. “Metabolism and Weight Loss: How You Burn Calories,” October 8, 2022. https://www.mayoclinic.org/healthy-lifestyle/weight-loss/in-depth/metabolism/art-20046508.
Mandal, MD Dr. Ananya. “What Is Anabolism?” News-Medical, November 26, 2009. https://www.news-medical.net/life-sciences/What-is-Anabolism.aspx.
The Editors of Encyclopaedia Britannica. “Catabolism.” Encyclopedia Britannica, July 20, 1998. https://www.britannica.com/science/catabolism.