This Topic deals with the balance between energy intake from food and energy output. We will look at the energy content of different food types, at the importance of energy balance (energy in = energy out) and at what factors influence the amount of energy we expend. Finally we will look at different methods of measuring energy expenditure.
At the end of this topic, you should know that :
- Food energy comes from carbohydrates, lipids and proteins.
- The energy in food is stored as potential energy, which is released during respiration and used to synthesise ATP.
- Energy is measured in kilojoules (kJ).
- Energy balance is considered as: energy in – energy out = change in energy stores. When the change in energy is zero, a person is said to be in energy balance. A positive energy balance will lead to an increase in weight over time, while a negative energy balance will lead to a decrease in weight.
- Fat contains 9 kcal of energy per gram, protein 4 kcal per gram and carbohydrates 3.8 kcal per gram. A diet rich in high fat foods will lead to a positive energy balance which can lead to overweight and obesity.
- High fat diets have also been linked to coronary heart disease by causing increased cholesterol levels which in turn may lead to an increase in the rate of formation of atherlosclerosis.
- The basal metabolic rate (BMR) is the minimum amount of energy required to maintain essential metabolic processes during a given period of time when at rest. It is usually measured in MJ/day or kcal/day.
- Physical activity uses energy in addition to the BMR. The more strenuous the activity, the more energy is expended.
- Every time you eat, some energy is used up in digesting, absorbing, metabolising and storing the food. This energy is eventually converted to heat. The process is known as dietary-induced thermogenesis. About 17% of energy in protein, 10% in carbohydrate and 3% in fat is burned off as heat energy.
- Increasing body size increases total energy expenditure. The greater the proportion of muscle to fat, the more energy is expended, since muscle burns more energy per gram than does fat.
- Males tend to expend more energy than females of the same age and body size. Energy requirements increase with age up to late teens, when it begins to decrease.
- Fasting, or strict dieting, causes the body to use up lean muscle tissue as well as fat tissue to meet the body’s energy requirements. Since muscle tissue uses a lot of energy, losing some of it lowers the total energy expended.
- During pregnancy a woman needs to consume about 837 kJ (200 kcal) a day more than normal during the last three months of pregnancy. A woman also has higher nutrient and energy requirements when she is breast feeding (lactating).
- Energy expenditure can be measured in various ways, including direct calorimetry, indirect calorimetry and heart rate recording.
- Direct calorimetry measures the amount of heat energy given out by an individual.
- Indirect calorimetry depends on the assumption that there is a relationship between oxygen consumption and energy expended. If oxygen consumption can be measured, energy expended can be estimated (but not measured directly).
- Heart rate monitoring depends on the assumption that there is a linear relationship (which can be graphed) between heart rate and oxygen consumption. Heart rate can be measured, oxygen consumption read off from the graph and energy expended estimated (but again not measured directly).
- The ratio of fat to lean tissue is known as body composition.
- Determining body composition by densitometry depends on the fact that fat is less dense than lean tissue. Density = mass (g) / volume cm3. Volume is measured by submerging the person in water and measuring the volume of water displaced. Percentage body fat can then be calculated. Advantage – it is an accurate method; disadvantages – it requires specialist equipment, individual needs to be confident in water.
- A skinfold caliper is used to measure the layer of fat (in mm) under the skin at various places on the body. These measurements are used to estimate total body fat. Advantages – it is quick, cheap and fairly accurate; disadvantages – it takes practice to master the accurate use of the calipers, and it wrongly assumes that body fat is distributed in the same way in everyone.
- Bioelectrical impedance analysis depends on the following facts:
- that an electrical conductor allows electricity to pass through it easily, offering little resistance, but that an insulator offers resistance (or impedance) to the flow of electricity;
- that lean tissue, like muscle, is a good conductor while fat is an insulator.
A small electric current is passed through the body and the electrical resistance measured. The greater the resistance, the greater the body fat percentage. Advantages – it is quick, the equipment is portable and easy to use: disadvantage – not as accurate as other methods – changes in hydration levels and skin temperature will affect electrical conduction and therefore the estimation of body fat.
- Body Mass Index (BMI) is a calculation which is used to determine whether someone is a normal weight or overweight for their height. BMI (kg m-2) = weight (kg) / height (m)2. A BMI of less than 20 is classed as underweight, 20-25 is classed as ideal weight, 25-30 is overweight and more that 30 is obese. Advantage – it is very easy to calculate: disadvantage- someone may be classified as overweight or obese when the additional weight is not fat but muscle or bone mass.
- If a person is overweight their waist:hip ratio can be used to determine whether they are an ‘apple’ or ‘pear’ shape. Apple shaped people have more fat laid down in their abdominal area and have a higher risk of developing coronary heart disease, high blood pressure, Type 2 diabetes than those who are ‘pears’.
- The incidence of obesity is rising in the
UK. Obese people are at a higher risk of developing cardiovascular diseases, including coronary heart disease, and high blood pressure; Type 2 diabetes and osteoarthritis. The consequences of such health risks include time off work due to illness and reduced life expectancy. - A person will become obese if they remain in a positive energy balance. The rapid increase in obesity in recent years is mainly due to a more sedentary lifestyle and an increase in the intake of high fat foods. Treatments for obesity include reducing energy intake and increasing energy expenditure.
- Exercise is important as part of a weight-control programme because it increases energy expenditure.
- Exercise should be regular, about 30 minutes of aerobic exercise most days, and strenuous enough to increase the heart rate to between 55% and 70% of the maximum heart rate (220 – age). Examples of such exercise include brisk walking, jogging, swimming and dancing.
- Some of the benefits regular exercise has on body composition and weight control include:
- increased muscle:fat ratio;
- increased BMR;
- increased energy expenditure.
- Osteoporosis occurs when calcium and other minerals are removed more quickly from bone than can be added. The bones become more porous and brittle with an increased risk of fractures.
- Osteoporosis affects men, women and children but is most common in post-menopausal women.
- Risk factors for osteoporosis include increasing age, being female, being post-menopausal, a diet inadequate in calcium and vitamin D, family history of osteoporosis, smoking, excessive alcohol consumption, lack of exercise and excessive exercise (in women).
- Bone becomes stronger and denser as a result of weight bearing activities such as brisk walking, jogging, playing badminton and dancing.
- Non-weight bearing activities, like swimming, are thought to have little effect on increasing bone density.
- Insulin and glucagon are hormones produced in the islets of Langerhans in the pancreas. Insulin is produced by β cells while glucagon is produced by α cells.
- When blood sugar levels are high, insulin is released into the blood and causes glucose to be converted to glycogen in the liver, thus returning blood glucose levels to normal.
- When blood sugar levels are low, glucagon is released causing glycogen in the liver to be converted to glucose. Again the blood glucose level returns to normal.
- Non-insulin dependent diabetes mellitus (NIDDM), or Type 2 diabetes, is linked to obesity. More than 80% of people with NIDDM are or have been overweight.
- Heredity is also thought to be a factor in the development of the NIDDM since the disease tends to run in families.
- Insulin works by binding to specific insulin receptors in the cell membranes of its target cells (liver cells, skeletal muscle cells and fat cells).
- As a result of insulin binding to the receptors, the cell membranes allow glucose to pass through into the cells more easily.
- People with NIDDM have normal (or above normal) levels of insulin, but the target cells have become less sensitive to its effects.
- When someone is obese the number of insulin receptors decreases. This reduced number of receptors makes the cells less sensitive to insulin and there is a reduced uptake of glucose by them.
Exercise improves the uptake of glucose in people with NIDDM. This is thought to be due to an increase in the sensitivity of and an increase in the number of insulin receptors on the cell membranes of target cells, particularly skeletal muscle cells