A-level Biology/Human Health and Disease/diet

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Dietary Reference Values

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Dietary reference values are just that – they are values that can help you plan your diet and ensure you are receiving everything you need in the right proportions. Dietary reference values are calculated for the following dietary components;

  • Estimated average requirement (energy/nutrients) – this is an estimate of the average requirements of a population
  • Reference nutrient intake – enough or more for nearly all the population
  • Low reference nutrient intake – sufficient for those with low needs.

Dietary reference values are used by people cooking for large groups of people - school caterers, hospitals and prisons for example. However, people suffering from disease will require different diets, so it is important to remember that a dietary reference value is an average, and so applies to groups and not individuals.


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Circumstances affect how much should be consumed in accordance with dietary reference values.

Pregnancy and lactation

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Contrary to popular belief, pregnancy does not affect an increase in a woman's intake of energy - she uses her fat stores and utilises micro nutrients more effectively to achieve this. However, pregnant women or those trying to conceive are advised to take folic acid to protect against defects such as neural tubes developing in the fetus. Vitamin D is recommended to be increased in both pregnancy and lactation.

Age, gender and activity

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All these things can affect how much food you require - for example, men require more energy than women, the young require certain things for growth. Those with a high activity level such as athletes, who will need higher levels than the population average and thus cannot easily utilise dietary reference values.

Food Labels

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Dietary reference values are also used on food labels to allow customers to make the choice that they want. Manufacturers may publish Recommended daily allowance information, and this must be accurate by law - for a food to be a source of a nutrient it must have at least 17% of the recommended daily allowance for that nutrient, and if it is 'rich in' a nutrient, it must contain over 50% of the recommended daily allowance for that nutrient.

A balanced diet

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A balanced diet is one which contains adequate amounts of all the necessary energy and nutrients required for healthy growth and activity. This includes both macro nutrients (carbohydrates, fats and proteins) and micro nutrients (vitamins and minerals). Sometimes cells can convert one compound to another, but this is not the case with a few things, and these are known as essential since we must intake them in our food to survive. They include essential amino acids, essential fatty acids and most minerals.

Table 1: Components of a balanced diet (Macronutrients)
Macronutrient Functions in humans
Protein Provide essential amino acids for making proteins such as collagen, enzymes, hormones and antibodies. Also provides energy.
Carbohydrates Provide energy.
Fats Provide energy, and essential fatty acids for plasma membranes and some hormones.
Table 2: Components of a balanced diet (Micronutrients and others)
Component Functions in humans
Vitamin A Proper functioning of the retina.
Vitamin B (inc folic acid) Required for respiration, protein synthesis, nerve function, red blood cell production.
Vitamin C To form collagen, make the skin function correctly, antioxidant, aids wound healing and stimulates absorption of iron from the gut.
Vitamin D Stimulates calcium uptake from the gut deposition in the bone.
Vitamin E Antioxidant
Vitamin K Formation of substances that promote blood clotting
Calcium Strengthen bones and teeth, used in muscle contraction and nerve impulses.
Chloride Osmotic balance, acidic environment in the stomach
Iodine Formation of thyroid hormones
Iron Haemoglobin and myoglobin
Magnesium Bone and teeth development
Phosphorus Used in bones and teeth as part of calcium-phosphate. ATP/DNA/RNA synthesis
Potassium Nerve impulses, muscle function.
Sodium Nerve impulses, muscle function, osmotic balance.
Zinc Wound healing, part of some enzymes, functioning of insulin depends on zinc.
Copper, cobalt and manganese Enzyme co-factor
Fluoride Tooth enamel aiding.
Fibre (excluding starch) Prevents constipation, protection against gut diseases, lowers cholesterol and aids peristalsis.
Water Solvent, coolant, removal of waste and transport medium.

Additional information

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The tables above do not cover all you need to know about the components of a balanced diet.


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There are 20 different amino acids, and we must take some in our diet as we can not convert others to them - these are known as essential amino acids and there are 8 of them. A balanced diet will include these 8 and a good supply of the non-essential 12. Vegetarians and vegans may have to take protein supplements to ensure they receive their essential amino acids.


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Only vitamin D and K can be made by the body, so the rest are essential components.

There are only two essential fatty acids, linoleic acid and linolenic acid - again these are fatty acids we cannot synthesise from other molecules. We do not require many of these, especially since well-nourished people will have nearly a year's supply in their fat stores. Fats usually provide energy.


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Plants often provide this in the form of cellulose, large complex compounds that we cannot digest so they simply pass through, providing bulk to the food. This bulk aids in peristalsis and retains water, the latter of which aids in passing faeces.


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Water is lost in the breath, sweat and faeces. It must be replaced, as death can occur from dehydration after a few days - it is an essential solvent.


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Malnutrition is the general term for a medical condition caused by an improper or insufficient diet. It is usually caused by inadequate consumption, but as with obesity can be caused by over-consumption.


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Anorexia nervosa, a psychological disease with physical consequences, and in extreme cases it can be fatal. Anorexia develops from extreme dieting, causing a weight well below normal, but the anorexic will continue to diet, sometimes until death. Symptoms include muscle wasting, thin sparse hair, low blood pressure, little to none sexual development (periods may stop) and a significant loss of body fat. Anorexics are also particularly susceptible to infection.


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In stark contrast to anorexia, obesity lies at the other end of the spectrum and is caused by eating more energy than used. The energy is stored as fat, and as weight increases, risk of other diseases such as CHD (see Smoking and Disease) and diabetes. The obese usually have high cholesterol and blood pressure, and are also are risk of several cancers, arthritis (from the increased strain on the skeleton), hernias and gallstones.


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This is both a lack of energy and nutrients, and the body can last a rather large amount of time without food (as opposed to without water), since it will drop its metabolic rate and utilise its reserves of carbohydrates, fats and proteins. The body uses glycogen stores in the liver (for less than a day), then fat stores (for 4–6 weeks, depending on the person) and finally protein in muscles and other tissues. However, as long as people are well fed before the starvation, they are usually ok for a while - but this is not true of those with previous deficiency, especially vitamin A deficiency.

Protein energy deficiencies

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In kwashiorkor, deficiency disease known to cause low weight, oedema, muscle waste, brittle hair, bloated appearance and loss of appetite, the body cannot make the proteins it needs during a period of low food because of the lack of energy and those already in the body are broken down for energy. This includes protein in the muscle and blood plasma, reducing solute concentration in blood plasma, increasing water potential. This causes water to not be absorbed into the blood from tissue fluid, causing tissue to swell (known as oedema).

Those who suffer from kwashiokor are often stunted as a result, and permanently cannot grow to their full potential.

Vitamin A deficiency

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Vitamin A deficiencies in children cause xerophthalmia, which is a drying or scarring of the cornea, causing night blindness, since vitamin A is converted into the pigment rhodopsin, which is used by rod cells at night. Also, epithelial cells use retinol from vitamin A to make retinoic acid, a chemical that aids cell growth, and without it the epithelia are not maintained properly and the body becomes susceptible to infections in the gut or gaseous exchange system, where epithelial cells protect it.

Vitamin A is found in some animal foods such as milk, eggs and fish-liver oils, and some fruits.

Vitamin D deficiency

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Vitamin D is usually synthesised in the skin using the suns energy in combination with cholesterol, and thus people usually receive enough from this source, but dark-skinned people and those whose religion requires them to cover their skin are advised to take vitamin D supplements. A vitamin d deficiency can cause rickets in children, since it regulates the deposition of calcium. In adults, it causes osteomalacia, which is a condition where the bones soften, causing them to be susceptible to fracture.

Blood tests have to be done and d-forte tabs 25000 iu has to be used once weekly for 1 month then every 2 weeks for one month and continue with 1 cap every month

In individuals where the reading is less than 100, the capsule needs to be taken twice weekly for a month. 25000 I U = 1.25 MG