Skip to Content

Food energy

« Back to Glossary Index

**Measurement of Food Energy:**

– Initial determinations of food energy involved burning samples in a bomb calorimeter.
– Direct calorimetry method tends to overestimate energy due to indigestible components.
– Energy content is now determined indirectly using chemical analysis.
– Modified Atwater system excludes fiber content for accurate energy calculation.
– Merrill and Watt proposed calorie conversion factors to improve the system.

**Sources of Dietary Energy:**

– Human diet mainly consists of carbohydrates, fats, proteins, water, and ethanol.
– Ruminants utilize bacteria to extract energy from cellulose.
– Minor components like organic acids and polyols contribute to energy.
– Nutrients like leucine regulate protein metabolism and appetite.
– Essential fatty acids are crucial for various biochemical processes.

**Utilization of Energy in the Human Body:**

Food energy is utilized for basal metabolism, maintaining body temperature, and muscular activity.
– Brain metabolism consumes approximately 20% of the energy obtained from respiration.
– Efficiency of converting energy into mechanical power depends on food type and muscle usage.
– Muscles have low overall efficiency due to various energy conversion losses.
– Each kilogram of body fat is equivalent to 32,300 kilojoules of food energy.

**Recommended Daily Intake and Guidelines:**

– Various countries and health organizations provide daily intake recommendations.
– The US government suggests 8,400 and 10,900 kJ for women and men aged 26-45, respectively.
– Recommendations are based on physical activity levels and body measurements.
– Guidelines aim to maintain a balance between energy intake and expenditure.
– Specific activities have reference daily energy needs for a man and a woman.

**Regulations and Impact on Energy Balance:**

– Legislation in various countries mandates nutrition labeling for food products.
– Dietary Guidelines for Americans provide recommendations for caloric intake.
– Various countries have established regulations and guidelines for food labeling and nutritional information.
– Efficiency, economy, and endurance performance are influenced by energy balance.
– Leucine, mental effort, and substrates play roles in regulating protein metabolism, brain energy consumption, and energy balance.

Food energy (Wikipedia)

Food energy is chemical energy that animals (including humans) derive from their food to sustain their metabolism, including their muscular activity.

Most animals derive most of their energy from aerobic respiration, namely combining the carbohydrates, fats, and proteins with oxygen from air or dissolved in water. Other smaller components of the diet, such as organic acids, polyols, and ethanol (drinking alcohol) may contribute to the energy input. Some diet components that provide little or no food energy, such as water, minerals, vitamins, cholesterol, and fiber, may still be necessary to health and survival for other reasons. Some organisms have instead anaerobic respiration, which extracts energy from food by reactions that do not require oxygen.

The energy contents of a given mass of food is usually expressed in the metric (SI) unit of energy, the joule (J), and its multiple the kilojoule (kJ); or in the traditional unit of heat energy, the calorie (cal). In nutritional contexts, the latter is often (especially in US) the "large" variant of the unit, also written "Calorie" (with symbol Cal, both with capital "C") or "kilocalorie" (kcal), and equivalent to 4184 J or 4.184 kJ. Thus, for example, fats and ethanol have the greatest amount of food energy per unit mass, 37 and 29 kJ/g (9 and 7 kcal/g), respectively. Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g), though there are differences between different kinds. For example, the values for glucose, sucrose, and starch are 15.57, 16.48 and 17.48 kilojoules per gram (3.72, 3.94 and 4.18 kcal/g) respectively. The differing energy density of foods (fat, alcohols, carbohydrates and proteins) lies mainly in their varying proportions of carbon, hydrogen, and oxygen atoms. Carbohydrates that are not easily absorbed, such as fibre, or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids contribute 10 kJ/g (2.4 kcal/g) and 13 kJ/g (3.1 kcal/g) respectively.

The energy contents of a complex dish or meal can be approximated by adding the energy contents of its components.

« Back to Glossary Index