What is Homeostasis?

Homeostasis is the body's ability to maintain a stable internal environment. Introduced by Claude Bernard in 1854, this concept includes:

  • Body temperature regulation
  • pH balance
  • Energy balance

Understanding Bioenergetics

Bioenergetics is the study of how energy flows in living organisms. The body maintains energy balance through:

  1. Energy Intake – Calories from food and the amount consumed
  2. Energy Expenditure – Energy used for physical activity and internal heat production

Internal heat includes:

  • Basal Metabolic Rate (BMR) – Energy required for vital functions at rest
  • Thermic Effect of Food – Energy needed for digestion and storage

Energy Imbalance and Weight Changes

When energy intake and expenditure are not equal, imbalances occur:

  • Excess energy intake → Stored as fat → Leads to weight gain
  • Insufficient energy intake → Weight loss due to under-eating, digestive disorders, or disease

Measuring Energy: Calories vs. Joules

The International System of Units (SI), established in 1960, standardized energy measurement:

  • Joule (J) / Kilojoule (kJ) – Used in most countries
  • Calorie (Cal) / Kilocalorie (kcal) – Common in the U.S. and EU food labels
  • 1 Cal = 1 kcal = 4.2 kJ
  • 1 Calorie raises the temperature of 1 kg of water by 1ºC

The calorie was first used in nutrition by Nicholas Clément in 1824, though its exact origins are debated.


The international unit of energy, the joule, is named after the English physicist James Prescott Joule (1818–1869), who in 1845 devised a “heat apparatus” (shown) that was able to estimate the “mechanical equivalent of heat”—that is, the work required to raise the temperature of a fixed volume of water by 1.8°F (1°C).