D1 - Human nutrition

A nutrient

A chemical substance found in foods that is used in the human body

Five classes of nutrients

Carbohydrates, proteins, lipids, minerals and water

Essential nutrients

Those that cannot be synthesized 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

Are carbohydrates an essential nutrient?

No, carbohydrates are not considered essential nutrients as human diets can obtain energy from other sources without ill effect, even though the body does not synthesize carbohydrates

Malnutrition

A health condition caused by a deficiency, imbalance or excess of nutrients in the diet

What causes malnutrition?

* Improper dietary intake of nutrients
* Overnutrition or undernutrition
* Can be caused by the inadequate utilization of nutrients by the body - e.g. due to illness or disease

Signs of malnutrition

* stunted growth
* wasting (undernutrition)
* obesity (overnutrition)

How can the energy content of food be estimated?

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 is **4.18** J – this is the specific heat capacity of water

Equation for energy content

Energy (joules) = Mass of water (g) × 4.2 (J/gºC) × Temperature increase (ºC)

Biggest source of error in calorimetry & how to increase reliability

* unwanted loss of heat
* 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 cm3 or 1 ml)

Three types of nutrients that are commonly used as energy sources

* carbohydrates - 1,760 kJ per 100 grams - preferential because easy to digest and transport


* lipids - 1,720 kJ per 100 grams - can store more energy but hard to digest and transport


* proteins - 4,000 kJ per 100 grams - produces nitrogenous waste which must be removed

How many universal amino acids

20

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

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

PKU

* Phenylkeronuria (PKU)
* 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

How is PKU 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

Two essential fatty acids and why

* ^^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

Result of not consuming essential fatty acids

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

Foods rich in essential fatty acids (omega-3 and omega-6) include fish, leafy vegetables and walnuts

Transport of 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 disposal (hence lower blood cholesterol levels)

Fatty acids in blood

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 
* T*rans* fats increase LDL levels and lower HDL levels, significantly raising blood cholesterol levels
* C*is*-polyunsaturated fats raise HDL levels, lowering blood cholesterol levels

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

organic molecules with complex chemical structures that are quite diverse and hence categorized by groups

Water soluble and fat soluble vitamins

* 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)

ascorbic acid

form of vitamin C

Function ascorbic acid

* 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

What is ascorbic acid made of?

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 effect

Vitamin C deficiency

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

Symptoms of vitamin C deficiency (scurvy)

* Skin discoloration & bruising
* hemorrhaging
* Anaemia
* Dental issues
* Exhaustion/fatigue
* Swelling of joints (edema)

Function vitamin D

absorption of calcium and phosphorus by the body – which contribute to bone mineralisation

Deficiency of vitamin D

* 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 synthesis

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

Who gets vitamin D deficiencies

Vitamin D deficiencies are usually restricted to individuals with highly limited sun exposure (e.g. elderly, certain ethnicities)

Risk of too much sun exposure

Skin cancer

Are dietary minerals an essential nutrient?

* 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)

Important functions played by minerals

* 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)

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

How is appetite 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)

How can the release of hormones that control appetite be triggered?

* 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 that trigger a hunger response

Hormones that trigger a hunger response include ghrelin (from stomach) and glucagon (from pancreas)

Hormones that trigger a lowered hunger response?

Hormones that trigger a satiety response include leptin (from adipose tissue) and CCK (from intestine)

Causes of obesity

Combination of following 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)

Hypertension

Abnormally high blood pressure

Why do individuals who are overweight or obese are more likely to suffer from hyper tension?

* 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)

Why are individuals who are overweight or obese more likely to suffer from type II diabetes?

* 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

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)

eating disorder in which individuals severely limit the amount of food they intake

Anorexia nervosa, most common in young females with body image anxiety and can potentially be fatal if left untreated

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

RDI

Recommended daily intake for a nutrient, the daily dietary level required to meet the requirements of health

Consideration RDI

It is an **estimate** only and will vary according to age, gender, activity levels and medical conditions