Food energy
Food energy is chemical energy that animals (including humans) derive from their food to sustain their metabolism, including their muscular activity.[1]
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.[2] 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.[3] 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.[4]
The energy contents of a complex dish or meal can be approximated by adding the energy contents of its components.
History and methods of measurement[edit]
Direct calorimetry of combustion[edit]
The first determinations of the energy content of food were made by burning a dried sample in a bomb calorimeter and measuring the temperature change in the water surrounding the apparatus, a method known as direct calorimetry.[5]
Recommended daily intake[edit]
Many countries and health organizations have published recommendations for healthy levels of daily intake of food energy. For example, the United States government estimates 8,400 and 10,900 kJ (2,000 and 2,600 kcal) needed for women and men, respectively, between ages 26 and 45, whose total physical activity is equivalent to walking around 2.5 to 5 km (1+1⁄2 to 3 mi) per day in addition to the activities of sedentary living. These estimates are for a "reference woman" who is 1.63 m (5 ft 4 in) tall and weighs 57 kg (126 lb) and a "reference man" who is 1.78 m (5 ft 10 in) tall and weighs 70 kg (154 lb).[19] Because caloric requirements vary by height, activity, age, pregnancy status, and other factors, the USDA created the DRI Calculator for Healthcare Professionals in order to determine individual caloric needs.[20][21]
According to the Food and Agriculture Organization of the United Nations, the average minimum energy requirement per person per day is about 7,500 kJ (1,800 kcal).[22] Although the U.S. has changed over time with a growth in population and processed foods or food in general, Americans today have available roughly the same level of calories as the older generation. [1]
Older people and those with sedentary lifestyles require less energy; children and physically active people require more. Recognizing these factors, Australia's National Health and Medical Research Council recommends different daily energy intakes for each age and gender group.[23] Notwithstanding, nutrition labels on Australian food products typically recommend the average daily energy intake of 8,800 kJ (2,100 kcal).
The minimum food energy intake is also higher in cold environments. Increased mental activity has been linked with moderately increased brain energy consumption.[24]