Human metabolism refers to the complex network of chemical reactions that occur within our bodies to maintain life. These reactions involve the conversion of food and oxygen into energy, the synthesis of biomolecules, and the elimination of waste products. Understanding the basics of human metabolism and the factors that influence it is crucial for maintaining overall health and managing weight effectively. This article provides an overview of human metabolism, focusing on its key processes and the factors that impact an individual’s metabolic rate as well as how our metabolism changes as we age.
Human Metabolism Basics
Metabolism can be divided into two main processes: catabolism and anabolism. Catabolism involves the breakdown of complex molecules, such as carbohydrates, fats, and proteins, into smaller units, releasing energy in the process. Anabolism, on the other hand, is the synthesis of complex molecules from smaller components, requiring energy (food!) intake. Together, these processes maintain the body’s energy balance and support essential functions.
Carbohydrate Metabolism: Carbohydrates are broken down into glucose, which serves as the primary energy source for cells. Glucose can be utilized immediately or stored as glycogen in the liver and muscles for later use. When glycogen stores are full, excess glucose is converted into fat for long-term storage.
Fat Metabolism: Fats are hydrolyzed into fatty acids and glycerol, which can then enter the bloodstream. Fatty acids are utilized by various tissues for energy production or converted into triglycerides for storage in adipose (fat) tissue.
Protein Metabolism: Protein breakdown results in amino acids, which are either used for protein synthesis or converted into glucose or fatty acids. Protein metabolism also plays a vital role in maintaining tissue integrity and repairing damaged cells.
Energy Production: Energy is generated through cellular respiration, primarily occurring in the mitochondria, often referred to as the “powerhouse” of the cell. This process involves the conversion of glucose, fatty acids, and amino acids into adenosine triphosphate (ATP), the “currency” of cellular energy.
Metabolic rate refers to the speed at which the body carries out metabolic processes.
Several factors influence an individual’s metabolic rate, including:
A person’s age: Metabolic rate tends to decrease with age due to the loss of muscle mass and hormonal changes. Aging also affects the efficiency of cellular processes, leading to a decline in metabolic rate.
A person’s body composition: Muscle tissue has a higher metabolic rate than fat tissue. Individuals with a higher proportion of lean muscle mass will tend to have a higher basal metabolic rate (BMR). Regular physical activity and strength training can help increase muscle mass and boost metabolic rate.
A person’s gender: Men generally have a higher metabolic rate than women due to differences in body composition, hormonal profiles, and muscle mass. However, these differences can vary among individuals.
Genetics: Genetic factors can influence metabolic rate and the efficiency of metabolic processes. Some individuals may have a naturally higher or lower metabolic rate, making weight management more challenging for some individuals.
Hormonal Factors: Hormones play a crucial role in regulating metabolism. Thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3), control the metabolic rate. Disorders affecting the thyroid gland, such as hypothyroidism or hyperthyroidism, can lead to metabolic imbalances.
A person’s body size and weight: Larger individuals tend to have a higher BMR due to increased body surface area and energy requirements. Additionally, individuals with excessive weight or obesity often experience metabolic dysfunction, including insulin resistance, which affects energy utilization
How Metabolism Changes as We Age
As the human body ages, several changes occur in the metabolism that can have significant impacts on overall health and well-being. Metabolism refers to the complex set of biochemical processes that occur within the body to convert food into energy and facilitate various physiological functions. While these changes are a natural part of the aging process, understanding them is crucial for maintaining good health and making appropriate lifestyle adjustments. In this summary, we will explore the key aspects of how human metabolism changes with age.
Basal Metabolic Rate (BMR): Basal metabolic rate refers to the amount of energy expended by the body at rest to maintain basic bodily functions such as breathing, circulation, and cell production. With age, BMR tends to decrease, primarily due to the loss of lean muscle mass and a decrease in physical activity. This decline in BMR makes it easier to gain weight and harder to lose it, as fewer calories are burned at rest.
Muscle Mass and Strength: Aging is often associated with a gradual loss of muscle mass and strength, a condition known as sarcopenia. This decline in muscle mass can be attributed to a combination of factors, including hormonal changes, decreased physical activity, and inadequate protein intake. Since muscle tissue is more metabolically active than fat tissue, reduced muscle mass contributes to the decline in BMR mentioned earlier.
Hormonal Changes: Hormonal changes play a significant role in metabolic alterations during aging. The decline in certain hormones, such as growth hormone, testosterone (in men), and estrogen (in women), can impact metabolism. These hormones are involved in regulating muscle mass, fat distribution, and overall energy balance. Their decrease can contribute to the age-related decline in muscle mass and increase in body fat.
Body Composition: The distribution of body fat also changes with age. Older adults tend to experience an increase in visceral fat, which accumulates around the organs, compared to subcutaneous fat, which is found just beneath the skin. This change in fat distribution is associated with a higher risk of metabolic disorders, including insulin resistance, diabetes, and cardiovascular disease.
Insulin Sensitivity and Glucose Metabolism: Insulin sensitivity, the body’s ability to respond to and utilize insulin effectively, decreases with age. This decrease can lead to impaired glucose metabolism and an increased risk of developing type 2 diabetes. Age-related insulin resistance may result from various factors, including increased inflammation, reduced physical activity, and changes in body composition.
Metabolic Rate during Physical Activity: Our energy expenditure during physical activity also declines with age. Older individuals may experience decreased aerobic capacity and reduced efficiency in oxygen utilization, leading to a decrease in overall endurance and stamina. This decline in physical activity can contribute to weight gain and further exacerbate the age-related metabolic changes.
Nutrient Absorption and Metabolism: Aging can affect the body’s ability to absorb and metabolize nutrients efficiently. For example, there may be a decrease in the production of digestive enzymes, leading to impaired absorption of certain nutrients, such as vitamin B12, calcium, and iron. Additionally, age-related changes in the gastrointestinal tract can impact nutrient absorption and digestion.
Hydration and Kidney Function: Maintaining proper hydration becomes increasingly important as we age. Older adults may have a reduced sensation of thirst, leading to inadequate fluid intake and an increased risk of dehydration. Furthermore, age-related changes in kidney function can affect the body’s ability to regulate fluid and electrolyte balance, potentially impacting overall metabolism.
All of this taken together provides a framework within which to consider lifestyle factors, energy intake and our level of activity to assist us in leading a more sustainable and generally healthy life.
Berardi, J. Essentials of Sport and Exercise Nutrition, 1st Ed. 2010
Bernadot, D. ACSM’s Nutrition for Exercise Science, 1st Ed. 2019
The National Dietetic Association https://www.eatright.org/