How does your metabolism change depending on your nutritional and health status? Metabolism adapts based on the body’s condition and can be categorized into an absorptive and postabsorptive state.  It can also be measured and formulated to help determine caloric needs for individuals.

Absorptive Metabolism

During this phase, a meal is digested and absorbed. The absorbed nutrients are mainly glucose, triglycerides (in chylomicrons), and amino acids. During this time, glucose, triglycerides, and amino acids are catabolized for energy. Approximately 50% of glucose from a meal is metabolized for ATP.¹ Amino acids are deaminated and enter the Krebs cycle, and fatty acids and glycerol backbones from triglycerides are also metabolized or transported to adipose tissue. The conversion to ATP for lipids and proteins is less than glucose energy metabolism.

Glycogenesis and lipogenesis for energy stores are pivotal reactions during absorption. Amino acids also form new proteins. The predominant hormone responsible for absorptive metabolism reactions is insulin, which promotes glycogenesis, lipogenesis, and protein synthesis (thyroid hormones and insulin-like growth factors also stimulate protein synthesis).¹ Insulin transports glucose from blood after a meal into cells via facilitated diffusion with glucose transporter (GLUT) molecules.

Postabsorptive Metabolism

The postabsorptive state is approximately four hours after a meal is consumed. To maintain normal blood glucose levels, postabsorptive metabolism reactions involve the breakdown of stored nutrients to supply ATP and the conversion of lipid and protein molecules into glucose. Glycogenolysis occurs within liver cells to directly supply blood glucose; glucose from glycogenolysis within muscle cells is used for muscle contraction. Gluconeogenesis allows glycerol from lipolysis to form into glucose within hepatocytes to be released into the bloodstream (glycerol can also enter the Krebs cycle once converted into pyruvic acid).¹ Fatty acids are catabolized to enter the Krebs cycle after beta-oxidation into acetyl CoA, or converted into ketone bodies used for ATP production.¹

Gluconeogenesis also allows certain amino acids to be converted into glucose via amino acid catabolism, and lactic acid is converted to glucose via lactic acid catabolism.¹ Due to low blood glucose levels, the hormone glucagon is responsible for allowing glucose release from stored or newly created glucose. Hormones norepinephrine and epinephrine are also stimulated due to low blood glucose levels, promoting glycolysis and lipolysis. Cortisol, during stress situations, stimulates glycolysis, lipolysis, and protein catabolism.¹

Fasting or Starving

Fasting is defined as no food consumption ranging between hours or a few days.¹ Starvation is when no food is consumed for weeks or months (or without proper food intake), generally sustaining life for two months with water consumption.¹ The brain and red blood cells use glucose for ATP production, and protein catabolism produces amino acids used for gluconeogenesis within the first day of fasting.¹ Lipolysis produces glucose and ATP via gluconeogenesis and the Krebs cycle using glycerol and fatty acid components. Ketone bodies are also formed from lipid catabolism and can enter the Krebs cycle once converted to acetyl CoA. Ketones eventually outpace glucose for ATP production, decreasing gluconeogenesis and protein catabolism.¹

Energy Expenditure or Metabolic Rate 

Energy expenditure is the amount of energy used by the body, and the metabolic rate is the amount of energy used within the body but measured within a specific period.  Energy, mostly in the form of ATP (some GTP), is formed when catabolizing nutrients (carbs, protein, fats) to synthesize molecules, transport molecules, or perform work within cells.² Energy released is in the form of heat, and excess heat is released into the environment to maintain 98.6F body temperature. Heat energy is measured by direct calorimetry via an insulated chamber designed to measure heat dissipation from the body.² Indirect calorimetry uses oxygen inhaled and carbon dioxide exhaled. This can be used because metabolism requires oxygen and produces carbon dioxide, releasing heat. 

Metabolic rate is influenced by several factors, including hormones, physical activity, food intake, age, and body temperature. Several factors influence the metabolic rate, as well as different classifications. 

Total Energy Expenditure (TEE)/Total Metabolic Rate (TMR) - energy used for all body functions. Composed of basal energy expenditure (BEE)/basal metabolic rate (BMR), thermic effect of activity (TEA; physical activity), thermic effect of food (TEF), and adaptive thermogenesis (AT).²

Basal energy expenditure (BEE)/Basal Metabolic Rate (BMR) - 50-65% of TEE²

Energy used for normal body functions and processes during rest.² Specifically, it is measured in a room temperature environment 10-12 hours after a meal.² Another measurement, known as resting energy expenditure (REE)/resting metabolic rate (RMR) is similar to BEE/BMR but is done without fasting. It represents 65-75% of TEE.²

• Common equations used to calculate approximate BMR and RMR include Harris-Benedict and Mifflin-St. Jeor. 

Harris Benedict Equation

Male: BMR = 66.5 + (13.75 x BW) + (5.0 x Ht) - (6.78 x age)

Female: BMR = 655.1 + (9.56 x BW) + (1.85 x Ht) - (4.68 x age) 

BW - body weight in kilograms (kg)

Ht - height in centimeters (cm)

Mifflin-St. Jeor Equation

Male: RMR = (10 x BW) + (6.25 x Ht) - (5 x age) + 5

Female: RMR = (10 x BW) + (6.25 x Ht) - (5 x age) - 161 

Thermic Effect of Activity (TEA)  - 

Energy used for posture and position of skeletal muscle, typical daily movements, and physical activity.

Thermic Effect of Food (TEF) - 5-15% of BMR²

Energy used to digest, absorb, process, and store food components. Protein has a higher TEF than carbs, and unsaturated fatty acids have a higher TEF than saturated fatty acids.²

Adaptive Thermogenesis (AT) - 

Energy used to maintain core body temperature during changes in environmental temperature using mechanisms such as sweating or shivering, as well as other heat-releasing or conserving measures. This is crucial to maintain homeostasis of the body. 

Calories and Body Weight 

Energy intake is provided from food, and energy from the catabolism of nutrients is used to perform cellular work and converted into heat.¹  Calories (technically spelled with the capital letter C) are defined as the amount of heat energy required to increase a gram of water by 33.8F or 1C. Kilocalories (kcals) are used to quantify the energy content within food and are equal to 1000 Calories.¹ In general, carbohydrates and protein provide 4 kcals per gram, and fat provides 9 kcals per gram. Alcohol provides 7 kcals per gram. Kilocalories represent the number of Calories found on nutrition labels; the words calories and kilocalories are used interchangeably in nutrition. 

When energy intake exceeds energy expenditure, weight gain is a common bodily response. Coupled with a sedentary lifestyle, health conditions such as obesity arise. Conversely, when energy expenditure exceeds energy intake, weight loss occurs. Prolonged imbalance can lead to being underweight or malnourished. An equation, known as body mass index (BMI), classifies body size from underweight to obese. BMI should be interpreted as appropriate based on fat-free mass or fat mass of an individual. For example, an adult with higher proportions of skeletal muscle may have a high BMI but not be considered overweight or obese. 

BMI = weight (kg) / height (m)2 

Underweight = < 18.5

Normal = 18.5-24.9

Overweight = 25-29.9

Obesity = > 29.9 

Source(s):

1. Jerry Tortora and Bryan Dickerson, Principles of Anatomy & Physiology, 16th ed. (New Jersey: John Wiley & Sons, 2021).

2. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019)