The virtual laboratory: Energy balance
copyright © 1982 - 2006 David A Bender
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The program will then run in a separate window, and at any time you can minimise the program window to check the theory from this page.
As you run the program, you are asked after each simulated experiment whether you wish to save the results to print out. If you save results, they are saved in a file called simout.txt in your temporary file area. The program automatically locates your temporary file area, and displays the path on the opening screen. When you close the program you are given the option of printing out the results you have chosen to save for printing. You cannot print out results until you end the program.
Energy balance: Food intake and physical activity
Definitions in energy metabolism
Changes in food intake and changes in body weight
Measuring energy expenditure and fuel utilisation by indirect calorimetry
Energy balance: Food intake and physical activity
There is obviously a relationship between food intake, energy expenditure in physical activity and body weight; the figure shows the results of studies conducted by McCance and Widdowson in 1946, in post-war Germany. There was a large amount of rubble to be cleared from bomb damage, a large number of people had to be found employment, and food was scarce. They investigated the effects of increasing food intake from an initial 10.5 MJ (2450 kcal) /day to 12.5 MJ (2990 kcal) /day on work output and body weight:
Initially, when the people were given the extra food their work output increased, and they gained body weight. In other words, their energy expenditure in physical work was less than their energy intake from food, and they were able to store the surplus as fat.
When they were offered a financial inducement to work harder, they did so; at the highest level work output they lost weight. In other words, at this high level of energy expenditure their food intake was inadequate, and they were calling on body reserves of fat.
In this simulation you can investigate the relationship between food intake and physical activity in three ways:
by constructing an activity diary - a table of the time spent in different types of activity throughout the day, so that you can see how physical activity affects energy requirements
by varying food intake, so that you can see how increased or decreased food intake affects body weight at a constant level of physical activity
by measuring oxygen consumption and carbon dioxide production by a subject walking on a treadmill at various speeds, so that you can see not only the effect of different levels of physical activity on energy expenditure, but also how the intensity of activity affects the relative amounts of fat and carbohydrate that are used.
Definitions in energy metabolism
Units of energy
The SI unit of energy is the Joule. 1 Joule is a small amount of energy, and in nutrition and physiology it is usual to work in kJ (1,000 Joules) or MJ (1,000,000 Joules)
Many people prefer to use the calorie. 1 calorie is the amount of heat required to raise the temperature of 1 g of water through 1 degree C, from 14.5C to 15.5C. Again in nutrition and physiology it is usual to work in kcal (1,000 cal); this is sometimes shown as Cal with a capital C, to differentiate it from a single calorie with a lower case c.
Basal metabolic rate (BMR)
The basal metabolic rate is the energy cost of maintaining metabolic homeostasis, nerve and muscle tone and circulation and breathing.
It is the energy expenditure measured when completely at rest, but awake, at a comfortable temperature (i.e. under conditions of thermal neutrality, so that the subject is not expending energy to keep warm or cool down), and several hours after the last meal, so that the subject is not expending energy on processing the products of digestion to synthesise body reserves of fat and glycogen.
The importance of being awake is that some people show an increase in metabolic activity when they are asleep, as a result of partial uncoupling of electron transport from ATP synthesis - this is a mechanism for regulating body weight as food intake varies. Other people show a fall in metabolic activity when they are asleep; such people are metabolically efficient, but are more likely to gain weight as a result of increased food intake.
Basal
metabolic rate depends on metabolically active lean body tissue; mainly muscle,
but all tissues make a contribution - although it is largely inert reserves
of triacylglycerol, adipose tissue also makes a contribution to metabolic rate.
Body weight is therefore an important factor in determining BMR, but gender
and age are also important.
Women have a higher proportion of body weight as fat than do men. This means that a woman has a lower BMR than a man of the same body weight.
With increasing age, even if body weight does not change, lean muscle tissue is gradually replaced by fat, so that the proportion of fat in the body increases with age. This means that an older person has a lower BMR than a younger person of the same age.
Resting metabolic rate (RMR)
It is not always possible to determine BMR under strictly controlled conditions. When the conditions are not completely controlled, the resultant measurement of energy expenditure is called the resting metabolic rate; for practical purposes, resting metabolic rate can be considered to be approximately equivalent to basal metabolic rate.
Physical activity ratio (PAR)
PAR is the energy cost of an activity, expressed as a multiple of BMR. Values range from only just > 1.0 for very gentle activities up to 8 x BMR or even higher for very intense activity.
PAR
1.0 – 1.4
lying, standing or sitting at rest, e.g. watching TV, reading, writing, eating,
playing cards
PAR
1.5 – 1.8
sitting: sewing, knitting, playing piano, driving, standing: preparing vegetables,
washing dishes, ironing, office and lab work
PAR
1.9 – 2.4
standing: mixed household chores, cooking, playing snooker or bowls
PAR
2.5 – 3.3
standing: dressing, undressing, showering, vacuum cleaning, walking: 4 –
6 km /h, playing cricket,
occupational: tailoring, shoemaking, electrical and machine tool, industry,
painting and decorating
PAR
3.4 – 4.4
standing: mopping floors, gardening, cleaning windows, walking: 4 – 6
km /h, playing golf,
occupational: motor vehicle repairs, carpentry and joinery,
chemical industry, bricklaying
PAR
4.5 – 5.9
standing: polishing furniture, chopping wood, heavy gardening, walking: 6 -
7 km /h,
exercise: dancing, moderate swimming, gentle cycling
occupational: labouring, hoeing, road construction, digging
and shovelling, felling trees
PAR
6.0 – 7.9
walking: uphill with load or cross-country, climbing stairs,
exercise: jogging, cycling, skiing, tennis, football
Physical activity level (PAL)
Some-one's overall physical activity level is calculated by summing the various activities during the day, multiplied by the time spent in each activity as a proportion of the day. Again it is expressed as a multiple of BMR. The UK Department of Health classification of occupational work during the typical 8 hours working day (excluding leisure activities) are as follows:
Light
work: PAR = 1.7
professional, clerical and technical workers, administrative and managerial
staff, sales representatives, housewives
Moderate
work: PAR = 2.2 (women) – 2.7 (men)
sales staff, domestic service, students, transport workers, joiners, roofing
workers
Moderately
heavy work – PAR 2.3 (women) – 3.0 (men)
machine operators, labourers, agricultural workers, bricklaying, masonry
Heavy
work: PAR = 2.8 (women) – 3.8 (men)
labourers, agricultural workers, bricklaying, masonry, where there is little
or no mechanisation
These figures exclude leisure activities; a desirable physical activity level for cardiovascular and general health is 1.7 x BMR, a figure that is achieved in UK by only some 22% of men and 13% of women. The average PAL in UK is 1.4 x BMR.
In the simulation program you will complete an activity diary for your subject, which will permit you to determine PAL and total energy requirement.
Diet-induced thermogenesis (DIT)
The increase in metabolic activity after a meal, as a result of the energy cost of digestion and absorption, and, more importantly, the energy cost of synthesising reserves of glycogen, triacylglycerol and protein. It may be as much as 10 - 15% of the energy yield of the meal.
Total energy expenditure (TEE)
The total expenditure of energy (and hence the total energy requirement to maintain body weight) in kJ or kcal, calculated from BMR x PAL + an estimate of DIT.
Changes in food intake and changes in body weight
Calculating in kJ, adipose tissue is:
15% water
5% protein at 17 kJ /g = 0.85 kJ /gram adipose tissue
80% triacylglycerol at 37 kJ /gram = 29.6 kJ /g adipose tissue
Hence 1 gram of adipose tissue yields 30 kJ.
From this we can calculate a theoretical weight change as:
33 grams /MJ energy imbalance /day
230 grams /MJ energy imbalance /week
Calculating in kcal, adipose tissue is:
15% water
5% protein at 4 kcal /g = 0.2 kcal /gram adipose tissue
80% triacylglycerol at 9 kcal /gram = 7.2 kcal /g adipose tissue
Hence 1 gram of adipose tissue yields 7.4 kcal
From this we can calculate a theoretical weight change as:
135 grams /1000 kcal energy imbalance /day
945 grams /1000 kcal energy imbalance /week
Assuming an energy requirement of 10 MJ (2500 kcal) /day , this means that total starvation would result in a maximum possible weight loss of:
330 grams /day
2.3 kg /week
In practice we do not see this theoretical change in weight with changes in food intake because there is adaptation.
As
food intake increases above requirements there is adaptation, and the rate of
weight gain decreases until the subject is again in energy balance, but with
a higher body weight, and a higher energy requirement.
This is because, as food intake increases, so there is:
a higher energy cost of digestion and absorption of foods as more is eaten
a higher energy cost of synthesis of triacylglycerol and glycogen, because
there is more food available for synthesis of body reserves
an increased rate of protein turnover as a result of greater availability
of energy
a higher BMR as body weight increases
a higher energy cost of physical activity with greater body weight because
it requires more energy to move a heavier body
As food intake decreases
below requirements there is again adaptation, and the rate of weight loss decreases
until the subject is again in energy balance, but with a lower body weight and
a lower energy requirement.
This is because, as food intake decreases, so there is:
a lower energy cost of digestion and absorption of foods as less is eaten
a much lower energy cost of synthesis of triacylglycerol and glycogen
because
there is less surplus food available for synthesis of body reserves
a decreased rate of protein turnover as a result of lower availability
of energy
a lower BMR as body weight decreases
a lower energy cost of physical activity with lower body weight because
it requires less energy to move a lighter body
During the first few days of severe energy restriction the rate of weight loss is greater than that calculated from loss of adipose tissue. This is because there is a considerable depletion of glycogen reserves in muscle and liver, and when the glycogen is depleted the water trapped within the glycogen molecules is lost.
Measuring energy expenditure and fuel utilisation by indirect calorimetry
Ideally, energy expenditure is measured by measuring heat output from the body. This is possible only using a thermally insulated chamber, which severely restricts the types of activity that can be performed, and it is usual to estimate energy expenditure indirectly, by measuring oxygen consumption.
To first approximation (and certainly within the limits of measuring oxygen consumption experimentally), there is an energy yield of 20 kJ (5 kcal) /L of oxygen consumed, regardless of the fuel being metabolised.
| energy yield | energy yield | oxygen | carbon dioxide | RQ | energy yield | energy yield | |
| kJ /g | kcal /g | L /g | L /g | kJ /L oxygen | kcal /L oxygen | ||
| carbohydrate | 16 | 4 | 0.829 | 0.829 | 1.000 | ~20 | ~5 |
| protein | 17 | 4 | 0.966 | 0.782 | 0.809 | ~20 | ~5 |
| fat | 37 | 9 | 2.016 | 1.427 | 0.707 | ~20 | ~5 |
Click here to open a printable version of the table.
This means that by measuring oxygen consumption during various activities we can estimate energy expenditure.
If we also measure carbon dioxide production, we can estimate the relative amounts of fat and carbohydrate being metabolised. This assumes that we ignore any oxidation of amino acids - for precision it would be necessary to estimate protein and amino acid metabolism by measurement of urinary urea excretion.
The percentage of energy derived from carbohydrate = ((RQ - 0.707) / (1 - 0.707))
x 100
and the percentage derived from fat = 100 - percentage from carbohydrate
In the simulation you will be shown oxygen consumption and carbon dioxide production for a subject walking at various speeds on a treadmill; using the data above, and the subject's BMR (which appears on the screen) you should be able to calculate the energy cost and PAR, as well as the relative amounts of carbohydrate and fat being metabolised.