Memory tricks for Metabolism & Energy Balance

BMR represents the calories the body burns doing nothing but essential functions — breathing, circulation, cell repair, brain activity — measured under strict resting conditions. BMR is influenced by muscle mass (muscle burns more at rest than fat), age (BMR declines with age), sex (men typically have higher BMR due to more lean mass), and genetics. It typically accounts for 60-75% of total daily energy expenditure.

Total Energy Expenditure breaks into three parts: BMR (the largest share, 60-75%), the Thermic Effect of Food (TEF, roughly 10%, the energy cost of digesting what you eat), and Physical Activity (the most variable share, ranging from 15% in sedentary people to much higher in athletes). Understanding these three components is essential for calculating calorie needs for weight management.

TEF is the energy the body spends digesting and processing food itself. Protein has by far the highest thermic effect, using 20-30% of its own calories just for digestion. Carbohydrates use about 5-10%, and fat has the lowest thermic effect at only 0-3%. This is one reason higher-protein diets are often associated with slightly higher calorie burn.

Energy balance compares calories consumed (Calories In) against calories burned (Calories Out, i.e., TEE). A sustained surplus leads to weight gain (stored as fat), a sustained deficit leads to weight loss, and balance maintains weight. While the equation is simple, the practical factors influencing each side (hormones, metabolic adaptation, food composition) make real-world application more complex.

Glycolysis breaks down glucose into pyruvate, releasing energy (ATP) — it's the first step in extracting energy from carbohydrate. Gluconeogenesis runs in the opposite direction: creating new glucose from non-carbohydrate sources (amino acids, glycerol, lactate) — critical during fasting or low-carb states when dietary glucose isn't available, primarily occurring in the liver.

Glycogen is the storage form of glucose, kept mainly in the liver and muscles. Liver glycogen can be broken down and released into the bloodstream to maintain blood glucose levels for the whole body. Muscle glycogen lacks the enzyme (glucose-6-phosphatase) needed to release glucose into the blood — it can only be used locally by that muscle for its own energy needs.

When carbohydrate intake is very low (or during prolonged fasting), the body's glucose stores run low and it shifts to breaking down fat for energy. The liver converts fatty acids into ketone bodies, which can fuel the brain and other tissues when glucose is scarce. This is the metabolic basis of ketogenic diets and also occurs, in a more extreme and dangerous form, in diabetic ketoacidosis.

Anabolic reactions build larger molecules from smaller ones (like building muscle protein from amino acids), requiring energy input. Catabolic reactions break larger molecules into smaller ones (like breaking glycogen into glucose), releasing energy. The body constantly balances both processes — muscle growth requires anabolism to outpace catabolism over time.

ATP (adenosine triphosphate) stores energy in its phosphate bonds and releases it when broken down to ADP. Whether energy originally comes from carbohydrate, fat, or protein, it's ultimately converted into ATP before the cell can use it — for muscle contraction, nerve signaling, or any active cellular process. This makes ATP the universal 'energy currency' regardless of which macronutrient supplied the original fuel.

Leptin is released by fat cells and signals fullness/satiety to the brain — the more fat stores, the more leptin, theoretically reducing appetite. Ghrelin is released by the stomach when empty and signals hunger, rising before meals and falling after eating. In obesity, the body can develop leptin resistance, where the fullness signal is present but the brain stops responding to it effectively — one reason obesity isn't simply a matter of willpower.

Insulin, released by the pancreas when blood sugar rises (after eating), helps cells take up glucose and promotes glycogen storage — lowering blood sugar. Glucagon, released when blood sugar drops (between meals or during fasting), triggers the liver to break down glycogen and release glucose into the blood — raising blood sugar. Type 1 diabetes involves insufficient insulin production; type 2 involves insulin resistance.

Set point theory proposes that the body has a genetically and physiologically defended weight range, and actively resists deviations from it through mechanisms like adaptive thermogenesis (metabolism slowing more than expected during dieting) and hormonal shifts (leptin dropping, ghrelin rising) that increase hunger and reduce energy expenditure. This helps explain why weight loss often plateaus despite continued calorie deficits, and why maintaining weight loss requires ongoing effort rather than a fixed endpoint.