ENERGY EXPENDITURE
In this background document we will be discuss how energy is expended in the body, how measurements of energy expenditure are made and how body composition is determined. How these measurements are used to develop simple indicators of healthy body weight will also be discussed. This document is based on content from chapter 7.
The energy content of food which we consume is measured in kcalories (or Calories) that we see on nutrition facts tables (1). Macronutrients, such as carbohydrates (2) and fat (3) are the main sources of energy for the human body. Protein (4), can also be used as a source of energy, when present in excess amounts, but is not as important a contributor to energy as carbohydrates and fat, so most of our discussion will be focussed on carbs and fats. In the body, energy from these macronutrients is stored in the chemical bonds of ATP (5) which is used to fuel the many chemical reactions required to support human metabolism.
Here we can see the different steps in cellular metabolism that led to the formation of ATP. In step 1 a small amount of ATP is produced by the breakdown of glucose to a compound called pyruvate. Pyruvate is then converted to acetyl CoA. Fats can also be broken down to acetyl CoA (step 2), by a process called beta oxidation. Proteins are digested to amino acids which are then deaminated (step 3) and they too can be used by the cell to produce energy.
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Acetyl CoA enters into a series of chemical reactions called the citric acid cycle, (step 4), which transfer electrons from compound to compound ultimately resulting in the formation of the energy molecule ATP (step 5). The body then uses ATP as a source of energy to support many biological functions.
Another component of energy expenditure call the thermic effect food (1). This refers to the amount of energy that is expended in the process of digestion absorption and storage of a nutrient and is estimated to be about 10% of TEE. And as you can see here from the numbers different macronutrients result in the expenditure of different levels of energy. Protein, is often promoted for weight loss, because of its high thermic effect, but its impact is not great, because of individual variation and the relatively small number of kcalories involved.
The energy used by the body is referred to as total energy expenditure. The pie chart shows the three main components of total energy expenditure. Part of the energy is used to support basal metabolism (shown in red); in fact about 60 to 75% of total energy expenditure is for basal metabolism. Basal metabolism is the amount of energy required to keep the body alive. It is the energy required to keep all organs functioning at a complete rest. It is usually measured in the morning in a warm room before a person rises after waking up and after a person has been without food or exercise for 12 hours. The basal metabolic rate, refers to the amount of energy expended per day; because it¡¯s measured per unit of time it is called a rate.
Physical activity typically represents 15 to 30% of total energy expenditure and is made up of intentional exercise (1), which is not a major component for most people. Most of the population is sedentary and don¡¯t regularly exercise. Non intentional physical activity,
resulting from the movement of the body during day to day activities, are more important and results in the expenditure of energy called non exercise activity thermogenesis or NEAT. This NEAT varies a great deal from individual to individual. The addition of intentional exercise to most people¡¯s routines, would increase energy expenditure and result in many health benefits.
The benefits of exercise are well known and are listed here. These include benefits for cardiovascular health, diabetes prevention and bone health. Exercise is believed to reduce the risk of breast cancer because its results in lower estrogen levels and estrogen can support the growth of tumors. Similarly, observational studies have consistently shown that have individuals with high levels of physical activity have a reduced risk of colon cancer.
Up to this point we have discussed how energy is expended in the body. This slide illustrates the concept of energy balance.
Someone is in energy balance when the amount of energy consumed as food equals the energy expended by the body for metabolism and physical activity.
When the energy content of food exceeds the body¡¯s energy expenditure then someone is in positive energy balance and will gain weight.
When energy content is less than energy expenditure someone is a negative energy balance and is losing weight.
We know that the body stores energy and this table shows that part of it is in the form of glycogen or glucose and that this carbohydrate component last for about 24 hours. Triglycerides are stored in adipose tissue and represents a substantial supply of energy as we can see here 57 days and this is actually calculated based on the fat composition of a relatively lean 70 kg male. So we can see that the body has considerable capacity to maintain energy stores. When someone is in positive energy balance the proportion of the total body weight made up of adipose tissue increases i.e. % body fat increases.
Up to this point we have discussed energy expenditure in the body. This expenditure, in turn, whether resulting in positive or negative energy balance will determine body composition, notably % body fat. This next section will look briefly at how energy expenditure and body composition are measured.
This slide shows that all macronutrients in food can be converted to fat but that the major contribution is from dietary fats. We know this because when the composition of triglycerides in the body is analyzed it reflects dietary fat intake. The body converts carbohydrates to fat and this may become an important additional source of body fat when there is substantial overconsumption.
You may recall these equations from earlier in the term. The energy expenditure for a sample of individuals, in energy balance, is measured and co- related to variables like gender, age, weight, height and physical activity to the generate equations shown. Note that if someone is in energy balance their energy expenditure is equal to their energy requirements. Energy balance is indicated by a stable body weight. These equations are then be used to estimate the energy requirements for members of the general population.
Pg 303 (3e)
This slide lists three common laboratory methods for measuring energy expenditure. Please read the pages shown in the textbook so you can distinguish between the three methods. The doubly-labelled water method was used to create the EER equations on the previous slide.
Another important variable used to assess health is the body composition, particularly the proportion of body weight that is fat tissue and the proportion that is lean. Lean tissue is all body tissue except fat. In this graphic shown here we are comparing a MRI thigh cross section of two individuals of approximately the same weight but one is young and has a much higher percentage of body weight as muscle as compared to the older person where you can see more fat, as indicated by the white regions. This loss of muscle mass with aging is called sarcopenia.
The methods used to measure body composition, are listed here and are part of the required reading for this section. Be sure that you can distinguish between the three methods listed here.
Pg 309-310 (3e); 284-285 (2e)
In addition to % body fat it is recognized that the location of body fat is also important. One of the unhealthiest locations to carry fat is as visceral fat. Visceral fat is fat deposited in the abdomen near internal organs as shown in the MRI on this slide. We also see subcutaneous fat, in the MRI, which is sitting outside of the organs.
Visceral fat is associated with an increased risk of developing type 2 diabetes and heart disease. It¡¯s a type of fat that releases free fatty acids more readily than subcutaneous fat and is a source of fat that is prone to becoming insulin resistant. It also releases proteins that promote inflammation throughout the body.
Doing an MRI is the gold standard for determining visceral fat; but MRIs are very expensive and so researchers are always looking for simpler ways of doing these measurements that can be easily conducted in a doctor¡¯s office or in the community. Waist circumference has been found to correlate well with MRI measurements of visceral fat making it a simple measurement that provides useful information.
Here we see two individuals one that is carrying his weight centrally and he¡¯s considered an apple shaped individual, i.e. an individual with a large waist circumference, whereas other individuals are pear shaped and carry their fat body fat subcutaneously on their hips. The apple-shape is considered more detrimental to health.
As noted earlier waist circumference has been found to correlate well with MRI measurements of visceral fat and as shown here, to also be correlated with disease risk. Waist circumference cut-offs have been developed. For example, Health Canada recommends the cut-offs shown here. Individuals with weights circumferences greater than these numbers are at increased risk of disease.
The most common tool for the assessment of healthy body weights is BMI or body mass index. As the figure shows there is an increased risk to health with both increased and decreased BMI. The middle of the curve represented by BMI range from 18.5 to 24.9 is classified as normal weight as this range has the lowest health risk. In the scientific literature the terms overweight and obese refer to specific ranges of BMI. The BMI ranges that define underweight, normal, overweight, and obese are very important and you should learn these numbers.
One limitation of BMI is that is does not consider body composition e.g. a very muscular person will have a high BMI, while being very healthy and the health risk for a person with a high % body fat and hence high BMI, will differ depending the location of the body fat. The solution to these issues is to use both BMI and waist circumference in the assessment of body weight.
This conclude our discussion of energy expenditure, how body composition is determined and how BMI and waist circumference are used as indicators of healthy body weight and composition. To help consolidate your learning remember to do the associated self assessment questions.
Self-Assessment Questions
1. This question is based on a required reading. An estimate of % body fat is determined by measuring subcutaneous fat: What method is being described?
a) Underwater weighing
b) Skinfold thickness
c) Bioelectrical impedance
2. This question is based on a required reading. When the heat, given off by the body, is measured to determine energy expenditure, what method is being used?
a) Bioelectrical impedance
b) Indirect calorimetry
c) Direct calorimetry
3. Which dietary macronutrient most contributes to adipose tissue in the body?
a) Dietary fat
b) Carbohydrates
c) Dietary fat and carbohydrates equally
4. Your professor spends 2 hours a day on her feet lecturing. What category of physical activity is this?
a) Intentional exercise
b) Thermic effect of movement
5. Which is most associated with an increased risk of disease?
a) Visceral fat, as indicated by waist circumference
b) Both visceral and subcutaneous fat have equivalent risk.
c) Subcutaneous fat
6. Sacropenia describes the loss of what tissue?
a) Body fat
7. The energy required to keep the liver functioning would fall into which category of energy expenditure?
a) Thermic effect of food
b) Physical activity
c) Basal metabolism
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