D1. Human Nutrition
Essential Nutrients and Energy
1. Outline why nutrients cannot be synthesised by the body, and why they have to be
included in the diet
A nutrient is a chemical substance found in foods that is used in the human body.
There are six classes of nutrients – carbohydrates, proteins, lipids, vitamins, minerals
and water. Essential nutrients are those that cannot be synthesised by the body and
must be ingested as part of the diet. Non-essential nutrients can be made by the body
or have a replacement nutrient which serves the same dietary purpose. Carbohydrates
are not considered essential nutrients as human diets can obtain energy from other
sources without ill effect.
2. Describe how malnutrition may be caused by a deficiency, imbalance or excess of
nutrients in the diet
Malnutrition is a health condition caused by a deficiency, imbalance or excess of
nutrients in the diet. It can be caused by an improper dietary intake of nutrients – e.g.
overnutrition (too much) or undernutrition (not enough). It can be caused by the
inadequate utilisation of nutrients by the body – e.g. due to illness or disease.
The symptoms of malnutrition will vary according to the specific nutrient and the type
of imbalance involved. Common signs of malnutrition included stunted growth and
wasting (undernutrition), as well as obesity (over nutrition).
Determining Energy Content
3. Determine the energy content of food by combustion
The energy content of food can be estimated by burning a sample of known mass and
measuring the energy released via calorimetry. Combustion of the food source causes
the stored energy to be released as heat, which raises the temperature of water. The
amount of energy required to raise 1 g of water by 1ºC is4.18 J – this is the specific
heat capacity of water.
The equation for calculating the energy content of a food source via calorimetry is as
follows:
Energy (joules) = Mass of water (g) ​× 4.2 (J/gºC) × Temperature increase (ºC)
The biggest source of error in calorimetry is usually caused by the unwanted loss of
heat to the surrounding environment. The food sources should be burnt at a constant
distance from the water to ensure reliability of results. The initial temperature and
volume of water should also be kept constant (1 g of water = 1 cm​3​ or 1 ml).
Comparing Energy Content
The three types of nutrients that are commonly used as energy sources are
carbohydrates, lipids (fats) and proteins.
- Carbohydrates are preferentially used as an energy source because they are
easier to digest and transport
- Lipids can store more energy per gram but are harder to digest and transport
(hence are used for long-term storage)
- Protein metabolism produces nitrogenous waste products which must be
removed from cells
The relative energy content of carbohydrates, proteins and fats are as follows:
- Carbohydrates – 1,760 kJ per 100 grams
- Proteins – 1,720 kJ per 100 grams
- Fats – 4,000 kJ per 100 grams
Amino Acids and Lipids
4. Explain why some amino acids are essential and how a lack of essential amino
acids affects the production of proteins
Amino acids are the monomeric building blocks from which proteins are constructed.
There are 20 different amino acids which are universal to all living organisms.
Amino acids can be either essential, non-essential or conditionally non-essential
according to dietary requirements. Essential amino acids cannot be produced by the
body and ​must​ be present in the diet. Non-essential amino acids can be produced by
the body and are therefore ​not required​ as part of the diet. Conditionally non-essential
amino acids can be produced by the body, but at rates lower than certain conditional
requirements (e.g. during pregnancy or infancy) – they are essential at ​certain times
only.
A shortage of one or more essential amino acids in the diet will prevent the production
of specific proteins. This is known as protein deficiency malnutrition and the health
effects will vary depending on the amino acid shortage.
5. Outline the cause and treatment of phenylketonuria (PKU)
Phenylketonuria (PKU) is a genetic condition that results in the impaired metabolism of
the amino acid phenylalanine. It is an autosomal recessive disease caused by a
mutation to the gene encoding the enzyme phenylalanine hydroxylase. Phenylalanine
hydroxylase (PAH) normally converts excess phenylalanine within the body into
tyrosine. In people with PKU, the excess phenylalanine is instead converted into
phenylpyruvate (also known as phenylketone). This results in a toxic build up of
phenylketone in the blood and urine (hence phenylketonuria).
Untreated PKU can lead to brain damage and mental retardation, as well as other
serious medical problems. Infants with PKU are normal at birth because the mother is
able to break down phenylalanine during pregnancy. Diagnosis of PKU is made by a
simple blood test for elevated phenylalanine levels shortly after birth.
PKU is treated by enforcing a strict diet that restricts the intake of phenylalanine to
prevent its build up within the body. This low-protein diet should include certain types
of fruits, grains, vegetables and special formula milk. This diet should be
supplemented with a medical formula that contains precise quantities of essential
amino acids. Patients who are diagnosed
early and maintain this strict diet can have
a normal life span without damaging
symptoms.
6. Explain why some fatty acids are essential
Humans can synthesise most fatty acids from carbohydrates, but two
(cis)-polyunsaturated fatty acids are considered essential. Alpha-linolenic acid (an
omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid) cannot be synthesised by
the body. This is because humans lack the enzyme required to introduce double
bonds at the required position of the carbon chain.
Essential fatty acids are modified by the body to make important lipid-based
compounds (such as signalling molecules). There is evidence to suggest dietary
deficiencies of these fatty acids may be linked to impaired brain development (e.g.
depression) and altered maintenance of cardiac tissue (e.g. abnormal heart function) –
although this evidence is contested.
Foods rich in essential fatty acids (omega-3 and omega-6) include fish, leafy
vegetables and walnuts.
7. Explain the use of Cholesterol in blood as an indicator of the risk of coronary heart
disease
Fats and cholesterol cannot dissolve in the bloodstream and so are packaged with
proteins (to form lipoproteins) for transport.
- Low density lipoproteins (LDLs) carry cholesterol from the liver to the body
(hence raise blood cholesterol levels)
- High density lipoproteins (HDLs) carry excess cholesterol back to the liver for
t disposal (hence lower blood cholesterol levels)
and
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The mix of fatty acids consumed as part of a diet directly influences the levels of
cholesterol in the bloodstream:
- Saturated fats increase LDL levels within the body, raising blood cholesterol
levels
- Trans fats increase LDL levels and lower HDL levels, significantly raising blood
cholesterol levels
- Cis-polyunsaturated fats raise HDL levels, lowering blood cholesterol levels
worst
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High cholesterol levels in the bloodstream lead to the hardening and narrowing of
arteries (atherosclerosis). When there are high levels of LDL in the bloodstream, the
LDL particles will form deposits in the walls of the arteries. The accumulation of fat
within the arterial wall leads to the development of plaques which restrict blood flow. If
coronary arteries become blocked, coronary heart disease (CHD) will result – this
includes heart attacks and strokes.
Vitamins and Minerals
8. Describe why vitamins are chemically diverse carbon compounds that cannot be
synthesised by the body
Vitamins are organic molecules with complex chemical structures that are quite
diverse and hence categorised by groups. Water soluble vitamins need to be
constantly consumed as any excess is lost in urine (e.g. vitamins B, C). Fat soluble
vitamins can be stored within the body (e.g. vitamins A, D, E, K).
The functions of vitamins are as diverse as their structure, although many function as
cofactors, antioxidants or hormones. Many vitamins are essential as they cannot be
synthesised by the body and their absence may cause a deficiency disease.
9. Outline why the production of ascorbic acid is practiced by some mammals, but
not others that need a dietary supply
Ascorbic acid is a form of vitamin C that is required for a range of metabolic activities
in all animals and plants. In mammals it functions as a potent antioxidant and also
plays an important role in immune function. It is also involved in the synthesis of
collagen (a structural protein) and in the synthesis of lipoproteins.
Ascorbic acid is made internally by most mammals from monosaccharides – but it is
not produced by humans. Consequently, human must ingest vitamin C as part of their
dietary requirements in order to avoid adverse health effects
A deficiency in vitamin C levels will lead to the development of scurvy and a general
weakening of normal immune function. Common food sources of vitamin C include
citrus fruits and orange juice.
10. Explain why a lack of Vitamin D or calcium can affect bone mineralization and
cause rickets or osteomalacia
Vitamin D is involved in the absorption of calcium and phosphorus by the body –
which contribute to bone mineralisation. In the absence of sufficient amounts of this
vitamin, these elements are not absorbed but instead excreted in the faeces. This can
lead to the onset of diseases such as osteomalacia (where bones soften) or rickets
(where bones are deformed).
Vitamin D can be naturally synthesised by the body when a chemical precursor is
exposed to UV light (i.e. sunlight). The vitamin D may be stored by the liver for when
levels are low (e.g. during winter when sun exposure is reduced). Individuals with
darker skin pigmentation produce vitamin D more slowly and hence require greater
sun exposure.
Vitamin D deficiencies are usually restricted to individuals with highly limited sun
exposure (e.g. elderly, certain ethnicities). While excess sun exposure is beneficial for
vitamin D production, it also increases the risks of developing skin cancers.
11. Outline why dietary minerals are essential chemical elements
Dietary minerals are chemical elements required as essential nutrients by organisms.
Minerals present in common organic molecules are not considered essential – e.g. C,
H, O, N, S. Minerals include calcium (Ca), magnesium (Mg), iron (Fe), phosphorus (P),
sodium (Na), potassium (K) and chlorine (Cl).
Minerals in Human Development
Some of the important functions played by minerals are listed below:
- Major constituents of structures such as teeth and bones (e.g. Ca, P, Mg)
- Important components of body fluids (e.g. Na, K, Cl)
- Cofactors for specific enzymes or components of proteins and hormones (e.g.
Fe, P, I)
A deficiency in one or more dietary mineral can result in a disorder (e.g. lack of
calcium can affect bone mineralisation)
Minerals in Plant Development
Minerals are also important in plant development, making fruits and vegetables a good
source of certain dietary minerals. Magnesium is an important component of
chlorophyll (required for photosynthesis). Potassium is an inorganic salt found within
the sap of a plant (maintains water potential). Calcium is important for plant root and
shoot elongation.
Appetite and Dietary Intake
12. Describe how appetite is controlled by a centre in the hypothalamus
Appetite is controlled by hormones produced in the pancreas, stomach, intestines and
adipose tissue. These hormones send messages to the appetite control centre of the
brain (within the hypothalamus). Hormonal signals will either trigger a feeling of hunger
(promote feasting) or satiety (promote fasting).
The release of hormones can be triggered in a number of ways:
- Stretch receptors in the stomach and intestine become activated when
ingested food distends these organs
- Adipose tissue releases hormones in response to fat storage
- The pancreas will release hormones in response to changes in blood sugar
concentrations
Hormones will either stimulate or inhibit the appetite control centre to promote
sensations of hunger or satiety. Hormones that trigger a hunger response include
ghrelin (from stomach) and glucagon (from pancreas). Hormones that trigger a satiety
response include leptin (from adipose tissue) and CCK (from intestine).
Hint: Ghrelin Grows Hunger ; Leptin Lowers Hunger
13. Explain why overweight individuals are more likely to suffer hypertension and type
II diabetes and how starvation can lead to breakdown of body tissue
Changes in diet and appetite control may result in individuals over-indulging or
under-indulging during meals. Individuals who overeat are likely to gain weight and
develop obesity-related illnesses. Individuals who undereat are likely to lose weight
and exhibit starvation symptoms.
Obesity
Clinical obesity (BMI > 30) describes a significant excess in body fat and is caused by a
combination of two factors:
- Increased energy intake (i.e. overeating or an increased reliance on diets rich
in fats and sugars)
- Decreased energy expenditure (i.e. less exercise resulting from an increasingly
sedentary lifestyle)
Individuals who are overweight or obese are more likely to suffer from
hypertension(abnormally high blood pressure)
- Excess weight places more strain on the heart to pump blood, leading to a
faster heart rate and higher blood pressure
- High cholesterol diets will lead to atherosclerosis, narrowing the blood vessels
which contributes to raised blood pressure
- Hypertension is a common precursor to the development of coronary heart
disease (CHD)
Individuals who are overweight or obese are also more likely to suffer from type II
diabetes (non-insulin dependent). Type II diabetes occurs when fat, liver and muscle
cells become unresponsive to insulin (insulin insensitivity). This typically results from a
diet rich in sugars causing the progressive overstimulation of these cells by insulin.
Hence overweight individuals who have a high sugar intake are more likely to develop
type II diabetes.
Starvation
Starvation describes the severe restriction of daily energy intake, leading to a
significant loss of weight. As the body is not receiving a sufficient energy supply from
the diet, body tissue is broken down as an energy source. This leads to muscle loss
(as muscle proteins are metabolised for food) and eventually organ damage (and
death).
14. Explain the breakdown of heart muscle due to anorexia
Anorexia nervosa is an eating disorder in which individuals severely limit the amount
of food they intake. It is most common in young females with body image anxiety and
can potentially be fatal if left untreated
In severe anorexia, the body begins to break down heart muscle, making heart
disease the most common cause of death. Blood flow is reduced and blood pressure
may drop as heart tissue begins to starve. The heart may also develop dangerous
arrhythmias and become physically diminished in size.
15. Use a databases of nutritional content of foods and software to calculate intakes
of essential nutrients from a daily diet
The recommended daily intake for a nutrient (RDI) is the daily dietary level required to
meet the requirements of health. It is an ​estimate​ only and will vary according to age,
gender, activity levels and medical conditions.
The recommendations are based on a daily energy intake of 8400 kJ (2000 kcal) for
healthy adults. On food packages, this information is usually presented as a
percentage of a daily total (based on identified serving size).
Dietary intake can be recorded and compared against levels of energy expenditure in
order to monitor weight change. There are a variety of online databases and software
programs that can be used to calculate dietary intake and expenditure.