7.4 Metabolism and Energy Balance

Metabolism and Energy Balance

Metabolism refers to the sum of all chemical reactions in the body that keep you alive. It covers everything from breaking down the food you eat into usable energy to building the complex molecules your cells need. Understanding how metabolism works and how energy balance is maintained connects directly to topics like nutrition, hormonal regulation, and disease states you'll encounter throughout this course.

Definition and Role in Energy Production

Metabolism is the total of all chemical reactions occurring within an organism to maintain life. It has two major branches:

• Catabolism: the breakdown of complex molecules into simpler ones, releasing energy
• Anabolism: the synthesis of complex molecules from simpler ones, requiring energy input

The primary purpose of metabolism is converting nutrients into usable energy in the form of adenosine triphosphate (ATP). Hormones, enzymes, and other regulatory factors coordinate these reactions to maintain homeostasis and keep energy utilization efficient.

Basal Metabolic Rate (BMR)

BMR is the minimum amount of energy your body needs to maintain vital functions (breathing, circulation, cell production) while completely at rest. It typically accounts for 60–75% of your total daily energy expenditure.

Several factors influence BMR:

• Age: BMR generally decreases with age as lean muscle mass declines
• Sex: Males tend to have a higher BMR due to greater muscle mass on average
• Body composition: More lean tissue means a higher BMR, since muscle is more metabolically active than fat
• Genetics: Inherited differences in metabolic enzyme activity and thyroid function affect baseline metabolic rate

Catabolism vs. Anabolism

Catabolism

Catabolic reactions break down complex molecules into simpler ones, releasing energy in the process. These reactions are exergonic, meaning they have a net release of energy.

• Glycolysis breaks glucose into pyruvate, yielding ATP and NADH
• Beta-oxidation breaks fatty acids into acetyl-CoA units, generating NADH and $FADH_2$

The energy released by catabolic reactions is captured in ATP and electron carriers (NADH, $FADH_2$), which then fuel other cellular processes.

Anabolism

Anabolic reactions build complex molecules from simpler precursors, and they require an input of energy. These reactions are endergonic.

• Protein synthesis assembles amino acids into functional proteins
• Glycogenesis links glucose molecules together to form glycogen for storage

Anabolic and catabolic pathways are tightly coupled: the ATP generated by catabolism powers the energy-demanding work of anabolism.

Energy Balance

Energy balance is the relationship between energy intake (food) and energy expenditure. Your total daily energy expenditure has three components:

1. Basal metabolism (BMR): the largest portion
2. Physical activity: the most variable component
3. Thermogenesis: energy used to digest, absorb, and process food (also called the thermic effect of food)

When these three components equal your caloric intake, you're in energy balance and your weight stays stable.

• Positive energy balance: intake exceeds expenditure, leading to weight gain. Chronic positive balance increases the risk of obesity and related conditions.
• Negative energy balance: expenditure exceeds intake, leading to weight loss. Prolonged negative balance can result in malnutrition and loss of lean tissue.

Metabolic Pathways for Macronutrients

Carbohydrate Metabolism

Carbohydrates are the body's preferred fuel source. Glucose moves through several linked pathways to produce ATP:

1. Glycolysis (cytoplasm): Glucose (6 carbons) is split into two molecules of pyruvate (3 carbons each), producing a net gain of 2 ATP and 2 NADH.
2. Pyruvate conversion: In the mitochondria, pyruvate is converted to acetyl-CoA (2 carbons), releasing $CO_2$ and generating NADH.
3. Citric acid cycle / Krebs cycle (mitochondrial matrix): Acetyl-CoA is oxidized through a series of reactions, producing 3 NADH, 1 $FADH_2$, and 1 ATP (as GTP) per turn. The cycle turns twice per glucose molecule.
4. Electron transport chain (ETC) and oxidative phosphorylation (inner mitochondrial membrane): NADH and $FADH_2$ donate electrons through a chain of protein complexes. The energy released pumps $H^+$ ions across the membrane, and ATP synthase uses that gradient to generate the bulk of ATP (approximately 30–32 ATP per glucose).

Gluconeogenesis is the reverse concept: the liver (and to a lesser extent the kidneys) synthesizes new glucose from non-carbohydrate precursors like amino acids, glycerol, and lactate. This pathway is critical during fasting to maintain blood glucose levels.

Lipid Metabolism

Fats are the most energy-dense macronutrient, yielding about 9 kcal per gram compared to 4 kcal/g for carbohydrates and proteins.

• Beta-oxidation (mitochondrial matrix): Fatty acid chains are cleaved two carbons at a time, producing acetyl-CoA, NADH, and $FADH_2$ with each cycle. The acetyl-CoA then enters the Krebs cycle. A single 16-carbon fatty acid (palmitate) can yield approximately 106 ATP.
• Ketogenesis (liver): When carbohydrate availability is low (prolonged fasting, uncontrolled diabetes), excess acetyl-CoA is converted into ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone). These can be used as fuel by the brain, heart, and skeletal muscle.
• Lipogenesis: When energy intake exceeds expenditure, excess acetyl-CoA (from any macronutrient source) is used to synthesize fatty acids, primarily in the liver. These fatty acids are stored as triglycerides in adipose tissue.

Protein Metabolism

Proteins are not a primary energy source under normal conditions, but amino acids can enter metabolic pathways when needed.

• Transamination: An amino group is transferred from one amino acid to an alpha-keto acid, producing a different amino acid and a different alpha-keto acid. This is how the body interconverts non-essential amino acids.
• Deamination: The amino group is removed from an amino acid, producing ammonia (which the liver converts to urea for excretion by the kidneys) and an alpha-keto acid carbon skeleton that can enter the Krebs cycle or other pathways.

Amino acids are classified by what their carbon skeletons can be converted into:

• Glucogenic amino acids (e.g., alanine, glutamine): can be converted to glucose via gluconeogenesis
• Ketogenic amino acids (e.g., leucine, lysine): can be converted to ketone bodies or acetyl-CoA
• Some amino acids are both glucogenic and ketogenic

Factors Influencing Energy Balance

Dietary Factors

The macronutrient composition of your diet affects energy balance beyond simple calorie counts. Protein has a higher thermic effect (20–30% of its calories are used just to digest and process it) compared to carbohydrates (5–10%) and fats (0–3%). This means high-protein meals increase energy expenditure slightly more than equivalent-calorie meals of fat or carbohydrate.

High-fat and high-sugar diets tend to promote excessive caloric intake because these foods are energy-dense and often less satiating per calorie.

Physical Activity and Exercise

Physical activity is the most controllable component of energy expenditure. Regular exercise increases daily caloric burn, preserves lean muscle mass (which supports BMR), and improves insulin sensitivity and overall metabolic health.

A sedentary lifestyle reduces energy expenditure and is a major risk factor for obesity and metabolic syndrome.

Hormonal Regulation

Several hormones coordinate appetite, energy storage, and fuel mobilization:

• Insulin (from pancreatic beta cells): promotes glucose uptake into cells and stimulates glycogen, fat, and protein synthesis. It's the primary "storage" hormone.
• Glucagon (from pancreatic alpha cells): stimulates glycogenolysis and gluconeogenesis in the liver, raising blood glucose. It opposes insulin's effects.
• Leptin (from adipose tissue): signals satiety to the hypothalamus. Higher body fat generally means higher leptin levels, though leptin resistance can develop in obesity.
• Ghrelin (from the stomach): stimulates hunger. Levels rise before meals and drop after eating.

Hormonal imbalances can disrupt energy balance. For example, insulin resistance is central to type 2 diabetes and is closely linked to chronic positive energy balance and obesity.

Genetic and Environmental Factors

Genetic variation affects metabolism, appetite regulation, and how efficiently you store or burn energy. Some people are genetically predisposed to higher or lower BMR, greater appetite drive, or more efficient fat storage.

Environmental and social factors also play a significant role:

• Access to healthy food options vs. reliance on calorie-dense processed foods
• Socioeconomic status, which influences both diet quality and opportunities for physical activity
• Cultural norms around food portions, meal timing, and exercise habits

Chronic imbalances in energy metabolism increase the risk of cardiovascular disease, type 2 diabetes, certain cancers, and other conditions. Maintaining energy balance through a balanced diet, regular physical activity, and stress management supports long-term metabolic health.