Nutrient Information: Energy

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Nutrient Key Information 
Nutrient Name: Energy
Nutrient Category: Macronutrients (Proximates)
Unit Name: kcal
Nutrient Summary: Energy is required to sustain the body’s various functions, including respiration, circulation, nerve transmission, physical work, metabolism, and protein synthesis.
Nutrient Function: Energy is supplied by carbohydrates, proteins, and fats in the diet. They are generally referred to as macronutrients. The general Calories Calculation Formula is:

Carbohydrate-gram x 4 + Protein-gram x 4 + Fat-gram x 9

Expert recommended calories range for each source is:
Carbohydrate: 45-65%
Protein: 10-35%
Total Fat: 20-35%

In a human body, energy is presented in the forms of:
Heat energy: such as keeping body in the constant temperature
Mechanical energy: such as moving muscles
Electrical energy: such as transmitting nerve signals
Chemical energy: such as performing anabolic reactions to build larger compounds from smaller molecules

Carbohydrate, Fat, and Protein generate compounds called acetyl CoA during their metabolism process. Acetyl CoA is used to generate energy compound ATP (Adenosine TriPhosphate) for immediate energy use. If the body has enough ATPs, the excessive acetyl CoA is converted into body fat as energy storage used later.

Energy Requirement:
A person's energy intake requirements and expenditure depend on this person's age, body composition, gender, and physical activity level. A person's main energy expenditure categories are:

  1. Basal Metabolism (~65% of a person's energy expenditure): it is the amount of energy required to keep the body functioning when a person is at complete digestive, physical and emotional rest (but in awake state). The basic functioning include breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, contraction of muscles, etc. Some factors impact a person's Basal Metabolic Rate (BMR):

    - Gender (men higher, women lower)
    - Age (the older the lower)
    - Height (the taller the higher, because of larger skin surface area)
    - Lean body mass (muscle) vs fat composition (more muscle the higher BMR)
    - Environment temperature (hot or cold increases BMR)
    - Mental stress (raise BMR)
    - Fasting or starving (lower BMR)
    - Acute illness, such as fever raises BMR
    - Genetic

    Some studies have suggested that a lower BMR is associated with longer life expectancy. One possible explanation for this relationship is that lower BMR may be associated with slower aging rate. This is because a lower BMR may result in less oxidative stress and cellular damage, which can contribute to the aging process. However, more research is needed to fully understand the relationship between Basal Metabolic Rate and human longevity and its underlying mechanisms.

  2. Physical Activity: it is the energy used to move and contract muscles, and the extra energy used to deliver nutrients and oxygen to heart and lungs. The longer, more frequent, and more intense activities need more energies. It accounts for about 20-50% of energy expenditure for a person, and the actual amount depends on the intense and duration of the activities.

  3. Thermic Effect of Food TEF (accounts for 10% of energy intake): it is the energy required to process food, such as digesting and metabolizing foods, absorbing nutrients, transporting foods and nutrients, storing ingested nutrients. The relative TEF value order for macronutrients are: protein, alcohol, carbohydrate, fat. This means high-protein foods need more energy to break them foods, digest them and absorb their nutrients.

  4. Adaptive Thermogenesis: when a body is experiencing some extreme conditions, such as extreme cold or trauma, it needs to do extra work and uses extra energy to build body tissues or produce enzymes and hormones necessary to cope with the demand.

Energy Measuring Unit:
The measuring unit of energy is kcal (kilocalorie or Calorie in upper case C, also called large calorie). 1 kcal is the amount of energy required to raise the temperature of 1 kg water by 1°C (one degree Celsius). United States and Canada food nutrition labels use "Calories" to refer to kilocalorie. Another measuring unit of energy is kilojoule (kJ). 1 kcal = 4.184 KJ. Europe and China food nutrition labels usually use KJ.

Fasting or Starving:
When a person is fasting or starving, the body draws reserved glycogen (mainly in liver, and it is for glucose) and fat (in adipose tissue) from body storage to provide glucose, glycerol, and fatty acids for energy. When use up body glycogen (in about 24 hours) or use up fat, body will break down protein tissues (such as muscle) to generate glucose and other energy source to fuel body cells. Protein tissues are vital to living bodies, you don't want to use them as the main energy source.
During fasting, the biggest challenge is the limited availability of glucose which is required for brain, nerve system, and red blood cells to function properly. Fatty acids in the fat tissues cannot convert to glucose. For emergent needs, liver can convert fats to ketone bodies to fuel brain (but it can cause side effects).

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Food Sources

Carbohydrate, Fat, and Protein are the sources of energy.

Most of cells can use glucose or fatty acids as fuel. However, brain and central nervous system and red blood cells must have glucose as fuel. Brain and nervous system use about 500 kilocalories glucose per day. Carbohydrate is the primary source for glucose, and protein is the secondary source for glucose.

Implication of excessive intake of the 3 energy food sources:

  • Excessive dietary fat intake can be converted into fat in body's adipose tissue very efficiently because of the fatty acids' chemistry structure.
  • Excessive dietary carbohydrate intake requires more metabolic steps to convert them into body fat. However, excessive carbohydrate makes body to use energy generated from the carbohydrate instead of dietary or body fat.
  • Excessive protein intake requires more metabolic steps to convert them into body fat. Excessive intake of protein will increase the excretion of urea by kidney and also increase the excretion of calcium.

In summary, we must have balanced diets for Carbohydrate, Fat, and Protein.

Sample Foods High in: Energy
View Additional Food Sources
Food Description Nutrient Amount1 Daily Value%2
Bread, whole-wheat, commercially prepared
Category: Baked Products
254 kcal 12.70%
Rice, white, medium-grain, enriched, cooked
Category: Cereal Grains and Pasta
130 kcal 6.50%
Oil, olive, salad or cooking
Category: Fats and Oils
884 kcal 44.20%
Nuts, almonds, dry roasted, with salt added
Category: Nut and Seed Products
620 kcal 31.00%
Beef, round, bottom round, steak, separable lean and fat, trimmed to 1/8" fat, all grades, cooked, braised
Category: Beef Products
247 kcal 12.35%
Fish, tuna, fresh, bluefin, cooked, dry heat
Category: Finfish and Shellfish Products
184 kcal 9.20%
1 Nutrient amount is in 100 gram food
2 Use Female 31-50 years old as Daily Value reference

Additional Nutrient Information
Nutrient Summary Energy is required to sustain the body’s various functions, including respiration, circulation, nerve transmission, physical work, metabolism, and protein synthesis.
Deficiency Health Effects If energy intake is lower than energy needs, the body adapts by reducing voluntary physical activity, reducing growth rates for children, and mobilizing energy reserves. It will lead to weight loss. For children, chronic undernutrition will cause slow growth weight, delayed bone growth, and impact brain development. For adult, it will decrease work capacity. Learn more in the Fasting paragraph of the Nutrient Function section.
Effects if Above Upper Limit If energy intake is higher than energy needs, the excess energy cannot be eliminated and they will be deposited in the form of body fat. As a result, weight gain occurs. It consequently increases chronic disease risk, including risk of Type II diabetes, hypertension, coronary heart disease (CHD), stroke, gallbladder disease, osteoarthritis, and some types of cancer.
External References
Additional Information Measuring Unit for Energy:
- One kilocalorie (kcal): the amount of heat energy needed to raise the temperature of 1 kilogram (kg) of water 1°C.
- International unit One Joule: the amount of work energy that moves a mass 1 meter distance using 1 newton force.

1 kilocalorie (kcal) = 4.184 kilojoules (KJ, 1000 Joule)

The Mifflin-St Jeor Basal Metabolic Rate (BMR) calculation equation:
BMR kcal/day = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + s
For men: s = 5
For women: s = -161

For example: a woman, weight 58kg, height 160cm, age 50, the BMR is:
(10 x 58kg) + (6.25 x 160cm) - (5 x 50years) - 161
= 580 + 1000 - 250 - 161 = 1169 kcal per day

A list of common physical activities ordered by their relative energy expenditure rates (kCal/kg/min):

  • Cross-country skiing 8 mph .229

  • Basketball (full court) .213

  • Soccer (vigorous) .213

  • Running (6 mph) .163

  • Aerobic dance .136

  • Swimming (45 yd/min) .128

  • Bicycling (15 mph) .108

  • Gardening .099

  • Golf .099

  • Walking (3.5 mph) .077

  • household tasks .066

  • Weight lifting (light-to-moderate) .053

  • Studying .024

Daily Value Age Group Recommended Daily Values
Toddler 1 to 3 years old: 1,000 kcal
Child 4 to 8 years old: 1,400 kcal
Male 9 to 13 years old: 1,800 kcal
Male 14 to 18 years old: 2,600 kcal
Male 19 to 30 years old: 2,800 kcal
Male 31 to 50 years old: 2,600 kcal
Male 51 to 70 years old: 2,400 kcal
Male Senior 71 or older: 2,200 kcal
Female 9 to 13 years old: 1,800 kcal
Female 14 to 18 years old: 2,000 kcal
Female 19 to 30 years old: 2,200 kcal
Female 31 to 50 years old: 2,000 kcal
Female 51 to 70 years old: 1,800 kcal
Female Senior 71 or older: 1,800 kcal
Female Pregnancy (>18): 2,200 kcal
Female Lactation (>18): 2,200 kcal
FDA (Based on 2000 calorie daily diet): 2,000 kcal

Dietary Reference Intakes The nutrient Dietary Reference Intakes and nutrition facts is from Institute of Medicine of National Academies 2006. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. Washington, DC: The National Academies Press.
US FDA Nutrition Education Nutrition facts knowledge are based on U.S. FOOD & DRUG Administration Nutrition Education Resources & Materials.
National Institutes of Health Nutrition facts knowledge are based on National Institutes of Health Dietary Supplement Fact Sheets.
Disclaimer The nutrient information provided here should not take the place of medical advice. We encourage you to talk to your healthcare providers (such as your doctor) about your dietary requirements which are best for your overall health. We also recommend you to read organization or professional reference documents or articles mentioned, but not limited to, in this page. Any mentions and reference links in this page don't represent our endorsement of their services and advice.

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